Design Guidance for Intersection Auxiliary Lanes (2014)

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112 Recommended Revisions to AAshto Green Book Overview This appendix presents the research team’s suggested changes to the AASHTO Policy on Geometric Design of Highways and Streets (commonly known as the Green Book) (2). Proposed additions to existing content are shown with double underlines, and proposed deletions are shown with strikethroughs. The proposed revisions are based on research conducted as part of this NCHRP project along with research recently completed. Some of the proposed revisions include adding specific ref- erences to research reports or other sources; those sources can be found in the References list at the end of this document, just as the Green Book is referenced above. When revisions are for- mally added to the Green Book, the actual reference numbers of those sources should change to coincide with the existing numbering sequence in Chapter 9 of the Green Book. Proposed new or revised Green Book figures and tables are assigned a figure or table number within this appendix so that they will appear in the list of figures and list of tables at the front of this report. These titles also include the Green Book figure or title number that they would replace or they include an “X” number to indicate that they are new figures or tables. 9.3 Types and Examples of Intersections, Introductory Section (pages 9-8 to 9-10) Proposed Revision to Green Book The basic types of intersections are three-leg (T), four-leg, multi- leg, and roundabouts. Further classification of the basic inter- section types includes such variations as unchannelized, flared, and channelized intersections as shown in Figure 9-3. Additional variations include offset intersections, which are two adjacent T-intersections that function similar to a four-leg intersection, and indirect intersections that provide one or more of the intersection movements at a location away from the primary intersection. At each particular location, the intersection type is determined primarily by the number of intersecting legs; the topography; the character of the intersecting highways; the traf- fic volumes, patterns, and speeds; and the desired type of oper- ation. These characteristics are also related to the type of traf- fic control (e.g., traffic signal, two-way or all-way stop, yield on minor approach). Variations of these intersection types to improve capacity by providing indirect left-turn move- ments are addressed in Section 9.9 on “Indirect Left Turns and U-Turns.” . . . Although many of the intersection design examples are in urban areas, the principles involved apply equally to design in rural areas. Some minor design variations occur with dif- ferent kinds of traffic control, but all of the intersection types shown lend themselves to cautionary or non-stop control, stop control for minor approaches, four-way stop control, and both fixed-time and traffic-actuated signal control. Right- turn roadways without stop or yield control are sometimes provided at channelized intersections. Such free-flow channel- ized right-turn lanes should be used only where an adequate merge is provided. Where motor vehicle conflicts with pedes- trians or bicyclists are anticipated, provisions for pedestrians and bicycle movements must be considered in the design. Channelized right-turn lanes have a definite role in improving operations and safety at intersections; however, at locations with high pedestrian volumes In built-up areas, the use of free-flow channelized right-turn lanes should be considered only where significant traffic capacity or safety problems may occur without them and adequate pedestrian crossings can be provided. Discussion The results from NCHRP Project 3-89 (Design Guidance for Channelized Right-Turn Lanes) (28) need to be inte- grated into this section of the Green Book. For example, NCHRP Project 3-89 recommended that the term “free- flow right turn” be replaced with the term “channelized right-turn lane.” Another suggestion provided by others is to be more precise with the type of traffic control present, e.g., yield control, signal control, free flow (i.e., no traffic control). A P P E N D I X A

113 9.3 Types and Examples of Intersections, 9.3.1 Three-Leg Intersections (pages 9-10 to 9-14) Proposed Revision to Green Book Channelized Three-Leg Intersections Channelization is often desirable for some reasons described in Section 9.6.2. Where channelization is provided, islands and turning roadways should be designed to accommodate the wheel tracks of each vehicle movement while providing optimum cross- ing paths and storage for pedestrians within the proposed inter- section. The simplest form of channelization is accomplished by increasing the corner radius between the two roadways sufficiently to permit a separate turning roadway that is separated from the normal traveled ways of the intersecting approaches by an island as shown in Figure 9-5A and 9-5C. The approach roadway may include a separate right-turn lane leading to the turning roadway for the accommodation of right-turn traffic. Often the provi- sion for a separate lane for left turns or for through movements to bypass left-turning traffic is appropriate on two-lane highways where right-turning roadways are justified. Left-turning traffic can be accommodated by the flaring of the through highway as shown in Figure 9-5B and 9-5C. The right-turning roadways should be designed to discourage wrong-way entry while providing suffi- cient width for anticipated turning trucks. Figure 9-5B depicts a channelized intersection incorporating one divisional island on the crossroad. Space for this island is made by flaring the pavement edges of the crossroad and by using larger- than-minimum pavement edge radii for right-turning movements. Figure 9-5C shows an intersection with a divisional island and right-turning roadways, a desirable configuration for intersections on important two-lane highways carrying intermediate to heavy traffic volumes (e.g., peak-hour volumes greater than 500 vehicles on the through highway with substantial turning movements). All movements through the intersection are accommodated on separate lanes. Where the traffic demand at an intersection approaches or exceeds the capacity of a two-lane highway and where signal con- trol may be needed in rural areas, it may be desirable to convert the two-lane highway to a divided section through the intersection, as shown in Figure 9-5C. In addition to adding auxiliary lanes on the through highway, the intersecting road (i.e., the stem of the three-leg intersection) may be widened on one or both sides for better maneuverability and increased capacity on the crossroad. The right-turn lane in the upper right quadrant accommodates a non-restricted exit from the major route. Figures 9-4B and 9-5B provide examples of bypass lanes, which are added to the outside edge of the approach, allowing through vehicles to pass left-turning vehicles on the right, while Figures 9-4C and 9-5C show traditional left-turn lanes. Regardless of the treatment, consideration of traffic demand, delay savings, crash reduction, and construction costs are all key factors in deter- mining whether to install a left-turn lane or a bypass lane. Research on left-turn accommodations at unsignalized intersections (9) pro- duced warrants for the installation of left-turn lanes and bypass lanes that account for those factors. Dimensions for turning roadways (e.g., lane width, taper length, deceleration and storage length) are provided in Section 9.7. Bypass lanes for through traffic should be designed with the same lane width as the width of the travel lane upstream and downstream of the intersection; the taper rate recommended in Section 9.7.2 for turning roadways can also be used for bypass lanes. Warrants for the installation of bypass lanes and left-turn lanes are provided in Section 9.7.3. Discussion The existing language on bypass lanes in this section does not provide information on installation warrants or design dimensions. While the warrants could be inserted here, it seems better suited for inclusion in Section 9.7.3. Therefore, the rec- ommended additions to this section include reference to the recently completed research in NCHRP Report 745 (9) that developed the warrants and a cross-reference to Section 9.7.3 where the new information on warrants is proposed. In addi- tion, a cross-reference to Section 9.7 is also provided to give guidance on appropriate design dimensions. During the review of this section, researchers observed that revisions to some existing diagrams would also be beneficial. Specific suggestions for revision include: • The left through lanes on the major road in Figure 9-3B appear to lead into the opposing left-turn lanes. The through lanes should be realigned. • The lane line separating the eastbound through and left- turn lanes in Figure 9-4C appears to be missing, and the eastbound-to-northbound left-turn movement arrow in Figure 9-4B appears to turn into oncoming traffic. The figure should be revised accordingly. • All of the figures in this section could benefit from inclusion of the conflict points in each intersection configuration. A more detailed review and revision of these figures was not a point of emphasis in this review, but these suggested revisions are provided as information to be considered in a more formal review by AASHTO in the future. 9.3 Types and Examples of Intersections, 9.3.2 Four-Leg Intersections (pages 9-14 to 9-19) Proposed Revision to Green Book A flared intersection, illustrated in Figures 9-6B and 9-6C, has additional capacity for through and turning movements at the intersection. Auxiliary lanes on each side of the normal pavement at the intersection illustrated in Figure 9-6B enable through vehicles to pass slow-moving vehicles preparing to turn right. Depending on the relative volumes of traffic and the type of traffic control used, flaring of the intersecting roadways can be accomplished by parallel auxiliary lanes, as on the highway shown horizontally, or by pave- ment tapers, as shown on the crossroad. Flaring generally is simi- lar on opposite legs. Parallel auxiliary lanes are essential where traffic volume on the major highway is near the uninterrupted-

114 flow capacity of the highway or where through and cross-traffic volumes are sufficiently high to warrant signal control. Auxiliary lanes are also desirable for lower volume conditions. The length of added pavement should be determined as it is for speed-change lanes, as shown in the subsection on Auxiliary Lanes in Sec- tion 9.7, and the length of uniform lane width, exclusive of taper, should normally be greater than 45 m [150 ft] on the approach side of the intersection. The length of the lane-addition and lane- drop tapers needed to accomplish the flaring can be determined from Equations 3-37 and 3-38 in Section 3.34. . . . Typical configurations of four-leg intersections with simple channelization are shown in Figure 9-7. Right-turning roadways as shown in Figure 9-7A are often provided at major intersections for the more important turning movements, where large vehicles are to be accommodated, and at minor intersections in quadrants where the angle of turn greatly exceeds is substantially below 90 degrees as shown in Figure 9-8A. A configuration with right-turn roadways in all four quad- rants of the intersection as illustrated in Figure 9-7A is suitable where sufficient space is available and right-turn volumes are high. Where one or more of the right-turning movements need separate turning roadways, additional lanes are generally needed for the complementary left-turning movements. The intersection with divisional islands on the crossroad illustrated in Figure 9-7B fits a wide range of volumes and its capacity is governed by the roadway widths provided through the intersection. For an intersection on a two-lane highway operating near capacity or carrying moderate volumes at high speeds, a con- figuration with channelized left-turn lanes as shown in Fig- ure 9-7C may be considered. The auxiliary lanes are used for speed changes, maneuvering, and storage of turning vehicles. The form of channelization on the crossroad should be deter- mined based on the cross and turning volumes and the sizes of vehicles to be accommodated. Where roadways cross one another at an angle other than 90 degrees, it is desirable to realign one or both highways to reduce the skew angle. Drivers may have difficulty seeing cross traffic at an intersection with a severe skew because of the added difficulty in turning their heads and the reduced visibility often created by parts of the vehicle. These effects are most pronounced for right-turn-on-red (RTOR) maneuvers at signalized inter- sections and for any maneuver from a minor road at two-way stop-controlled intersections. Older drivers in particular have difficulty with skewed inter- sections, due to restricted range of motion and diminished reac- tion time. The Highway Design Handbook for Older Drivers and Pedestrians (29) recommends: 1. In the design of new facilities or redesign of existing facilities where right-of-way is not restricted, all intersecting roadways should meet at a 90-degree angle. 2. In the design of new facilities or redesign of existing facilities where right-of-way is restricted, intersecting roadways should meet at an angle of not less than 75 degrees. 3. At skewed intersections where the approach leg to the left intersects the driver’s approach leg at an angle of less than 75 degrees, the prohibition of RTOR is recommended. Figure 9-8A shows use of right-turn islands and roadways at an intersection in quadrants where the angle of intersection is substantially below 90 degrees. Figure 9-8B shows an oblique intersection that has been modified to reduce the skew with separate turning roadways in the acute angle quadrants. When realignment cannot be obtained, extensive application of appro- priate signing and signal control is recommended. The simplest form of intersection on a divided highway has paved areas for right turns and a median opening conforming to designs discussed throughout this chapter. Sections 9.4 through 9.11 include guidelines to be used for intersection design. Often the speeds and volumes of through and turning traffic justify a higher type of channelization suitable for the predominant traf- fic movements. Channelization is often used at intersections on divided highways as shown in Figure 9-9. Right-turning roadways with speed-change lanes and median lanes for left turns afford both a high degree of efficiency in operation and high capacity and permit through traffic on the highway to operate at reasonable speed. Figure 9-9B shows an intersection configuration with dual left-turn lanes for each of the left-turning movements. This configuration needs traffic signal control with a separate signal phase for the dual left-turn movement. Dual left-turn lanes may be used for any one approach or a combination of approaches for which the left-turn volumes are high. The auxiliary lanes in the median may be separated from the through lanes by pave- ment markings or by an elongated island, as shown for the east- west direction in Figure 9-9B. Furthermore, pavement markings, contrasting pavements, and signs should be used to discourage through drivers from entering the median lane inadvertently. Left-turning vehicles typically leave the through lane to enter the median lane in single file but, once within it, are stored in two lanes. On receiving the green signal indication, left-turn maneuvers are accomplished simultaneously from both lanes. The median opening and the crossroad pavement should be suf- ficiently wide to receive the two side-by-side traffic streams. Where roadways cross one another at an angle other than 90 degrees, the effects of the skew can be mitigated by providing right-turn roadways or realigning the cross street to reduce the impact of the skew. Figure 9-8A shows use of right-turn islands and roadways at an intersection in quadrants where the angle of intersection greatly exceeds 90 degrees. Drivers have difficulty seeing cross traffic at an intersection with a severe skew because of the difficulty drivers, particularly older drivers, have in turning their heads and the reduced visibility often created by parts of the vehicle. It is desirable to realign one or both highways to reduce the skew angle. Figure 9-8B shows an oblique intersection that has been modified to reduce the skew with separate turning roadways in the acute angle quadrants. When realignment cannot be obtained, extensive application of appropriate signing and signal control is recommended. Discussion The text mentions briefly that there is a suggested length for auxiliary lanes at flared intersections but does not really specify what that is or how to determine it, except to say, “The length of added pavement should be determined as it is for speed-change lanes and the length of uniform lane width, exclusive of taper, should normally be greater than 45 m [150 ft] on the approach

115 side of the intersection.” Speed-change lanes, it is assumed, refer to auxiliary lanes at interchanges, but no cross-reference is made; one could assume that instead of referring to inter- section auxiliary lanes in Section 9.7, it refers to Tables 10-3 through 10-5 and/or the discussion on interchange auxiliary lanes on pp. 10-76. The guidance given to the reader could be much improved, to describe not only the needed lengths of the full-width lane upstream and downstream of the intersection but also the appropriate lane-addition and lane-drop tapers, which could be drawn from the L = WS equations (3-37 and 3-38) on pp. 3-134. The discussion of the effects of skew and how it can be mitigated could lead to confusion or incorrect con- clusions (compare with assessment of Section 9.4.2). In addition, the supporting text on skew should be moved closer to Figure 9-8, where the examples are given. Finally, the text incorrectly describes turning roadways being pro- vided where the turning angle is greater than 90 degrees. Rewriting the text and redrawing the figures will resolve the discrepancies. The suggested revisions correct the error in the text about turning roadways on quadrants greater than 90 degrees, revise the discussion on skew to be consistent with the High- way Design Handbook for Older Drivers and Pedestrians (27), and revise text in Section 9.4.2 to have the information on skew closer to the figure that illustrates it. In addition to the Highway Design Handbook for Older Drivers and Pedestrians, see references under Section 9.4.2. 9.4 Alignment and Profile, 9.4.2 Alignment (page 9-27) Proposed Revision to Green Book Once a decision has been made to realign a minor road that intersects a major road at an acute angle, the angle of the realigned intersection should be as close to 90 degrees as practical. Although a right-angle crossing is normally desired, some deviation from a 90-degree angle is permissible. Recon- struction of an intersection to provide an angle of at least 60 75 degrees provides most of the benefits of a 90-degree inter- section angle while reducing the right-of-way takings and con- struction costs often associated with providing a right-angle intersection. Discussion There is no citation in the existing text to back up the claim that “an angle of at least 60 degrees provides most of the ben- efits of a 90-degree intersection angle,” and review of exist- ing literature did not uncover any substantiating research. However, recent research into both older driver characteris- tics and driver field of view considerations contradicts this statement and, instead, recommends either 75 or 70 degrees. While some of the studies are older, their results are sub- stantiated by newer research and, thus, were included in the list below. Further, many states’ design manuals recommend either 75 or 70 degrees, and a sampling of these references are included below. Finally, another section of Chapter 9 specifically calls for an intersection angle of not less than 75 degrees. In Section 9.6.5, Turning Roadways with Cor- ner Islands, Subsection Oblique-Angle Turns with Corner Islands, on page 9-112, the Green Book states, “If practical, angles of intersection less than 75 degrees should not be used.” References that support the use of recommending a limit of 70 or 75 degrees include the following: • FHWA Highway Design Handbook for Older Drivers and Pedestrians (29), in I. Intersections (At-Grade), states, “In the design of new facilities or redesign of existing facili- ties where right-of-way is restricted, intersecting roadways should meet at an angle of not less than 75 degrees.” • ITE, Traffic Engineering Handbook (5th edition) (92) states on page 385, “Crossing roadways should intersect at 90 degrees if possible, and not less than 75 degrees.” The handbook also says, “Skew angles in excess of 75 degrees often create special problems at stop-controlled rural intersections. The angle complicates the vision triangle for the stopped vehicle; increases the time to cross the through road; and results in a larger, more potentially confusing intersection.” • Gattis and Low (106) suggested from a driver field of view study that 70 degrees is more appropriate because with greater skew angles portions of the vehicle block driver line of sight. • Son et al. (45) developed method to calculate sight distance available to drivers at skewed intersections and directly considered intersection angle, lane width, shoulder width, position of stop line, vehicle dimension, and driver’s field of view. Results include tables listing sight distance avail- able based on skew and support skew angle of 70 degrees. • Garcia and Libreros (107) and Garcia (108) conducted research based on driver’s field of view and recommended skew angles of no less than 70 degrees for crossing maneuvers. • Ohio DOT, Location and Design Manual Volume 1— Roadway Design (109), Section 401.3 Crossroad Alignment, states, “Intersection angles of 70 degrees to 90 degrees are to be provided on all new or relocated highways. An angle of 60 degrees may be satisfactory if: (1) the intersection is sig- nalized; or (2) the intersection is skewed such that a driver stopped on the side road has the acute angle (at center of intersection) on his left side (vision not blocked by his own vehicle).” • Wisconsin DOT, Facilities Development Manual (110), in Chapter 11 Design, Section 25 Intersections at Grade, Sub- section 2.8.1 (FDM 11-25-2.8.1), says that for intersections on a tangent or on outside of curve the desirable skew angle

116 is between 75 degrees and 105 degrees with a minimum of 70 degrees and maximum of 110 degrees. • Illinois DOT, Bureau of Local Roads and Streets Manual (111) in Chapter 34, Intersections, states, “Preferably, the angle of intersection should be within 15° of the perpen- dicular. This amount of skew can often be tolerated because the impact on sight lines and turning movements is not significant. Under restricted conditions where obtaining the right-of-way to straighten the angle of intersection would be impractical, an intersection angle up to 30° from the perpendicular may be used.” • California DOT, Highway Design Manual (112), in Section 403.3 Angle of Intersection, states, “When a right angle cannot be provided due to physical constraints, the inte- rior angle should be designed as close to 90 degrees as is practical, but should not be less than 75 degrees.” • Harkey (47) recommended that the minimum critical angle for intersections in roadway design policies be revised to 75 degrees. 9.4 Alignment and Profile, 9.4.2 Profile (page 9-27) Proposed Revision to Green Book The calculated stopping and accelerating distances for pas- senger cars on grades of 3% or less differ little from the cor- responding distances on the level. Grades steeper than 3% may need changes in several design elements to sustain operations equivalent to those on level roads. Most drivers are unable to judge the effect of steep grades on stopping or accelerating distances. Their normal deductions and reactions may thus be in error at a critical time. Accordingly, grades in excess of 3% should be avoided on the intersecting roads in the vicinity of the intersection. Where conditions make such designs too expensive, grades should not exceed about 6 percent, with a corresponding adjustment in specific geometric design elements. The United States Access Board provides proposed minimum design standards that are to be applied to all public rights-of-way, including sidewalks and crosswalks (113) and shared-use paths (57). These guidelines specify that the cross-slope of a sidewalk should not exceed 2% measured perpendicular to the direction of pedestrian travel. This may necessitate tabling of the intersection area, which will impact the vertical alignment of the roadway and may affect intersection drainage. Discussion What design elements need changed? How does one quantify “vicinity”? Which “specific geometric elements” apply? There is no citation given for this section to reference the supporting documentation for these terms. As such, a research project that generates a list of design elements, a quantification of vicinity, and a list of specific elements should be developed to support these statements, or they should be dropped or modified. Further, the Americans with Disabilities Act (ADA) requires that public rights-of-way, including sidewalks and crosswalks, be accessible to pedestrians with disabilities. ADA guidelines require that the cross-slope of a sidewalk should not exceed 2% measured perpendicular to the direction of pedestrian travel. This necessitates tabling of an intersection that impacts the vertical alignment of the roadway and can impact drainage. Text was added to cover this point. The ADA guidelines are evolving with the most current versions available at the fol- lowing links: • Published in the Federal Register on July 26, 2011: http:// www.access-board.gov/guidelines-and-standards/streets- sidewalks/public-rights-of-way/proposed-rights-of-way- guidelines • Published in the Federal Register on February 13, 2013: http://www.access-board.gov/guidelines-and-standards/ streets-sidewalks/shared-use-paths/supplemental-notice 9.6 Turning Roadways and Channelization, 9.6.1 Types of Turning Roadways (pages 9-55 to 9-56) Proposed Revision to Green Book General The widths of turning roadways for intersections are governed by the volumes of turning traffic and the types of vehicles to be accommodated. In almost all cases, turning roadways are designed for use by right-turning traffic. The widths for right-turning road- ways may also be applied to other roadways within an intersec- tion. There are three typical types of right-turning roadways at intersections: (1) a minimum edge-of-traveled-way design, (2) a design with a corner triangular island, and (3) a free-flow design using a simple radius or compound radii. The turning radii and the pavement cross slopes for free-flow right turns are functions of design speed and type of vehicles. For an in-depth discussion of the appropriate design criteria, see Chapter 3. Channelized Right-Turn Lanes Channelized right-turn lanes have a definite role in improving operations and safety at intersections. However, to achieve these benefits, they should have consistent design and traffic control and should be used at appropriate locations. Crosswalk location—A pedestrian crosswalk could poten- tially be placed at any location along a channelized right-turn roadway (e.g., upstream, center, or downstream). It is obviously desirable to place the crosswalk at whatever location would max- imize safety, presumably the location where pedestrians who are crossing or about to cross the right-turn roadway are most vis- ible to motorists and where motorists are most likely to yield to pedestrians. An evaluation of crosswalks at existing channelized right turns (26) revealed that the majority of the sites (nearly 70 percent) had marked crosswalks near the center of the chan- nelized right-turn lane; only about 30% of crosswalks were at

117 the upstream or downstream end of the channelized right-turn lane. Similarly, a highway agency survey (11) found that highway agencies prefer a crosswalk location near the center of a chan- nelized right-turn lane; over 70% of highway agencies reported in the survey that their practice was to place crosswalks near the center of channelized right-turn lanes. Consistency of crosswalk location at channelized right-turn lanes is important to pedestrians with vision impairment, and current highway agency practice indicates a preference for cross- walk locations near the center of a channelized right-turn lane. A crosswalk location at the center of the channelized right-turn lane moves vehicle-pedestrian conflicts away from both the diverge maneuver at the upstream end of the channelized right- turn lane and the merge maneuver at the downstream end of the channelized right-turn lane. The only potential exception to a center crosswalk location for channelized right-turn lanes is where a Stop sign or traffic signal control is provided at the entry to the cross street; the crosswalk should be beyond the stop line at that point. To summarize the recommended guidance for the placement of crosswalks at channelized right-turn lanes: • Where the entry to the cross street at the downstream end of the channelized right-turn lane has yield control or no control, place the crosswalk near the center of the channelized right- turn lane. • Where the entry to the cross street at the downstream end of the channelized right-turn lane has Stop sign control or traffic signal control, place the crosswalk immediately downstream of the stop bar, where possible. Where the channelized right- turn roadway intersects with the cross street at nearly a right angle, the stop bar and crosswalk can be placed at the down- stream end of the channelized right-turn roadway. Special crosswalk signing and marking—Marked crosswalks are the primary means of indicating the presence of a pedestrian crossing. However, drivers do not always yield the right of way to pedestrians simply because they are in a crosswalk. Other special crosswalk signing and marking treatments have been considered for use at pedestrian crossings on channelized right-turn road- ways to enhance crossing safety for pedestrians, in general, and for pedestrians with vision impairment. These include: • Use of a crosswalk to improve the visibility of the crosswalk for motorists and to better define crosswalk boundaries for pedes- trians (raised crosswalks are particularly helpful to pedestrians with vision impairment). • Addition of fluorescent yellow-green signs both at the cross- walk and in advance of the crossing location (to supplement the high-visibility markings). • Use of a real-time warning device to indicate to the motorist when a pedestrian is present in the area (may be activated via passive detection technologies such as microwave or infrared or via traditional methods such as push buttons). • Use of dynamic message signs (for real-time or static warning messages to motorists). Additional signing and pedestrian crosswalk treatments may improve the motorist yield behavior and pedestrian use of the crosswalk. Island type—A channelized right-turn lane consists of a right-turning roadway at an intersection, separated from the through travel lanes of both adjoining legs of the intersection by a channelizing island. At right-angle intersections, such chan- nelizing islands are roughly triangular in shape, although the sides of the island may be curved, where appropriate, to match the alignment of the adjacent roadways. Islands serve three pri- mary functions: (a) channelization—to control and direct traffic movement, usually turning; (b) division—to divide opposing or same-direction traffic streams; and (c) refuge—to provide refuge for pedestrians. Most islands combine two or all of these func- tions. Islands for channelized right-turn lanes typically serve all three functions. The edges of channelizing islands may be defined by raised curbs or may consist of painted pavement or turf that is flush with the pavement. Most channelizing islands in urban areas are defined by raised curbs. Curbed islands are considered most favorable for pedestrians because curbs most clearly define the boundary between the traveled way, intended for vehicle use, and the island, intended for pedestrian refuge. Curbed islands can improve the safety for pedestrians by allowing them to cross the street in two stages. Raised islands with cut-through pedes- trian paths are important to pedestrians with vision impairment because they provide better guidance and information about the location of the island than painted islands. Where curb ramps are provided, truncated dome detectable warnings are required at the base of the ramp, where it joins the street, to indicate the location of the edge of the street to pedestrians with vision impairment. Radius of turning roadway—Design criteria for the radii of channelized right-turn roadways are a function of turning speeds, truck considerations, pedestrian crossing distances, and resulting island sizes. Channelized right-turn lanes provide one method for accommodating larger turning radii without widen- ing the major-street pedestrian crossings and without increas- ing the intersection pavement area. Where right-turn volumes are high and pedestrian and bicycle volumes are relatively low, capacity considerations may dictate the use of larger radii, which enable higher-speed, higher-volume turns. However, small turn- ing radii, which promote low-speed right turns, are appropriate where such turns regularly conflict with pedestrians, as higher speeds have been shown to result in a decrease in yielding to pedestrians by motorists. Angle of intersection with cross street—The alignment of a channelized right-turn lane and the angle between the channel- ized right-turn roadway and the cross street can be designed in two different ways: • A flat-angle entry to the cross street. • A nearly right-angle entry to the cross street. The two designs differ in the shape of the island that creates the channelized right-turn lane. The flat-angle entry design has an island that is typically shaped like an equilateral triangle (often with one curved side), while the nearly right-angle design is typically shaped like an isosceles triangle. The flat-angle entry design is appropriate for use in channelized right-turn lanes with either yield control or no control for vehicles at the entry to the cross street. The nearly right-angle entry design can be used with Stop sign control or traffic signal control for vehicles at the entry to the cross street; yield control can also be used with this design where the angle of entry and sight distance along the cross street are appropriate.

118 Deceleration lanes—Drivers making a right-turn maneuver at an intersection are usually required to reduce speed before turning. Significant deceleration that takes place directly on the through traveled way may disrupt the flow of through traffic and increase the potential for conflicts with through vehicles. To minimize deceleration in the through travel lanes, deceleration lanes should be considered. Right-turn deceleration lanes pro- vide one or more of the following functions (5): • A means of safe deceleration outside the high-speed through lanes for right-turning traffic. • A storage area for right-turning vehicles to assist in optimiza- tion of traffic signal phasing. • A means of separating right-turning vehicles from other traffic at stop-controlled intersection approaches. The addition of a deceleration lane at the approach to a chan- nelized right-turn lane provides an opportunity for motorists to safely slow down prior to reaching the crosswalk area at the turning roadway. Acceleration lanes—Acceleration lanes provide an opportunity for vehicles to complete the right-turn maneuver unimpeded and then accelerate parallel to the cross-street traffic prior to merging. Channelized right-turn lanes with acceleration lanes appear to be very difficult for pedestrians with vision impairment to cross. Therefore, the use of acceleration lanes at the downstream end of a channelized right-turn lane should generally be reserved for locations where no pedestrians or very few pedestrians are present. Typically, these would be locations without sidewalks or pedestrian crossings; at such locations, the reduction in vehicle delay resulting from addition of an acceleration lane becomes very desirable. Pedestrian signals—Pedestrian signals can be used at pedes- trian crossings on channelized right-turn roadways to enhance crossing safety for pedestrians, particularly for pedestrians with vision impairment. Where a signal is provided for pedestrians to cross a channelized right-turn lane, a pedestrian-actuated signal should be considered. Minimum Edge-of-Traveled-Way Designs Where it is appropriate to provide for turning vehicles within minimum space, as at unchannelized intersections, the corner radii should be based on minimum turning path of the selected design vehicles. The sharpest turn that can be made by each design vehicle is shown in Sections 2.1.1 and 2.1.2, and the paths of the inner rear wheel and the front overhang are illustrated. The swept path widths indicated in Section 2.1.2, which are slightly greater than the minimum paths of nearly all vehicles in the class repre- sented by each design vehicle, are the minimum paths attainable at speeds equal to or less than 15 km/h [10 mph] and consequently offer some leeway in driver behavior. These turning paths of the design vehicles shown in Figures 2-1 through 2-9 and Figures 2-13 through 2-23 are considered satisfactory as minimum designs. Tables 9-15 and 9-16 summarize minimum-edge-of-traveled-way design values for various design vehicles. Discussion NCHRP 3-89 (28) recommends inserting a new subsection on channelized right-turn lanes, given that the proposed level of detail is currently not found in the Green Book. The proposed subsection would be inserted between the exist- ing subsections entitled “General” and “Minimum Edge-of- Traveled-Way Designs.” 9.6 Turning Roadways and Channelization, 9.6.1 Types of Turning Roadways (page 9-80) Proposed Revision to Green Book From the analysis of these maneuvers and corresponding paths, together with other pertinent data, the appropriate type of mini- mum design can be selected. Applications of minimum designs for turning movements are common, even in rural areas. Minimum designs are appropriate for locations with low turning speeds, low turning volumes, or high property values, or environmental issues. Discussion NCHRP Synthesis 422 (101) showed that states primarily use three factors for justifying tradeoffs in highway geometric design: safety, cost, and environmental issues. As such, this consideration should also be added to the provided list of factors. 9.6 Turning Roadways and Channelization, 9.6.1 Types of Turning Roadways (page 9-88) Proposed Revision to Green Book The dimensions presented in Figures 9-34 and 9-35 demonstrate why curb radii of only 3 to 4.5 m [10 to 15 ft] have been used in most cities. Curb radii should accommodate the expected amount and type of traffic and allow for safe turning speeds at intersections. Figures 9-34 and 9-35 show the impact on the right-turning path for typical curb radii designs used in many cities. A curb radius of 4.5 m [15 ft] is typically used for the intersection of a residential street with another residential street, collector, or arterial, while a curb radius of 7.5 m [25 ft] is typically used for the intersection of arterial streets or at locations that are truck or bus routes. Where larger radii are used, an intermediate refuge or median island is desirable or crosswalks may need to be offset so that crosswalk distances are not objectionable do not adversely impact intersec- tion operation and safety. Typically, refuge islands are provided when the crossing distance exceeds 18.2 m [60 ft]. In summary, the corner radii proposed at an intersection on urban arterial streets should satisfy the needs of the drivers using them, the amount of right-of-way available, the angle of turn between the intersection legs, the number of pedestrians using the crosswalk, the width and number of lanes on the intersecting street, and the posted speeds on each street. The following is offered as a guide: Discussion It is not clear how the figures demonstrate the reason for selected radii. What they do show is the right-turning paths

119 of various design vehicles superimposed upon typical curb radii found in many U.S. cities. This typical curb radius is 15 ft for residential street to residential/arterial street and 25 ft for intersections of arterial streets or locations that are truck/bus routes. The section cautions designers to ensure that crosswalk distances are not “objectionable” but does not quantify how this is determined. Sources for supporting information include the following: • AASHTO, Guide for the Planning, Design, and Operation of Pedestrian Facilities (114). • “A pedestrian pushbutton should be placed in the median of signalized mid-block crossings where the crossing distance exceeds 60 feet (18.2 meters).” Virginia DOT Guidelines for the Installation of Marked Crosswalks (115) on page 16. • “It has been proposed that pedestrian refuge islands be pro- vided wherever possible, when the total length of a cross- walk is greater than 75 feet, or in areas where there are many elderly or handicapped pedestrians.” ITE, Design and Safety of Pedestrian Facilities (116) on page 14. 9.6 Turning Roadways and Channelization, 9.6.2 Channelization (page 9-93) Proposed Revision to Green Book • Motorists should not be confronted with more than one deci- sion at a time; as such, sufficient median storage should be provided to permit through and left-turning traffic to make a two-stage maneuver. Discussion Crossing a divided highway or turning left from the crossroad onto a divided highway involves more than one decision if there is not sufficient median storage to break the movement into two phases. This maneuver is further complicated by the larger scan area created by a divided highway. 9.6 Turning Roadways and Channelization, 9.6.2 Channelization (page 9-93) Proposed Revision to Green Book • Where the distance to the downstream driveway or inter- section is less than the desirable distance for merging or weaving and where pedestrians are present, turning road- ways should be controlled with a yield, stop, or signal con- trol and the angle of the intersection should be greater than 60 75 degrees. • Traffic streams that intersect without merging and weaving should intersect at angles as close to 90 degrees as practical, with a range of 60 75 to 120 105 degrees acceptable. Discussion Intersection skew angle should be changed to 75 degrees. Please refer to detailed discussion above in Section 9.4 for details and supporting documentation. 9.6 Turning Roadways and Channelization, 9.6.2 Channelization (page 9-94) Proposed Revision to Green Book • Refuge areas for turning vehicles should be provided separate from through traffic. • For locations with sufficient turning volumes and/or safety concerns, separate storage lanes should be used to permit turning traffic to wait clear of through-traffic lanes. Discussion The use of “refuge” to describe the storage area for the turn- ing vehicles is confusing, as it is typically used in reference to areas to protect pedestrians or bicyclists. The use of sepa- rate storage lanes instead helps reduce that confusion and is more descriptive. Further, there needs to be sufficient traffic present to warrant the use of a separate turn lane before one should be required. 9.6 Turning Roadways and Channelization, 9.6.3 Islands (page 9-97) Proposed Revision to Green Book Widening a roadway to include a divisional island (Figure 9-37) should be done so that the proper paths to follow are unmistakably evident to drivers. The alignment should require no appreciable conscious effort in vehicle steering. Often the highway is on a tan- gent, and to introduce dividing islands, reverse-curve alignment would be needed. Tapers can be used, but should be consistent with lane shifts at the design speed. In rural areas, where speeds are generally high, reversals in curvature should preferably be with radii of 1165 m [3,825 ft] or greater. Sharper curves may be used on intermediate-speed roads (up to 70 km/h [45 mph]) with radii of 620 m [2,035 ft] or greater. Usually, the roadway in each direction of travel is bowed out, more or less symmetri- cally about the centerline as shown in Figure 9-37A. Widening may also be implemented on one side only with one of the roadways continuing through the intersection on a straight course (see Figure 9-37 B). When this arrangement is used for a two-lane road planned for future conversion to a divided highway, the traveled way on tangent alignment will become a permanent part of the ultimate development.

120 Widening on tangent alignment, even with flat curves, may produce some appearance of distorted alignment. Where the road is on a curve or on widening alignment, advantage should be taken of the curvature in spreading the traffic lanes with- out using reverse curves, as illustrated in sections C and D of Figure 9-37C. Discussion Figure 9-37 (reproduced as Figure A-1) needs to be improved. Currently, B makes it appear that the taper used is insuffi- cient, while D is drawn in such a way as to make it appear that the alignment for approaching traffic is directed into the opposite direction through lanes at the intersection, which would promote wrong-way movements. As such, B (and per- haps C, which also shows poor alignment) should be replaced with better representations of the concepts being illustrated. The word “tangent” in A should be changed to “centerline” to match the word used in the text. 9.6 Turning Roadways and Channelization, 9.6.3 Islands (page 9-104) Proposed Revision to Green Book Delineation is especially pertinent at the approach nose of a divisional island. In rural areas, the approach should consist of a gradual widening of the divisional island as indicated in Figure 9-41. Although not as frequently obtainable, this same design also should be striven for in urban areas. Preferably, the approach should gradually change to a raised surface with tex- ture or to jiggle bars that may be crossed readily even at consid- erable speed. The transition taper length should be computed with Equation 3-37 {page 3-134 in 2011 Green Book} where the posted or statutory speed limit is 70 km/h [45 mph] or greater and with Equation 3-38 (pp. 3-134 in 2011 Green Book) where the posted or statutory speed limit is less than 70 km/h [45 mph]. If this distance cannot be met, this transition section should be as long as practical. The cross sections in Figure 9-41 demonstrate the transition. The face of curb at the approach island nose should be offset at least 0.5 m [2 ft] and preferably 1 m [3 ft] from the normal edge of traveled way, and the wid- ened pavement gradually should be transitioned to the normal width toward the crossroad. Discussion There is no specific guidance given regarding the length of transition except to say that it “should be as long as practical.” Most state design manuals use a form of the taper equations contained on page 3-134 as Equation 3-37 and Equation 3-38 as shown in the following sources: • “An approach taper directs traffic to the right. Approach taper lengths are calculated using the following: Design Speed of 50 mph [80 km/h] or more: L = WS [L = 0.6 WS]. Design Speed less than 50 mph [80 km/h]: L = WS2/60, (L = WS2/156). Where: L = Approach taper length in feet [m] W = Offset width in feet [m] S = Design Speed [km/h].” Ohio DOT, Location and Design Manual Volume 1— Roadway Design (109) in Section 401.6 Approach Lanes. • “The length of the Approach Taper varies depending on the operating speeds. Guidelines for determining lengths are: For speeds of 45 mph (70 kph) and over: L = WS, (L = 0.6 WS). For speeds under 45 mph (70 kph): L = WS2/60, (L = WS2/100). Where: L = Length of entering taper, ft (m) W = Width to be tapered, ft (m) S = Operating Speed, mph (kph).” South Dakota DOT, Road Design Manual (117) in Chapter 12 Intersection, on page 12-22. • In Iowa DOT Design Manual, Chapter 6: Geometric Design: “The procedure for determining minimum taper ratios for redirecting through lanes is the same as shown in Table 1 for lane drops; however, for design speeds over 45 mph (70 km/h), the use of reverse curves rather than tapers is recommended.” Text support for Table 1 [reproduced as Table A-1 below] following: “When dropping a through lane, the minimum length of taper can be determined by the following formulas: L = WS2/60 for speeds of 40 mph or less (L = WS2/155 for speeds of 70 km/h or less) L = S × W for speeds of 40 mph or more (L = 0.62 × S × W for speeds of 70 km/h or more) where L = minimum length of taper, S = posted speed limit or 85th percentile speed, W = width of lane to be dropped or redirection offset. Preferably, taper Source: A Policy on Geometric Design of Highways and Streets (2011) by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission. Figure A-1. Green Book Figure 9-37 with suggested deletion of image D.

121 ratios should be evenly divisible by 5 (15:1, 20:1, etc.). Cal- culations that result in odd ratios should be rounded up to the next increment of 5” (118). “Table 1 utilizes the formu- las to determine the appropriate taper ratios for dropping a 12-foot (3.6 meter) wide lane. The ratio remains constant for a given design speed, while the length varies with the lane width” (118). • Georgia DOT, in Section 4.2.4 Lane Width Transitions and Shifts, states, “Lane width transitions can occur at several locations including: . . . Mainline lane shifts in advance of a typical section change such as a change in median width. There are two methods by which an alignment transition or “shift” may be accomplished: The first method is to treat the transition or shift as though it were any other alignment change. With this approach, a transition or shift would be accomplished through the use of a series of reverse curves. Quite often, the use of curve radii which do not require superelevation result in a length of transition greater than that required by providing a taper. Superelevation should be used if warranted by normal procedures. The second method of accomplishing a transition or “shift” involves the use of tapers. Tapers are acceptable provided the fol- lowing two conditions exist: The alignment shift is con- sistent with the cross-slope of the roadway and does not require “shifting” over the top of an existing pavement crown. The direction of the shift is not counter to the pavement cross-slope (from a superelevation or reverse- crown consideration). Taper lengths associated with shifts on Georgia roadways should be calculated as: Case 1— Design Speed ≥ 45 mph: L = W × S; Case 2—Design Speed < 45 mph: L = (W × S2)/60 Where L = distance needed to develop widening (ft), W = width of lane shift (ft), s = design speed (mph)” (119). • Washington State DOT Design Manual in Chapter 1310 Intersections, on page 1310-20 in Exhibit 1310-10a Median Channelization: Widening (120), provides a table of desir- able taper rates, reproduced as Table A-2. • While the formulas are not directly presented as part of the approach taper length discussion, the taper rates shown in the table are, for the most part, directly attributable to the formulaic methodology. 9.6 Turning Roadways and Channelization, 9.6.4 Free-Flow Turning Roadways at Intersections (page 9-106) Proposed Revision to Green Book An important part of the design on some intersection is the design of a free-flow alignment for right turns. Ease and smoothness of operation can result when the free-flow turning roadway is designed with compound curves preceded by a right-turn deceleration lane, as indicated in Figures 9-42B and 9-42C. The shape and length of these curves should be such that they: (1) allow drivers to avoid abrupt deceleration, (2) permit development of some superelevation in advance of the maximum curvature, and (3) enable vehicles to follow natural turning paths. The design speed of a free-flow turning roadway for right turns may vary between the end of the right- turn deceleration lane and the central section. The design speed of the turning roadway may be equal to, or possibly within 20 to 30 km/h [10 to 20 mph] less than the through roadway design speed. Refer to Tables 3-8 through 3-12 for minimum radii for right-turning traffic. Turning roadways at intersections should use the “upper range” design speed whenever practical although the “middle range” speeds may be used in constrained situations. Figure 9-42 Use of Simple and Compound Curves at Free-Flow Turning Roadways Discussion This section and Figure 9-42 are out of place here. This information deals more with the exit design from either a freeway or grade-separated arterial and is already covered in Chapter 10 (Section 10.9.6 Ramps). Further, it confuses the issue with relation to corner islands. As such, it should be removed. Table A-1. Length and taper ratio for dropping 12-ft lane (118). Table A-2. Desirable taper rates for widening for median channelization (120).

122 9.7 Auxiliary Lanes, 9.7.1 General Design Considerations (pages 9-124 to 9-125) Proposed Revision to Green Book Deceleration lanes are advantageous on higher-speed roads, because the driver of a vehicle leaving the highway has no choice but to slow down on the through-traffic lane if a deceleration lane is not provided. The failure to brake by the following drivers, because of a lack of alertness, may result in rear-end collisions. Acceleration lanes are not always desirable at stop-controlled intersections where entering drivers can wait for an opportunity to merge without disrupting through traffic. Acceleration lanes are advantageous on roads without stop control and on all high-volume roads even with stop control where openings between vehicles in the peak-hour traffic streams are infrequent and short, particularly on roads with higher operating speeds and/or higher volumes. Acceleration lanes are not desirable at all-way stop-controlled intersections where entering drivers can wait for an opportunity to merge without disrupting through traffic. For additional design guidance related to lengths and other aspects of deceleration and acceleration auxiliary lanes, refer to Section 10.9.6. Discussion The description of acceleration lanes at stop-controlled intersections is confusing; it looks like the word “always” should have been “all-way” to describe the traffic control. The follow- ing sentence about using acceleration lanes on stop-controlled approaches in peak periods seems to be a mixture of two differ- ent thoughts; if a roadway is stop-controlled, it does not need an acceleration lane because the Stop sign creates gaps, but if a roadway is not stop-controlled, then the high volumes and/or speeds in a peak period would be good cause for an accelera- tion lane for traffic entering from the minor road. The existing two-sentence discussion on acceleration lanes appears to be merely an introduction leading into the cross- reference for Section 10.9.6. It is recommended to separate it into its own paragraph to help distinguish it from the discus- sion on deceleration lanes. Interestingly, deceleration gets its own section immediately following in Section 9.7.2, while the remainder of discussion on acceleration lanes is pretty much confined to Chapter 10, where it is described in the context of interchange ramps, not intersections. This change removes confusion about using acceleration lanes at stop-controlled intersections and improves the vis- ibility of the text by placing it in its own paragraph. 9.7 Auxiliary Lanes, 9.7.2 Deceleration Lanes (pages 9-125 to 9-127) Proposed Revision to Green Book Figure 9-48 [reproduced as Figure 6-1 in this report] illus- trates the upstream functional area of an intersection in relation to the components of deceleration lane length, which consist of the perception-reaction (PR) distance, the lane change and decel- eration distance full deceleration length (also called the maneuver distance), and the storage length distance (also called the queue storage length distance). The physical length of a deceleration lane for turning vehicles consists of the entering taper length, L2, the deceleration length, L3, and the storage length, L4. Desirably, the total physical length of the auxiliary lane should be the sum of the length for these three components (lane change, deceleration, and storage distances). Common practice, however, is to accept a moderate amount of deceleration within the through lanes and to consider the taper length as a part of the deceleration within the through lanes. Each component of the deceleration lane length is discussed below. Perception-Reaction Distance The PR distance (d1) in Figure 9-48 represents the distance traveled while a driver recognizes the upcoming turn lane and prepares for the left-turn maneuver. It increases with perception- reaction time and speed. The perception-reaction time varies with the driver’s familiarity with the roadway segment and state of alert- ness; for example, an alert driver who is familiar with the roadway and traffic conditions has a smaller perception-reaction time than an unfamiliar driver. Traffic conditions on urban and suburban roadways could result in drivers having a higher level of alertness than those on rural highways. Therefore, a value of 1.5 sec is often used as the perception-reaction time for urban and suburban conditions, and 2.5 sec is often used for rural situations (83). Lane Change and Deceleration Length On many facilities, it is not practical to provide the full length of the auxiliary lane for deceleration due to constraints such as restricted right-of-way, distance available between adjacent intersections, and extreme storage needs. In such cases, at least part of the deceleration by drivers needs to be accomplished before entering the auxiliary lane. Research (98) has shown that crash potential increases as the difference in speed increases between vehicles in a traffic stream. In particular, the research concludes that a vehicle traveling 10 mph slower than other traf- fic (i.e., a vehicle with a 10-mph speed differential) is twice as likely to be involved in, or cause, a crash than when all vehicles are traveling at the same speed; the likelihood of a crash increases exponentially with greater speed differentials. Shorter auxiliary lane lengths will increase the speed differential between turning vehicles and through traffic. Therefore, the distances shown in Table 9-22 should be provided when possible, and designers are encouraged to provide sufficient deceleration length such that drivers do not need to decelerate more than 15 km/h [10 mph] within the through lane to accommodate the design. The decel- eration distances discussed are applicable to both left- and right- turning lanes, but the approach speed is usually lower in the right lane than in the left lane. Provision for deceleration clear of the through-traffic lanes is a desirable objective on arterial roads and streets and should be incorporated into design, whenever practical. Research (121) has shown that a two-stage deceleration process that uses rates of 4.2 ft/s2 while the driver changes lanes into the turning lane and 6.5 ft/s2 within the deceleration lane would accommodate most drivers. In locations with geometric con- straints, a design that incorporates a higher deceleration rate

123 could be offered as an alternative. While drivers may be able to negotiate the left-turn lane at higher deceleration rates, even up to the 11.2 ft/s2 rate described in Section 3.2.2 for stop- ping sight distance, a design that accommodates lower rates provides a more conservative design that is less demanding on drivers and contains more provision for storage of queues of left-turning vehicles. A design alternative for constrained conditions can be accomplished through the use of a constant 6.5 ft/s2 rate throughout the full deceleration length. Table 9-22 presents the estimated distances needed by drivers to maneuver from the through lane into the left-turn lanea turn bay and brake to a stop (6); typical lengths are based on a two-stage deceleration process with rates of 4.2 ft/s2 and 6.5 ft/s2, and constrained lengths are based on a constant 6.5 ft/s2 rate. Typical lengths should be used for new roadway projects and for reconstruction projects where sufficient right- of-way exists to provide the additional length. Constrained lengths may be considered at locations where the existing roadway is being changed and adjacent development prohibits the use of typical lengths. On many facilities, it is not practical to provide the full length of the auxiliary lane for deceleration due to constraints such as restricted right-of-way, distance available between adja- cent intersections, and extreme storage needs. In such cases, at least part of the deceleration by drivers needs to be accom- plished before entering the auxiliary lane. Inclusion of the taper length as part of the deceleration distance for an auxiliary lane assumes that an approaching turning vehicle can decel- erate comfortably up to 15 km/h [10 mph] before clearing a through lane. Shorter auxiliary lane lengths will increase the speed differential between turning vehicles and through traf- fic. A 15-km/h [10-mph] differential is commonly considered acceptable on arterial roadways. Higher speed differentials may be acceptable on collector highways and streets due to higher levels of driver tolerance for vehicles leaving or entering the roadway due to slow speeds or high volumes. Therefore, the distances discussed above should be accepted as a desirable goal and should be provided where practical. The deceleration distances discussed above are applicable to both left- and right- turning lanes, but the approach speed is usually lower in the right lane than in the left lane. Discussion Note 1 in Table 9-22 has a typo; it should be a + sign instead of an = sign. Reference #6 above the existing Green Book Table 9-22 is an NHI course on access management from 1998 (84), but the actual content of Table 9-22 and its notes are credited to Stover and Koepke’s Transportation and Land Development (83) in the TRB Access Management Manual (AMM) (13). It is unclear which of these is the correct source and whether this should this be the source for determining appropriate deceleration rates and lengths. Much of the existing material is very similar to that found in the AMM, so it is logical that the AMM’s sources are also the Green Book’s sources. In contrast, the Green Book’s discussion of a 10-mph dif- ference in speed does not have the same support as the cor- responding text in the AMM. The Green Book states that a 10-mph differential between turning and through vehicles is commonly considered acceptable on arterial roadways, though higher speed differentials may be acceptable on col- lector highways and streets due to higher levels of driver toler- ance for vehicles leaving or entering the roadway due to slow speeds or high volumes. Given that higher speed differentials also occur on high-speed arterials as found in this research, it would be useful for the Green Book to clarify on what basis that differential is “acceptable” and add the references to the research or other guidance documents that support the expla- nation. Referencing research by Soloman (98) or others, which describes the increased likelihood of crashes as speed differen- tial increases above 10 mph, establishes a basis for recommend- ing that designers provide left-turn lane designs that do not require speed differentials greater than 10 mph. This research also identified smaller speed differences for sites with longer combined taper and deceleration lengths. The results of the research conducted as part of this study generated the recommended language changes along with the replacement Figure A-2 and Table 2-1. Metric U.S. Customary Speed, km/h Distance,a m Speed [mph] Distancea [ft] 30 20 [20] [70] 50 45 [30] [160] 65 85 [40] [275] 80 130 [50] [425] 95 185 [60] [605] 110 245 [70] [820] a Rounded to 5m [5 ft] Notes: 1. The above full deceleration lengths are L2 = L3 in Figure 9-48. 2. Assumes a turning vehicle has “cleared the through lane” when it has moved laterally approximately 3 m [9 ft] so that a following through vehicle can pass without encroaching upon the adjacent traffic lane. 3. The speed differential between the turning vehicle and following through vehicles is 15 km/h [10 mph] when the turning vehicle “clears the through traffic lane.” 4. 1.8 m/s2 [5.8 ft/s2] deceleration while moving from the through lane into the turn lane; 2.0 m/s2 [6.5 ft/s2] average deceleration after completing lateral shift into the turn lane. Table 9-22. Desirable full deceleration lengths.

124 Figure A-2. Example graphic for replacing Green Book Figure 9-48. Functional area upstream of an intersection illustrating components of deceleration lane length. Metric U.S. Customary Speed, km/h Lane Change and Deceleration Distance,a m Speed [mph] Lane Change and Deceleration Distance a [ft] Typical Constrained Typical Constrained 30 29.0 21.3 [20] [95] [70] 40 42.7 32.0 [25] [140] [105] 50 59.4 45.7 [30] [195] [150] 55 79.2 62.5 [35] [260] [205] 65 100.6 80.8 [40] [330] [265] 70 125.0 103.6 [45] [410] [340] 80 152.4 126.5 [50] [500] [415] 90 181.4 153.9 [55] [595] [505] 95 213.4 182.9 [60] [700] [600] 105 246.9 213.4 [65] [810] [700] 110 283.5 248.4 [70] [930] [815] a Rounded to 5 ft and converted to equivalent values rounded to 0.1 m Notes: 1. The above full deceleration lengths are d2a + d2b in Figure 9-48. 2. The speed differential between the turning vehicle and following through vehicles is 15 km/h [10 mph] when the turning vehicle clears the through-traffic lane and completes the lane change (i.e., distance d2a). 3. Deceleration lengths for a Typical installation are based on 1.3 m/s2 [4.2 ft/s2] deceleration while moving from the through lane into the turn lane (distance d2a) and 2.0 m/s2 [6.5 ft/s2] deceleration after completing the lane change (distance d2b). 4. Deceleration lengths for a Constrained installation are based on a 2.0 m/s2 [6.5 ft/s2] deceleration throughout the entire length. 5. Typical lengths should be used for new roadway projects and for reconstruction projects where sufficient right-of-way exists to provide the additional length. 6. Deceleration rates are based on deceleration on dry, level pavement. Designs for approaches on downgrades of more than 2% and intersections at locations prone to wet pavement should account for the additional length necessary for vehicles to decelerate to a stop in those conditions. 7. Access points should not be allowed in the deceleration areas. Table A-3. Example material to replace Green Book Table 9-22. Desirable lane change and deceleration distances.

125 9.7 Auxiliary Lanes, 9.7.2 Deceleration Lanes (pages 9-127 to 9-130) Proposed Revision to Green Book Taper Length On high-speed highways it is common practice to use a taper rate between 8:1 and 15:1 (longitudinal:transverse or L:T). Long tapers approximate the path drivers follow when entering an auxiliary lane from a high-speed through lane. However, with exceptionally long tapers some through drivers may tend to drift into the deceleration lane—especially when the taper is on a hor- izontal curve. In addition, long tapers may constrain the lateral movement of a driver desiring to enter the auxiliary lanes. This situation primarily occurs on urban curbed roadways. As shown in Figure 9-48 and Table 9-22, the physical length of a deceleration lane for turning vehicles consists of the entering taper length, the length of the full-width deceleration lane, and the storage length. The distance over which the initial decelera- tion occurs, though it takes place during the lane change, does not necessarily coincide exactly with the taper length. The lon- gitudinal location along the highway where a vehicle will change lanes from the through lane to a full-width deceleration lane will vary depending on many factors. These factors include the type of vehicle, the driving characteristics of the vehicle operator, the speed of the vehicle, the number of vehicles already queued in the turn lane, weather conditions, and lighting conditions. Two common methods are available for a designer to deter- mine the actual length of the taper to be used for a deceleration lane. In the first method, the taper length is selected first, based on the guidance in the preceding paragraphs for a taper that is appropriate for the location being considered. Once the designer determines the appropriate taper rate and length, the length can then be related to Table 9-22 to determine the corresponding length of the full-width deceleration lane needed to provide the typical (or constrained) length shown in the table. Conversely, the designer may decide to first provide a specific length of full- width deceleration lane; if so, the designer would then subtract that length from the appropriate value in Table 9-22 to deter- mine the corresponding taper length. Drivers may complete their lane change downstream of the taper, particularly at locations with short or “squared-off” tapers. The difference between the deceleration distance and the physi- cal boundaries of the taper and full-width deceleration lane for a left-turn auxiliary lane is displayed in Figure 9-XA; dimensions for a right-turn lane are similar to those for a left-turn lane. For urbanized areas, short tapers appear to produce better “targets” for the approaching drivers and to give more posi- tive identification to an added auxiliary lane. Short tapers are preferred for deceleration lanes at urban intersections because of slow speeds during peak periods. The total length of taper and deceleration length should be the same as if a longer taper was used. This results in a longer length of full-width pavement for the auxiliary lane. Short tapers also allow more length for full-width deceleration distance and do not restrict the ability of drivers to complete their lane changes further upstream of the intersection. This type of design may reduce the likelihood that entry into the auxiliary lane may spill back into the through lane. Municipalities and urban counties Jurisdictions across the country are increasingly adopting the use of taper lengths such as short as 30 15 m [100 50 ft] for a single-turn lane and 45 30 m [150 100 ft] for a dual-turn lane for urban streets. Some agencies permit the tapered section of deceleration aux- iliary lanes to be constructed in a “squared-off” section at full paving width and depth. This configuration involves a painted delineation of the taper. The abrupt squared-off beginning of deceleration exits offers improved driver commitment to the exit maneuver and also contributes to driver security because of the elimination of the unused portion of long tapers. The design involves transition of the outer or median shoulders around the squared-off beginning of the deceleration lane. The squared-off design principle can be applied to median deceleration lanes, and it can also be used at the beginning of deceleration right-turn exit terminals when there is a single exit lane. When two or more exit lanes are used, the tapered designs discussed in Section 10.9.6 under “Speed-Change Lanes” are rec- ommended. Additional guidance for lengths of tapers may be found in the MUTCD (7). Figure A-3. Example graphic for new Green Book Figure 9-XA. Key dimensions for maneuvers and physical boundaries at left-turn auxiliary lanes.

126 The longitudinal location along the highway, where a vehicle will move from the through lane to a full-width deceleration lane, will vary depending on many factors. These factors include the type of vehicle, the driving characteristics of the vehicle operator, the speed of the vehicle, weather conditions, and lighting conditions. Straight-line tapers are frequently used, as shown in Figure 9-49A. The taper rate may be 8:1 [L:T] for design speeds up to 50 km/h [30 mph] and 15:1 [L:T] for design speeds of 80 km/h [50 mph] and greater. Straight-line tapers are particularly applicable where a paved shoulder is striped to delineate the auxiliary lane. Short, straight- line tapers should not be used on curbed urban streets because of the probability of vehicles hitting the leading end of the taper. A short curve is desirable at each end of long tapers as shown in Figure 9-49B, but may be omitted for ease of construction. Where curves are used at the ends, the tangent section should be about one-third to one-half of the total length. Symmetrical reverse-curve tapers are commonly used on curbed urban streets. Figure 9-49C shows a design taper with symmetrical reverse curves. A more desirable reverse-curve taper is shown in Figure 9-49D where the turnoff curve radius is about twice that of the second curve. When 30 m [100 ft] or more in length is provided for the tapers in Figure 9-49D, tapers 1 and 2 would be suitable for low-speed operations. All the example design dimensions and configurations shown in Figure 9-49 are applicable to right-turn lanes as well as left-turn lanes. Discussion The Green Book states, “The taper rate may be 8:1 for design speeds up to 30 mph and 15:1 for design speeds of 50 mph and greater.” This was approximately true for the sites in the deceleration study, though actual tapers varied between 6:1 and 18:1. The review of state design manuals for the interim report in this research showed that states gener- ally recommend a straight-line taper using one of the follow- ing guidelines: • Specific lengths, perhaps based on speed, between 60 and 180 ft. (Illinois guidelines state 135–310 ft, while Florida calls for 50 ft for all single left-turn lanes.) • Taper rate based on speed, generally between 8:1 and 15:1. • The values shown in the Green Book. Even though the taper rates were similar to Green Book guidelines, some deceleration took place upstream of the taper and the lane-change maneuver was not always completed within the taper at the deceleration study sites. Emphasizing that physical boundaries of turning lanes are not necessar- ily the same as the maneuver boundaries for drivers will add clarity to the guidelines. In addition, encouraging designers to use shorter tapers, such as Florida’s 50-ft length, facilitates longer full-width deceleration lanes and/or queue storage; it also facilitates more efficient lane-change maneuvers, remov- ing decelerating vehicles from the through travel lane more quickly and further upstream of the intersection. 9.7 Auxiliary Lanes, 9.7.3 Design Treatments for Left-Turn Maneuvers (pages 9-131 to 9-133) Proposed Revision to Green Book Guidelines for Design Installation of Left-Turn Lanes Many factors enter into the choice of type of intersection and the extent of design of a given type, but the principal controls are the design-hour traffic volume, the character or composition of traffic, and the design speed. The character of traffic and design speed affects many details of design, but in choosing the type of intersection they are not as significant as the traffic volume. Of particular significance are the actual and relative volumes of traf- fic involved in various turning and through movements. In designing an intersection, left-turning traffic should be removed from the through lanes, whenever practical. Therefore, provisions for left turns (i.e., left-turn lanes) have widespread application. Ideally, left-turn lanes should be provided at driveways and street intersections along major arterial and collector roads wherever left turns are permitted. In some cases or at certain loca- tions, providing for indirect left turns (jughandles, U-turn lanes, and diagonal roadways) may be appropriate to reduce crash fre- quencies and preserve capacity. The provision of left-turn lanes has been found to reduce crash rates anywhere from 20 to 65 7 to 48% (1) (9). Left-turn facilities should be established on roadways where traffic volumes are high enough or crash histories are sufficient to warrant them. They are often needed to provide adequate service levels for the intersections and the various turning movements. Warrants for Left-Turn Lanes and Bypass Lanes Figures 9-4B and 9-5B provide examples of bypass lanes, which are added to the outside edge of the approach, allowing through vehicles to pass left-turning vehicles on the right, while Figures 9-4C and 9-5C show traditional left-turn lanes. Regardless of the treatment, consideration of traffic demand, delay savings, crash reduction, and construction costs are all key factors in determining whether to install a left-turn lane or a bypass lane. Research on left- turn accommodations at unsignalized intersections (9) produced warrants for the installation of left-turn lanes and bypass lanes that account for those factors. Guidelines for where left-turn lanes should be provided are set forth in Table 9-X1 for urban and sub- urban roadways and Table 9-X2 and Table 9-X3 for rural highways (and Figure 9-XB, Figure 9-XC, and Figure 9-XD for urban and suburban roadways, rural two-lane highways, and rural four-lane highways, respectively). Sseveral documents for both signalized and unsignalized intersections provide guidance on left-turn lanes (10, 16, 19, 9). These guidelines discuss the need for left-turn lanes based upon (a) the number of arterial lanes, (b) design and operat- ing speeds, (c) left-turn volumes, and (d) opposing traffic volumes. The HCM (29) indicates that exclusive left-turn lanes at sig- nalized intersections should be installed as follows: • Exclusive left-turn lanes should be provided where exclusive left-turn signal phasing is provided; • Exclusive left-turn lanes should be considered where left-turn volumes exceed 100 veh/h (left-turn lanes may be provided for lower volumes as well based on the highway agency’s assess- ment of the need, the state of local practice, or both); and • Double left-turn lanes should be considered where left-turn volumes exceed 300 veh/h.

127 Left-Turn Lane Peak-Hour Volume (veh/hr) Three-Leg Intersection, Major Urban and Suburban Arterial Volume (veh/hr/ln) That Warrants a Left-Turn Lane Four-Leg Intersection, Major Urban and Suburban Arterial Volume (veh/hr/ln) That Warrants a Left-Turn Lane 5 450 50 10 300 50 15 250 50 20 200 50 25 200 50 30 150 50 35 150 50 40 150 50 45 150 < 50 50 or More 100 < 50 Table A-4. Example material for new Green Book Table 9-X1. Suggested left-turn lane warrants based on results from benefit-cost evaluations for urban and suburban arterials. (a) Three Legs (b) Four Legs 0 5 10 15 20 25 30 35 40 45 50 0 50 100 150 200 250 300 350 400 450 Le ft- Tu rn V ol um e (ve h/ hr /ln ) Major Arterial Volume (veh/hr/ln) Urban and Suburban Arterial, Three Legs Left-turn lane warranted Left-turn lane not warranted 0 5 10 15 20 25 30 35 40 45 50 0 50 100 150 200 250 300 350 400 450 Le ft- Tu rn Vo lu m e (ve h/ hr ) Major Arterial Volume (veh/hr/ln) Urban and Suburban Arterial, Four Legs Left-turn lane warranted Left-turn lane not warranted Figure A-4. Example graphic for new Green Book Figure 9-XB. Suggested left-turn lane warrants based on results from benefit-cost evaluations for intersections on urban and suburban arterials. Left-Turn Lane Peak-Hour Volume (veh/hr) Three-Leg Intersection, Major Two-Lane Highway Peak- Hour Volume (veh/hr/ln) That Warrants a Bypass Lane Three-Leg Intersection, Major Two-Lane Highway Peak- Hour Volume (veh/hr/ln) That Warrants a Left- Turn Lane Four-Leg Intersection, Major Two-Lane Highway Peak- Hour Volume (veh/hr/ln) That Warrants a Left- Turn Lane 5 50 200 150 10 50 100 50 15 < 50 100 50 20 < 50 50 < 50 25 < 50 50 < 50 30 < 50 50 < 50 35 < 50 50 < 50 40 < 50 50 < 50 45 < 50 50 < 50 50 or More < 50 50 < 50 Table A-5. Example material for new Green Book Table 9-X2. Suggested left-turn treatment warrants based on results from benefit-cost evaluations for rural two-lane highways.

128 (a) Three Legs (b) Four Legs Bypass lane warranted 0 5 10 15 20 25 0 50 100 150 200 250 Le ft- Tu rn V ol um e (ve h/ hr ) Major Highway Volume (veh/hr/ln) Rural, Three Legs, Two Lanes on Major Left-turn treatment not warranted Left-turn lane warranted 0 5 10 15 20 25 0 50 100 150 200 250 Le ft- Tu rn V ol um e (ve h/ hr ) Major Highway Volume (veh/hr/ln) Rural, Four Legs, Two Lanes on Major Left-turn treatment not warranted Left-turn lane warranted Figure A-5. Example graphic for new Green Book Figure 9-XC. Suggested left-turn treatment warrants based on results from benefit-cost evaluations for intersections on rural two-lane highways. Left-Turn Lane Peak-Hour Volume (veh/hr) Three-Leg Intersection, Major Four-Lane Highway Peak-Hour Volume (veh/hr/ln) That Warrants a Left-Turn Lane Four-Leg Intersection, Major Four-Lane Highway Peak-Hour Volume (veh/hr/ln) That Warrants a Left-Turn Lane 5 75 50 10 75 25 15 50 25 20 50 25 25 50 < 25 30 50 < 25 35 50 < 25 40 50 < 25 45 50 < 25 50 or More 50 < 25 Table A-6. Example material for new Green Book Table 9-X3. Suggested left-turn lane warrants based on results from benefit-cost evaluations for rural four-lane highways. (a) Three Legs (b) Four Legs 0 5 10 15 20 25 0 50 100 150 200 250 Le ft- Tu rn V ol um e (ve h/ hr ) Major Highway Volume (veh/hr/ln) Rural, Three Legs, Four Lanes on Major Left-turn lane not warranted Left-turn lane warranted 0 5 10 15 20 25 0 50 100 150 200 250 Le ft- Tu rn V ol um e (ve h/ hr ) Major Highway Volume (veh/hr/ln) Rural, Four Legs, Four Lanes on Major Left-turn lane not warranted Left-turn lane warranted Figure A-6. Example graphic for new Green Book Figure 9-XD. Suggested left-turn lane warrants based on results from benefit-cost evaluations for intersections on rural four-lane highways.

129 Table 9-23 is a guide to traffic volumes where left-turn facili- ties should be considered on two-lane highways. For the vol- umes shown, left turns and right turns from the minor street can be equal to, but not greater than, the left turns from the major street. Additional information on left-turn lanes, including their suggested lengths, can be found in Highway Research Record 211, NCHRP Report Synthesis 225, and NCHRP Report 279, and NCHRP Report 745 (10, 19, 17, 9). In the case of double left-turn lanes, a capacity analysis of the intersection should be performed to determine what traffic controls are needed in order for it to function properly. Local conditions and the cost of right-of-way often influ- ence the type of intersection selected as well as many of the design details. Limited sight distance, for example, may make it desirable to control traffic by yield signs, Stop signs, or traffic signals when the traffic densities are less than those ordinarily considered appropriate for such control. The alignment and grade of the intersecting roads and the angle of intersection may make it advisable to channelize or use auxiliary pavement areas, regardless of the traffic densities. In general, traffic ser- vice, highway design designation, physical conditions, and cost of right-of-way are considered jointly in choosing the type of intersection. For the general benefit of through-traffic movements, the number of crossroads, intersecting roads, or intersecting streets should be minimized. Where intersections are closely spaced on a two-way facility, it is seldom practical to provide signals for completely coordinated traffic movements at reasonable speeds in opposing directions on that facility. At the same time, the resultant road or street patterns should permit travel on road- ways other than the predominant highway without too much inconvenience. Traffic analysis is needed to determine whether the road or street pattern, left open across the predominate pre- dominant highway, is adequate to serve normal traffic plus the traffic diverted from any terminated road or street. Discussion Much of the subsection on “Guidelines for Design of Left- Turn Lanes” is really more about the installation of left-turn lanes. This subsection could be split into multiple parts, to include discussion of installation, warrants, and general design principles. The existing text in the first two paragraphs is on installation. The suggested revised text can be presented as a subsection on warrants to fill an information need. The remaining text can then be the subsection on “Guidelines for Design of Left-Turn Lanes.” Also, there is a minor typo in the last paragraph that begins on page 9-132. The revisions add the warrant information developed in NCHRP Project 3-91 (NCHRP Report 745) (9) and correct some typographical errors in the text. The sug- gested revised text is from Appendix A of NCHRP Report 745, updated to reflect changes between the 2004 Green Book used as the source then and the 2011 edition used now. 9.7 Auxiliary Lanes, 9.7.3 Design Treatments for Left-Turn Maneuvers (page 9-137) Proposed Revision to Green Book Offset Left-Turn Lanes For medians wider than about 5.4 m [18 ft], it is desirable to offset the left-turn lane so that it will reduce the width of the Metric US Customary Opposing Advancing volume (veh/h) Opposing Advancing Volume (veh/h) volume 5% 10% 20% 30% volume 5% 10% 20% 30% (veh/h) left turns left turns left turns left turns (veh/h) left turns left turns left turns left turns 60-km/h operating speed 40-mph operating speed 800 330 240 180 160 800 330 240 180 160 600 410 305 225 200 600 410 305 225 200 400 510 380 275 245 400 510 380 275 245 200 640 470 350 305 200 640 470 350 305 100 720 515 390 340 100 720 515 390 340 80-km/h operating speed 50-mph operating speed 800 280 210 165 135 800 280 210 165 135 600 350 260 195 170 600 350 260 195 170 400 430 320 240 210 400 430 320 240 210 200 550 400 300 270 200 550 400 300 270 100 615 445 335 295 100 615 445 335 295 100-km/h operating speed 60-mph operating speed 800 230 170 125 115 800 230 170 125 115 600 290 210 160 140 600 290 210 160 140 400 365 270 200 175 400 365 270 200 175 200 450 330 250 215 200 450 330 250 215 100 505 370 275 240 100 505 370 275 240 Table 9-23. Guide for left-turn lanes on two-lane highways (10)

130 divider to 1.8 to 2.4 m [6 to 8 ft] immediately before the intersec- tion, rather than to align it exactly parallel with and adjacent to the through lane. This alignment will place the vehicle waiting to make the turn as far to the left as practical, maximizing the offset between the opposing left-turn lanes, and thus providing improved visibility of opposing through traffic. The advantages of offsetting the left-turn lanes are (1) better visibility of oppos- ing through traffic; (2) decreased possibility of conflict between opposing left-turn movements within the intersection; and (3) more left-turn vehicles served in a given period of time, par- ticularly at a signalized intersection (122). Figure 9-XE provides suggested minimum widths for separator and offset islands for different conditions. Parallel offset left-turn lanes may be used at both signalized and unsignalized intersections. This left-turn lane configuration is referred to as a parallel offset left-turn lane and is illustrated in Figure 9-52A. Discussion The draft Access Management Manual (97) includes a graphic (see Figure A-7) showing minimum width for the offset island. This information could be added to the Green Book to provide additional guidance on offset dimensions. 9.7 Auxiliary Lanes, 9.7.3 Design Treatments for Left-Turn Maneuvers (page 9-139) Proposed Revision to Green Book Double or Triple Left-Turn Lanes Where two median lanes are provided as a double left-turn lane, left-turning vehicles leave the through lanes to enter the median lanes in single file, but once within the median lanes, the vehicles are stored in two lanes. On receiving the green indication, the left-turning vehicles turn simultaneously from both lanes. With three-phase signal control, such an arrangement results in Research (121) has shown an increase in capacity for double left-turn lanes of approximately 180 195% of that of a single median lane. Occasionally, the two-abreast turning maneuvers may lead to sideswipe crashes. These usually result from too sharp a turning radius or a roadway that is too narrow. The receiving leg of the intersection should have adequate width to accommo- date two lanes of turning traffic. A width of 9 m [30 ft] is used by several highway agencies. Capacity benefits can be achieved if the receiving leg width is greater than 36 ft; however, the tradeoff for a wider crossing distance and increase in costs are to be consid- ered. Capacity benefits for the left-turn operation on the order of 56 pcphgpl for each U-turning vehicle will also be achieved if U-turns are prohibited for the double left-turn maneuver. Triple Source: Access Management Manual (draft second edition), exhibit 17-7. Reproduced with permission of the Transportation Research Board. Figure A-7. Example graphic for new Green Book Figure 9-XE. Minimum width with offset island.

131 left-turn lanes have also been used at locations with very high left-turn volumes. Double and triple turning lanes should only be used with signalization providing a separate turning phase. Multiple left-turn lanes are becoming more widely used at signalized intersections where traffic volumes have increased beyond the design volume of the original single left-turn lane. The following are design considerations for double or triple left- turn lanes: • Width of receiving leg. • Width of intersection (to accommodate the two or three vehi- cles turning abreast). • Clearance between opposing left-turn movements if concur- rent maneuvers are used. • Turning path width for design vehicle. • Pavement marking visibility. • Location of downstream conflict points. • Weaving movements downstream of turn. • Potential for pedestrian conflict. Offtracking and swept path width are important factors in designing double and triple left-turn lanes. At such locations, vehicles should be able to turn side by side without encroaching upon the adjacent turn lane. A desirable turning radius for a dou- ble or triple left-turn lane is 27 m [90 ft], which will accommodate the P, SU-9 [SU-30], SU-12 [SU-40], and WB-12 [WB-40] design vehicles within a swept path width of 3.6 m [12 ft]. Larger vehicles need greater widths to negotiate double or triple left-turn lanes constructed with a 27 m [90 ft] turning radius without encroach- ing on the paths of vehicles in the adjacent lane. Table 9-24 illustrates the swept path widths for specific design vehicles making 90-degree left turns (51). Table 9-24 can be used to determine the width needed at the center of a turn where the maximum vehicle offtracking typically occurs. To help drivers maintain their vehicles within the proper lanes, the longitudinal lane line markings of double or triple left-turn lanes may be extended through the intersection area to pro- vide positive guidance (see MUTCD Section 3B.08, Extensions Through Intersections or Interchanges for guidance). This type of pavement marking extension is intended to provide a visual cue for lateral positioning of the vehicle as the driver makes a turning maneuver. Discussion The findings from this project’s double left-turn lane study (121) generated the following potential recommendations: • The Green Book states that the capacity of double left-turn lanes is approximately 180% of that of a single median lane. Per the Highway Capacity Manual (8) the base satu- ration flow rate for a metropolitan area with population of 250,000 is 1900 pcphgpl and the left-turn adjustment factor is 1/1.05. Comparing the single-lane saturation flow rate (1900/1.05 = 1810 pcphgpl) to the average saturation flow rates for the double left-turn lanes sites in this study (1774 + 1776 = 3550 pcphgpl) results in a value (3550/1810 = 1.96 or 196 percent) that is greater than 180 percent. • The Green Book states that the receiving leg of the inter- section should have adequate width to accommodate two lanes of turning traffic and that a width of 30 ft is used by several highway agencies. Early literature by Neuman (5) stated the throat width for the turning traffic is the most important design element, and that because of the offtracking characteristics of vehicles, a 36-ft throat width is desirable for acceptance of two lanes of turning traffic. In constrained situations, 30-ft throat widths are accept- able minimums. Within this study, the pattern of increas- ing saturation flow rate for increasing receiving leg width was examined to try to identify if there were dimensions where a sizable increase in saturation flow rate occurs. The method concluded that the change point occurs between receiving leg widths of 36 ft and 40 ft. • Additional discussions or cautions in the section on mul- tiple turn lanes: – Double left-turn vehicles, turning into a receiving leg of two lanes where a third lane is being added as a dedicated downstream lane from a channelized right- turn lane, were observed to move into the additional lane as soon as physically possible, even across a solid white line. – The number of U-turning vehicles has a significant impact on the operations of double left-turn lanes. The text in the Green Book was revised to reflect the higher saturation flow rate (rounded the 196% to 195 percent) and to note the potential increase in capacity available by using a wider leg width and prohibiting U-turns. Findings from this research project (121), previous liter- ature (7, 52, 73, 74), and the engineering judgment of the research team developed the list of design considerations for double or triple left-turn lanes. 9.8 Median Openings, 9.8.1 General Design Considerations (page 9-140) Proposed Revision to Green Book Medians are discussed in Section 4.11 chiefly as an element of the cross-section. General ranges in width are given, and median width at intersection is treated briefly. For intersection conditions, the median width, the location and length of the opening, and the design of the median end are developed in combination to fit the character and volume of through and turning traffic. Figure 9-50 illustrates the appropriate dimen- sions for the median width and the length of median opening. Median openings should reflect street or block spacing and the access classification of the roadway. In addition, full median opening should be consistent with traffic signal spacing crite- ria. In some situations, median openings should be eliminated or made directional.

132 When considering construction or elimination of a median opening, the following guidance can be considered: • For median widths less than 9 m [30 ft] in width, median openings should be constructed opposite driveways and cross- roads as appropriate. • For median widths greater than 9 m [30 ft] in width, median openings should be constructed every 400 m [1320 ft] in urban areas and 800 m [2640 ft] in rural areas. This spacing may be adjusted by 30 m [100 ft] either way to conform to driveways and crossroads. No two median opening should be closer than 150 m [500 ft] apart unless servicing a through crossroad. There are two types of median crossover intersections: bidirec- tional (sometimes called conventional) and directional (see Figure 9-XF). A bidirectional crossover allows vehicles to make a U-turn from either direction of travel, which creates additional points of conflict as compared to the directional crossover. Fur- ther, as turning volumes increase, an interlocking of travel paths can occur in a bidirectional crossover, which could limit sight dis- tance and result in unpredictable driver behavior. Several studies have shown that crash rates at directional crossovers are less than bidirectional crossovers for signalized corridors and that direc- tional crossovers provide better operational performance. If the median is wide enough to permit storage of vehicles, the use of a centerline and stop bar in the median storage area can communicate to drivers how the median should be negotiated and provide a sense of storage area. This can reduce undesirable maneuvers such as side-by-side queuing and lane encroachment. Further, it can communicate that it is allowable to make crossing and turning maneuvers in stages at this intersection. Discussion There is no guidance listed for under which conditions a median opening should be eliminated. Further, there is no guidance given on when it should be made directional. This is compounded by the fact that there is no discussion of the dif- ference between a bidirectional (conventional) crossover and a directional crossover. Additional sources for information on median openings are in the following: Michigan DOT Geo- metric Design Guide 670 (102), Michigan DOT Road Design Manual (103), FHWA Alternative Intersections/Interchanges: Informational Report (104), and NCHRP Report 650 (105). There is no discussion of the impact of providing mark- ings or a divisional island for vehicle storage in the median. Guidance regarding the impact of proper pavement markings on driver behavior should be added as available in NCHRP Report 650 (105). 9.8 Median Openings, 9.8.2 Control Radii for Minimum Turning Paths (page 9-144) Proposed Revision to Green Book The customary intersection on a divided highway does not have a continuous physical edge of traveled way delineating the left- turn path. Instead, the driver has guides at the beginning and at the end of the left-turn operation: (1) the centerline of an undivided crossroad of the median edge of a divided crossroad, and (2) the curved median end. For the central part of the turn the driver has the open central intersection area in which to maneuver. Under these circumstances for minimum design of the median end, the precision of compound curves does not appear to be needed, and simple curves for the minimum assumed edge of left turn have been found satisfactory. The larger the simple curve radius used, the better it will accommodate a given design vehicle, but the result- ing layout for the larger curve radius will have a greater length of median opening and greater paved areas than one for a minimum radius, These areas may be sufficiently large to result in erratic maneuvering by small vehicles, which may interfere with other traf- fic. To reduce the effective size of the intersection for most motor- ists, consideration should be given to providing an edge marking corresponding to the desired turning path for passenger cars, while providing sufficient paved area to accommodate the turning path of an occasional large vehicle. Minimizing the median opening length has shown to improve intersection safety performance for rural divided highways; as such, the minimum turning radius for the design vehicle should be used for design purposes. Discussion This section should also emphasize the safety impacts of minimizing the median opening length. Much of the discussion focuses on curve radii for the median design, but forgets that the overall length of the opening is important as well. NCHRP Report 650 provides discussion regarding median design (105). 9.8 Median Openings, 9.8.3 Minimum Length of Median Opening (page 9-149) Proposed Revision to Green Book The use of a minimum length of opening without regard to the width of median or the control radius should not be consid- ered except at very minor crossroads. Care should be taken not to make The median opening should not be longer than needed at rural unsignalized intersections to reduce undesirable median maneuvers. The minimum length of opening for U-turns is dis- cussed in Section 9.9 on “Indirect Left Turns and U-Turns.” Figure A-8. Example graphic for new Green Book Figure 9-XF. Bidirectional (conventional) and directional median openings.

133 Discussion The phrase “care should be taken” is awkward and vague and the second sentence should be reworded to eliminate it. The statement that “care should be taken not to make the median opening longer than needed” should be expanded to give the reason for the guidance. Further, a reference to NCHRP Report 650 (105) should be considered here so that users can find the reference material for the statement. 9.9 Indirect Left Turns and U-Turns, 9.9.1 General Design Considerations (pages 9-155 to 9-157) Discussion There is an ongoing research effort that may become a useful resource for alternative intersection design. This guidebook is due to be completed in December 2013. As such, a placeholder is being added as a reminder to review this resource when it becomes available and determine if it is something that should be added as a reference resource to this section. • Alternative Intersections Comparative Analysis, Rakha, Hesham, Virginia Polytechnic Institute, completion date December 2013. The purpose of this project is to develop a guidebook for the analysis of alternative intersection designs to be used by consulting engineers performing compara- tive analysis during preliminary engineering. Intersections considered: displaced left-turn intersection, median U-turn intersection, restricted crossing U-turn intersection, quad- rant roadway intersection, jughandle intersection, and mod- ern roundabout. 9.9 Indirect Left Turns and U-Turns, 9.9.2 Intersections with Jughandle or Loop Roadways (pages 9-157 to 9-158) Proposed Revision to Green Book Jughandles are one-way roadways in two quadrants of the intersection that allow for removal of left-turning traffic from the through stream without providing left-turn lanes. All turns—right, left, and U-turns—are made from the right side of the roadway. Drivers wishing to turn left exit the major roadway on the right and turn left onto the minor road at a terminus separated from the main intersection. There should be 30 m [100 ft] between the jughandle intersection and the stop bar for the primary intersec- tion. Less right-of-way is needed along the roadway because the left- turn lanes are not needed. However, more right-of-way is needed at the intersection to accommodate the jughandles. Each jughandle typically requires a triangle 120 m [400 ft] by 90 m [300 ft] in the quadrant deployed. Figure 9-60 illustrates a jughandle intersection with the diagonal connecting roadways in advance of the inter- section. The possible movements are illustrated in Figure 9-61. . . . Table 9-X4 shows the number of conflict points at a four-leg signalized intersection as compared to a four-leg signalized inter- section with two jughandles. The use of the jughandles reduces the crossing (left-turn) conflict points by 6. Discussion There is no guidance given on the spacing of jughandle intersection and the primary intersection. Further, there is no sense of the space that this type of intersection takes up. Ref- erences that discuss jughandles include the following: FHWA, Alternative Intersections/Interchanges: Informational Report (104), FHWA, Signalized Intersections: Informational Guide (7), Hummer et al. (123), and New Jersey DOT Roadway Design Manual (124). Figure 9-60 needs to be improved, as there are several issues with it as presented. First, the jughandle roadways should intersect the cross road at as near to 90 degrees as possible. Sec- ond, there should be a short description of the conflict points associated with the intersection type as compared to a standard intersection. The suggested figure shown in Figure A-9 is given as a guide to replace Figure 9-60 (shown in Figure A-10). 9.9 Indirect Left Turns and U-Turns, 9.9.2 Intersections with Jughandle or Loop Roadways (page 9-159) Proposed Revision to Green Book An alternative to providing a jughandle ramp in advance of the intersection is to provide a loop roadway beyond the intersec- tion. The loop design may be considered when the right-of-way Conflict type Four-Leg Signalized Intersection Four-Leg Signalized Intersection with Two Jughandles Merging/diverging 16 16 Crossing (left turn) 12 6 Crossing (angle) 4 4 Total 32 26 Table A-7. Example material for new Green Book Table 9-X4. Number of conflict points at a four-leg signalized intersection compared to a four-leg signalized intersection with two jughandles.

134 for the farside far-side quadrant is less expensive than that for the nearside near-side quadrant. Vertical alignment and comparative grading costs may also influence the intersection quadrant where the turning roadway is placed. The left-turn movement becomes a right-turn movement at the intersection of a far-side loop road- way with the crossroad, resulting in fewer conflicts and higher capacity for the left-turn movement. However, this increases the intersection’s entering volume and doubles the exposure of drivers at the primary intersection as drivers making this maneuver must navigate the primary intersection twice. If a right-turn lane is provided on the near side of the intersection, left-turn move- ments from the main roadway are eliminated. Figure 9-62 illus- trates the use of a loop roadway beyond the intersection. Discussion Within this paragraph both farside (one word) and far- side (with hyphen) is used. Also nearside (one word) and near side (two words) is used. These inconsistencies should be repaired. Further, wording of the description of where the left-turn maneuver is made is confusing. Additionally, the second to the last sentence does not make sense in this context. There is no discussion of one of the critical weaknesses of the far-side jughandle intersection, which is that drivers making that movement must navigate the primary intersec- tion twice. This increases the intersection’s entering volume and doubles the exposure of drivers making this maneuver to potential collisions within the primary intersection. Potential references on jughandles are listed above. Figure 9-62 (reproduced in this document as Figure A-11) needs improvement, as there are several issues with it as pre- sented. First, the jughandle roadways should better show state- of-the-art for employing free-flow turns. Second, there should be a better representation of the loop roadway along with typi- cal minimum design values. Figure A-12 shows an example of a better figure for the Green Book Figure 9-62. Source: FHWA Signalized Intersections: Informational Guide (7) Figure A-9. Example graphic for replacing Green Book Figure 9-60. Intersection with jughandle roadways for indirect left turns. Source: A Policy on Geometric Design of Highways and Streets (2011) by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission. Figure A-10. Copy of Green Book Figure 9-60. Intersection with jughandle roadways for indirect left turns. Source: A Policy on Geometric Design of Highways and Streets (2011) by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission. Figure A-11. Copy of Green Book Figure 9-62. Intersection with loop roadways for indirect left turns.

135 9.9 Indirect Left Turns and U-Turns, 9.9.2 Intersections with Jughandle or Loop Roadways (page 9-160) Proposed Revision to Green Book Additional information regarding design and operation of jughandle intersections is presented in Signalized Intersections: Information Guide (23) and Alternative Intersections/Interchanges: Informational Report (104). Discussion An additional source of information including some typical dimensions is in the Alternative Intersections/Inter- changes: Informational Report (104). Other potential refer- ences on jughandles are listed above. 9.9 Indirect Left Turns and U-Turns, 9.9.3 Displaced Left-Turn Intersections (page 9-160) Proposed Revision to Green Book A displaced left-turn intersection, also known as a continuous- flow intersection (CFI) or a crossover-displaced left-turn (XDL) intersection, removes the conflict between left-turning vehicles and oncoming traffic at the main intersection by introducing a left-turn bay placed to the left of oncoming traffic. Vehicles access the left-turn bay at a midblock sig- nalized intersection on the approach where continuous flow is desired. Figure 9-63 shows the design of an intersection with displaced left-turn roadways and Figure 9-64 illus trates some of the vehicle movements at such an intersection. Table 9-X5 shows the number of conflict points at a four-leg signalized intersection as compared to a CFI with displaced Source: New Jersey DOT Highway Design Manual (125) Figure A-12. Example graphic for replacing Green Book Figure 9-62. Intersection with loop roadways for indirect left turns.

136 left turns on the major street only. The use of the CFI separates the conflict points for left-turning traffic from the primary intersection. Discussion A discussion regarding conflict points (number and type) for CFI versus standard intersection would be helpful. A potential source of information and typical dimensions is available in the FHWA publication, Alternative Intersections/ Interchanges: Informational Report (104). 9.9 Indirect Left Turns and U-Turns, 9.9.3 Displaced Left-Turn Intersections (page 9-161) Proposed Revision to Green Book Additional information regarding the operation of displaced left-turn intersections is presented in Signalized Intersections: Infor- mational Guide (23) and Alternative Intersections/Interchanges: Informational Report (104). Discussion A potential source of information and typical dimensions is available in the FHWA publication, Alternative Intersections/ Interchanges: Informational Report (104). 9.9 Indirect Left Turns and U-Turns, 9.9.4 Wide Medians with U-Turn Crossover Roadways (page 9-162) Proposed Revision to Green Book Figure 9-65 illustrates an indirect left turn for two arterials where left turns are heavy on both roads. The north-south roadway is undivided and the east-west roadway is divided with a wide median. Because left turns from the north-south road would cause congestion because of the lack of storage, left turns from the north-south road are prohibited at the main intersection. Left-turning traffic turns right onto the divided road and then makes a U-turn at a one-way cross- over in the median of the divided road. Auxiliary lanes are highly desirable for the left-turn movements and the right- turn movements needed for the median U-turn operation. Figure 9-66 illustrates some of the vehicle movements at such an intersection. Figure 9-XG illustrates a variation of this design called a restricted crossing U-turn (RCUT) intersection, also called a Superstreet intersection or a J-turn intersection. The RCUT redirects both left-turn and through movements from the crossroad to a one-way crossover in the median of the divided roadway. Left-turning traffic from the major, divided roadway still uses the primary intersection. This configuration is gen- erally suited to higher-volume major roads in suburban and rural areas where relatively low traffic volumes enter from the crossroad. Pedestrians cross the divided roadway in a diago- nal fashion, going from one corner to the opposite corner between the channelized left turns for the divided roadway. A further variant of this approach also prohibits left turns from the divided roadway at the primary intersection directing that movement to the crossovers. Discussion Figure 9-65 (reproduced as Figure A-13) is a poor representation of the design. The figure should be nar- rowed and elongated such as the suggested figure shown in Figure A-14. Conflict type Four-Leg Signalized Intersection Merging/diverging 16 14 Crossing (left turn) 12 Crossing (angle) 4 Total 32 Intersection Continuous-Flow 6 10 30 Table A-8. Example material for new Green Book Table 9-X5. Number of conflict points at a four-leg signalized intersection compared to a continuous- flow intersection with displaced left turns on the major street only. Source: A Policy on Geometric Design of Highways and Streets (2011) by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission. Figure A-13. Copy of Green Book Figure 9-65. Typical Arrangement of U-turn roadways for indirect left turns on arterials with wide medians.

137 A modification to the median U-turn intersection is the restricted crossing U-turn intersection. A figure showing the RCUT, such as Figure A-15, should be added along with a brief description. The proposed revision is shown as a mod- est addition to this section, but another approach would be to create a separate section for the median U-turn intersection and another for the restricted crossing U-turn intersection. Potential sources for information are available in the following: FHWA Alternative Intersections/Interchanges: Informational Report (104), FHWA Signalized Intersections: Informational Guide (7), and Hummer et al. (123). 9.9 Indirect Left Turns and U-Turns, 9.9.4 Wide Medians with U-Turn Crossover Roadways (page 9-163) Proposed Revision to Green Book Due to their design, median U-turn crossovers need a wide median to enable the U-turn movement. For median widths of less than 20 m [60 ft], expanded paved aprons, often called “loons,” can be provided opposite a median crossover. Median U-turn roadways may be appropriate at intersections with high major-street through movements, low-to-medium left turns from the major street, low- to-medium left turns from the minor street and any amount of minor street through volumes. Locations with high left-turning vol- umes may not be good candidates because the out-of-direction travel incurred and the potential for queue spillback at the median U-turn roadway location could outweigh the benefits associated with remov- ing left turns from the main intersection. Median U-turn roadways can be applied on a single approach or multi ple approaches. Discussion A comment should be added about using loons to accom- modate larger vehicle U-turns with narrow medians. 9.9 Indirect Left Turns and U-Turns, 9.9.4 Wide Medians with U-Turn Crossover Roadways (page 9-164) Proposed Revision to Green Book Use of a median U-turn crossover intersection may result in fewer left-turn collisions and a minor reduction in merging and diverging collisions. There is a potential reduction in overall travel time and stops for mainline through movements. Find- ings are mixed with respect to overall stops. While the number of conflicting movement at the intersections is reduced, The dis- tance for pedestrians to cross is increased and turning paths of vehicles making median U-turns may encroach into bike lanes. Additional right-of-way and access may need to be restricted within the influence of the median U-turn locations. Sign- ing, visual cues, education, and enforcement may be needed to guide drivers to the intended turning path and minimize illegal turns. Table 9-X6 shows the number of conflict points at a four-leg sig- nalized intersection as compared to a four-leg signalized intersec- tion with a median U-turn configuration. The use of the median U-turn crossover configuration eliminates all left-turn crossing conflicts while also reducing the number of merging/diverging conflict points by 4. Typical crash reductions when compared to conventional intersections range from 20 to 50 percent. The table also shows the conflict points for a restricted crossing U-turn configuration. The number of left-turn crossing con- flicts is reduced to two and there are no angle crossing conflict points, though merging/diverging conflicts increase by two. Figures 9-XH and 9-XI provide conflict diagrams for the two intersection designs. Discussion A discussion of the conflict points for both a median U-turn intersection and a restricted crossing U-turn intersection should be added. Potential sources for additional information are available in: FHWA Alternative Intersections/Interchanges: Informational Report (104), FHWA Signalized Intersections: Informational Guide (7), and Hummer et al. (123). 9.9 Indirect Left Turns and U-Turns, 9.9.4 Wide Medians with U-Turn Crossover Roadways (page 9-164) Proposed Revision to Green Book Additional information regarding design and operation of intersections with median U-turn crossover roadways is contained in Signalized Intersections: Informational Guide (23) Figure A-14. Example graphic for replacement of Green Book Figure 9-65. Figure A-15. Example graphic for new Green Book Figure 9-XG. Typical restricted crossing U-turn intersection.

138 Conflict type Four-Leg Signalized Intersection Median U-Turn Configuration Restricted Crossing U-Turn Configuration Merging/diverging 16 12 18 Crossing (left turn) 12 0 2 Crossing (angle) 4 4 0 Total 32 16 20 Table A-9. Example material for new Green Book Table 9-X6. Number of conflict points at a four-leg signalized intersection compared to a four-leg signalized intersection with a median U-turn configuration and four-leg signalized intersection with a restricted crossing U-turn configuration. Figure A-16. Example graphic for new Green Book Figure 9-XH. Conflict diagram for median U-turn configuration. Figure A-17. Example graphic for new Green Book Figure 9-XI. Conflict diagram for restricted crossing U-turn configuration.

139 and Alternative Intersections/Interchanges: Informational Report (104). The location and design of median U-turn roadways is addressed in greater detail in Section 9.9.5. Discussion An additional source of information including some typi- cal dimensions is the Alternative Intersections/Interchanges: Informational Report (104). 9.9 Indirect Left Turns and U-Turns, 9.9.5 Location and Design of U-Turn Median Openings (pages 9-165 and 9-166) Proposed Revision to Green Book Medians of 5.0 5.5 m [1618 ft] and 15.6 m [5051 ft] or wider are needed to permit passenger and single-unit truck traffic, respectively, to turn from the inner lane (next to the median) on one roadway to the outer lane of a two-lane opposing roadway. Also, a median left-turn lane is highly desirable in advance of the U-turn opening to eliminate stopping on the through lanes. This scheme would increase the median width by approximately 3.6 m [12 ft]. {Within Table 9-30 (Minimum Designs for U-Turns), the val- ues on the Metric portion should be revised to show one place past the decimal (e.g. Passenger for Inner Lane to Outer Lane should be 5.5 not simply 5).} Discussion The Green Book states “Medians of 5.0 m [16 ft] and 15 m [50 ft] or wider are needed to permit passenger and single-unit truck traffic, respectively, to turn from the inner lane (next to the median) on one roadway to the outer lane of a two-lane opposing roadway.” However, Table 9-30 (p. 9-166) shows values of 5 m and 18 ft for passenger and 15 m and 51 ft for SU-30. This does not concur with the text. It appears that the decimal portion of the values listed for the metric chart were dropped (not rounded) from the table resulting in this discrepancy. As such, it is suggested that the text be corrected to read “5.5 m [18 ft] and 15.6 m [51 ft]” to address this discrepancy. Further, Table 9-30 Metric should be updated to include the decimal portion of the result so that the two tables agree. 9.9 Indirect Left Turns and U-Turns, 9.9.5 Location and Design of U-Turn Median Openings (pages 9-165 and 9-166) Proposed Revision to Green Book Wide medians are uncommon in highly developed areas. Conse- quently, Special U-turn designs, called loons, should be considered where right-of-way is restricted. speeds are low, and signal control is used downstream to provide sufficient gaps in the traffic stream Source: Michigan Department of Transportation Geometric Design Guide 670 Figure A-18. Example graphic for new Green Book Figure 9-XJ. Typical loon design to facilitate U-turning traffic on arterials with restricted median widths.

140 In conditions where the U-turn crossover is unsignalized, provisions for sufficient gaps in the traffic stream through the use of an upstream signal or due to natural gaps appearing in the traffic stream due to low volumes may be needed. Further, when establishing the clearance intervals for the signalized crossover, it is essential to provide additional time to account for the extra travel distance required for drivers to navigate the loon. Median widths of 2.4 to 12.5 m [7 8 to 40 41 ft] may be used for U-turn openings to permit passenger vehicles or single-unit trucks to turn from the inner lane in one direction onto the shoulder of a four-lane divided highway in the other direction. This special U-turn feature can be incorporated into the design of an urban roadway section by constructing a short segment of shoulder area along the outside edge of the traveled way across from the U-turn opening (see Figure 9-XJ). The outside curb- and-gutter section would then be carried behind the shoulder area and the shoulder would be designed as a pavement. Through the use of loons, agencies can realize the safety and operational benefits of a divided roadway using median U-turns without the high cost of acquiring enough land along the entire corridor to provide sufficient median width. Discussion The Green Book states that “Wide medians are uncom- mon in highly developed areas.” This statement is not sup- portable, as such, it should be removed. In locations where median roadways are common, such as Michigan, Florida, Texas, and North Carolina, it is not unusual to find wide medians in highly developed areas. Telegraph Road in Detroit has a median that varies from 60 ft to 100 ft for most of its length. The Green Book states that “Consequently, special U-turn designs should be considered where right-of-way is restricted, speeds are low, and signal control is used downstream to pro- vide sufficient gaps in the traffic stream.” There are several issues associated with this statement: • The common term for “special U-turn designs” is “loon.” • The use of loons has successfully been incorporated on many rural freeways as well as urban freeways. The sug- gestion that loons should be considered where “speeds are low” does not support this and should be cut. The work done by Sisiopiku and Aylsworth-Bonzelet (126, 127) showed that the operation of loons was inde- pendent of urban or rural placement and signal control strategies. • It is unclear how signal control “downstream” would create gaps in the traffic stream. As such, this should be changed to “upstream”. While this is helpful in the urban setting, in the rural setting it is also sufficient to have low enough traffic volumes for gaps to be naturally present. This should also be reflected. • The need to provide gaps is only necessary if the crossover is unsignalized. The Green Book states that “Median widths of 2 to 12 m [7 to 40 ft] may be used for U-turn openings to permit passenger vehicles or single-unit trucks to turn from the inner lane in one direction onto the shoulder of a four-lane divided high- way in the other direction.” However, Table 9-30 (p. 9-166) shows values of 2 m and 8 ft for passenger and 12 m and 41 ft for SU-30. This does not concur with the text. It appears that the decimal portion of the values listed for the metric chart were dropped (not rounded) from the table resulting in this discrepancy. As such, it is suggested that the text be corrected to read “2.4 to 12.5 m [8 to 41 ft]” to address this discrepancy. Further, Table 9-30 Metric should be updated to include the decimal portion of the result so that the two tables agree. The Green Book states “This special U-turn feature can be incorporated into the design of an urban roadway section by constructing a short segment of shoulder area along the out- side edge of the traveled way across from the U-turn opening. The outside curb-and-gutter section would then be carried behind the shoulder area and the shoulder would be designed as a pavement.” The provision of an illustration is critical as the use of loons is not typical and may be difficult to fully understand without one. It should be emphasized that through the use of loons, agencies can realize the safety and operational benefits of a divided roadway using median U-turns without the high cost of acquiring enough land along the entire corridor to provide sufficient median width. Sisiopiku and Aylsworth-Bonzelet (126, 127) found that when establishing the clearance intervals for the signalized crossover, it is essential to provide additional time to account for the extra travel distance required for drivers to navigate the loon. This should be noted. 9.9 Indirect Left Turns and U-Turns, 9.9.5 Location and Design of U-Turn Median Openings (page 9-165) Proposed Revision to Green Book Where U-turn openings are proposed for access to the oppo- site side of a multilane divided street, they should be 15 to 30 m [50 to 100 ft] in advance of the next downstream left-turn lane. For U-turn openings designed specifically for the pur- pose of eliminating left-turn movement at a major intersection, they should be downstream of the intersection, preferably. In an urban setting, they should be midblock between adjacent crossroad intersections. This type of U-turn opening should be designed with a median left-turn lane for storage. In a rural setting, they should be between 300 to 450 m [1000 to 1500 ft] apart. Additionally, a U-turn opening can be provided upstream of the major intersection to remove traffic wishing to make a U-turn from that intersection.

141 Discussion The Green Book states “For U-turn openings designed spe- cifically for the purpose of eliminating left-turn movement at a major intersection, they should be downstream of the intersection, preferably midblock between adjacent crossroad intersections.” This statement raises several issues: • Locating the crossover “midblock between adjacent cross- road intersections” implies that this application is in an urban setting. This should be specified. • Guidance should then be given for locating the crossover in a rural setting. Examples of values being used include the following: – Arizona (Section 1060—Median Openings): “In a rural area, the median opening is not less than 1320 feet from an intersection with an improved public road or another median opening.” (128) – Florida (Section 1.3 Department Policy on Medians and Median Openings): Minimum median opening spacing for directional crossovers for access class 2 and 3 road- ways is 1320 feet (400 m). (129) – Maryland (Section 10.8 Median Crossover Spacing): Identifies minimum median crossover spacing stan- dards for state highways in rural settings of 3000 ft for primary highways (partially controlled and uncon- trolled), 1500 ft for secondary highways (arterial routes) and 1000 ft for secondary highways (collector routes). (130) – Virginia (Appendix F—Access Management Design Standards for Entrances and Intersections page F-23): Identifies minimum median crossover spacing stan- dards of 1320 ft for a rural principal arterial with legal speed limit in excess of 35 mph. (131) • It should be pointed out that they can also be upstream of the intersection (e.g., Texas U-turn), which removes traffic wishing to make a U-turn from the primary intersection. 9.9 Indirect Left Turns and U-Turns, 9.9.5 Location and Design of U-Turn Median Openings (page 9-165) Proposed Revision to Green Book Normally, U-turns should not be permitted from the through lanes. However, where medians have adequate width to shield a vehicle stored in the median opening, through volumes are low and left-turn/U-turns are infrequent, this type of design may be permissible. Minimum widths of median to accommodate U-turns by different design vehicles turning from the lane adja- cent to the median are given in Table 9-30. These dimensions are for a four-lane divided facility. If the U-turn is made from a median left-turn/U-turn lane, the width needed is the separator width; the total median width needed would include an addi- tional 3.6 m [12 ft] for a single median turn lane. At major intersections, many jurisdictions allow both left turns and U-turns to be made around the curbed nose at the end of a left-turn lane. Where dual left-turn lanes are needed along a street with a raised-curb median and the turning volume of large trucks is high, left turns and U-turns may be permitted from the inside lane and left turns only may be allowed from the outside turn lane. However, when the turning volume of large trucks is low, a dual lane crossover maneuver may be permitted allowing both lanes to make a U-turn movement (Figure 9-XK). Under this condition, the minimum width of the median opening is 11 m [36 ft], which does not accommodate a large truck turning adjacent to another vehicle. Discussion The discussion of how multiple left turns impact the median opening design does not support the state of the practice. The Green Book states: “Where dual left-turn lanes are needed along a street with a raised-curb median, left turns and U-turns may be permitted from the inside lane and left turns only (emphasis added) may be allowed from the outside turn lane.” There are many locations that use dual U-turns through a directional median opening (e.g., Detroit, MI). The inside left-turn lane turns to the inside lane of the arterial, while the outside left-turn lane turns to the outside lane of the arterial. The width of the median opening for the cross- over must be increased to 11 m [36 ft] and the design does not accommodate a large truck turning adjacent to another vehicle. Further, dual left-turn can be provided at locations that do not have a raised-curb median. Sources of informa- tion include the following: Michigan DOT Geometric Design Guide 670 (102), Michigan DOT Road Design Manual (103), and FHWA Alternative Intersections/Interchanges: Informa- tional Report (104). The information presented in Source: Michigan Depart- ment of Transportation Geometric Design Guide 670. Figure A-19 and Source: Michigan Department of Trans- portation Geometric Design Guide 670. Figure A-20 should be combined into a single figure. 9.10 Roundabout Design (pages 9-167 to 9-169) Proposed Revision to Green Book {Add this paragraph to the end of the section.} The use of roundabouts is currently evolving in the United States. As such, public outreach and education are a vital factor in realizing the improvements in traffic operations and safety that can be achieved with these designs. Additional information regarding outreach resources to help obtain public support for roundabouts is presented in the Federal Highway Administra- tion Roundabout Outreach and Education Toolbox (132).

142 Discussion The use of roundabouts is evolving in the United States. Because of this, public outreach and education are still vital factors in realizing the improvements in traffic operations and safety that can be achieved with these designs. This should be called out and resources identified, such as the FHWA publication on Roundabout Outreach and Education Toolbox (132). 9.10 Roundabout Design, 9.10.2 Fundamental Principles (page 9-173) Proposed Revision to Green Book {The following paragraph should be added to the end of the section entitled “Lane Balance and Continuity.”} Right-turn bypass lanes, also called slip lanes, can be imple- mented in conventional and innovative roundabout intersec- tions to increase the capacity. A bypass lane is a separate right- turn lane that lies adjacent to the roundabout and allows right-turning movements to bypass the roundabout. There are three configurations for the bypass lane: slip lane without an acceleration lane stop, slip lane without an acceleration lane yield, and slip lane with free-flow entry. In areas with bicy- cle and pedestrian activity, bypass lanes should only be used where necessary as the entries and exits of bypass lanes can increase conflicts with pedestrians, bicyclists, and with merg- ing on the downstream leg. Discussion The use of a right-turn bypass is not covered in the Green Book. This would fit well at the end of the Lane Balance and Lane Continuity section. There are three configurations for the bypass lane: slip lane without an acceleration lane stop, slip lane without an acceleration lane yield, and slip lane with free-flow entry. Sources for additional information are avail- able in the following: Mauro et al. (133), Al-Ghandour et al. (134), and NCHRP Report 672 (135). 9.10 Roundabout Design, 9.10.2 Fundamental Principles (page 9-174) Proposed Revision to Green Book {The following paragraph and figure should be added to the end of the section entitled “Appropriate Natural Path.”} The turbo-roundabout concept has emerged as a viable alternative to conventional multilane roundabouts. This design addresses functional problems of conventional mul- tilane roundabouts by using raised splitter islands to divide traffic streams, which eliminates weaving maneuvers, forces drivers to stay in the correct lane and reduces driving speeds. Turbo-roundabouts force circulating traffic flows to use spiral pathways such that each entering lane has fixed destinations. This requires drivers to choose the correct lane for entry to Source: Michigan Department of Transportation Geometric Design Guide 670 Figure A-19. Example graphic for new Green Book Figure 9-XK. Dual U-turn directional crossover design (part A). Source: Michigan Department of Transportation Geometric Design Guide 670 Figure A-20. Example graphic for new Green Book Figure 9-XK. Dual U-turn directional crossover design (part B).

143 the turbo roundabout based on their destination. Because of this, turbo-roundabouts do not permit U-turn maneuvers. Figure 9-XL illustrates the conflict points for a conventional multilane roundabout as compared to a turbo roundabout. The reduction in conflict points from 24 to 14 represents a global reduction in crash probability, but those conflicts asso- ciated with the turbo roundabout may result in higher sever- ity crashes due to increased impact angle. Turbo-roundabouts have a 40 to 70% reduction in crash risk as compared to multi- lane roundabouts. Discussion Turbo roundabouts are not included. This would fit well at the end of the Appropriate Natural Path section. The physical separation between lanes helps prevent sideswipe crashes and provides positive guidance to drivers who may be unfamiliar with the operation of a roundabout. Published research shows between a 40% and 70% reduction in crash risk (136, 137). Fortuijn (136) found a 70% lower crash risk when a double-lane roundabout is converted into a turbo roundabout. Mauro et al. (137) reported 40–50% reduction in total accident rate based on conflict analysis techniques applied to nine layouts with different demand scenarios and 20–30% reduction in number of potential accidents with inju- ries. Sources for additional information are available in the following: Giuffre et al. (138), Giuffre et al. (139), Vasconcelos et al. (140), and Silva et al. (141). 9.10 Roundabout Design, 9.10.2 Fundamental Principles (page 9-175) Proposed Revision to Green Book {The following paragraph should be added to the end of the section entitled “Design Vehicle.”} On larger statewide facilities, it may be necessary to accom- modate large WB-67 trucks or even oversized vehicles, sometimes called superloads. Special consideration for the size and tolerances of these vehicles may need to be provided in design and construc- tion. Typical factors that may need to be considered include: • Modification to the truck apron and central island design. • Widened entry and exit lanes. • Inclusion of right-turn bypass lanes. • Use of gated pass-throughs or tapered center islands to sup- port through movements. • Review of lane striping. • Installation of removable signs with setbacks for permanent fixtures (light poles). • Identification of maximum heights for splitter islands. • Identification of under-clearance for lowboy vehicles. • Consideration of truck stability with regards to mountable aprons, curbing and islands. Discussion There is no discussion regarding accommodation of over- size/overweight vehicles. This discussion would fit well at the Source: Silva, Ana, Silvia Santos and Marco Gaspar, “Turbo-roundabout Use and Design,” CITTA 6th Annual Conference on Planning Research. Figure A-21. Example graphic for new Green Book Figure 9-XL. Conflict points in a conventional multilane roundabout as compared to a turbo roundabout.

144 end of the Design Vehicle section. At a minimum, a list of things to “watch for” as a designer should be added (e.g., con- sider lowboy vehicle designs which could hang up, maximum height of splitter islands). Sources for additional information are available in the following: Russell et al. (142), Park et al. (143), NCHRP Report 672 (135). 9.10 Roundabout Design, 9.10.2 Fundamental Principles (page 9-175) Proposed Revision to Green Book An overarching consideration at roundabouts is the accom- modation of visually impaired pedestrians. Pedestrians with vision impairments face several challenges at roundabouts. These challenges magnify the need to maintain slow vehicle speeds with the crosswalk area, to provide intuitive crosswalk alignments and to provide design elements that encourage driv- ers to yield to pedestrians in a predictable manner. The use of pedestrian hybrid beacons (previously known as HAWKs) and raised crosswalks have been shown to be effective (65). Discussion The information above does not provide any guidance as to which devices or design elements may encourage drivers to yield to pedestrians in a predictable manner. The discus- sion should be expanded to include the results of NCHRP Report 674 (65) as shown above.