Development of Left-Turn Lane Warrants for Unsignalized Intersections (2013)

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43 CHAPTER 3 LITERATURE REVIEW—LEFT-TURN LANE DESIGN This chapter contains information on the design of left-turn lanes, reviewing information from state design manuals and national reference documents, as well as recent research. AASHTO GREEN BOOK MATERIAL AASHTO’s Green Book (5) contains general guidance for use in determining the design values for left-turn lanes, also called auxiliary lanes in the book. Many details in the states’ design guidelines are based on the Green Book text and figures, with some variations. The Green Book text on these design guidelines is summarized in the remainder of this section; the original text is supported within the Green Book by Exhibits 9-95 to 9-98. Auxiliary lanes should be at least 10 ft wide and desirably should equal that of the through lanes. Where curbing is to be used adjacent to the auxiliary lane, an appropriate curb offset should be provided. The length of the auxiliary lanes for turning vehicles consists of three components: entering taper, deceleration length, and storage length. Desirably, the total length of the auxiliary lane should be the sum of the length of these three components. 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. Deceleration Length 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. The approximate total lengths needed for a comfortable deceleration to a stop from the full design speed of the highway are as follows: for design speeds of 30, 40, 45, 50, and 55 mph, the desirable deceleration lengths of the auxiliary lane are 170, 275, 340, 410, and 485 ft, respectively, based on grades of less than 3 percent. On many urban facilities, it is not practical to provide the full length of an auxiliary lane for deceleration, and in many cases the storage length overrides the deceleration length. In such cases, at least part of the deceleration must be accomplished 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 decelerate comfortably up to 10 mph in a through lane before entering the auxiliary lane. Shorter auxiliary lane lengths increase the speed differential between turning vehicles and through traffic. A 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 lengths given above should be accepted as a desirable goal and should be provided where practical. The lengths stated above are

44 applicable to both left- and right-turning lanes, but the approach speed is usually lower in the right lane than in the left lane. Storage Length The auxiliary lane should be sufficiently long to store the number of vehicles likely to accumulate during a critical period. The storage length should be sufficient to avoid the possibility of left-turning vehicles stopping in the through lanes waiting for a signal change or for a gap in the opposing traffic flow. At unsignalized intersections, the storage length, exclusive of taper, may be based on the number of turning vehicles likely to arrive in an average 2-minute period within the peak hour. Space for at least two passenger cars should be provided; with over 10 percent truck traffic, provisions should be made for at least one car and one truck. The 2-minute waiting time may need to be changed to some other interval that depends largely on the opportunities for completing the left- turn maneuver. These intervals, in turn, depend on the volume of opposing traffic. Where the volume of turning traffic is high, a traffic signal is often needed. Taper On high-speed highways it is common practice to use a taper rate that is between 8:1 and 15:1 (longitudinal:transverse [L:T]). Long tapers approximate the path drivers follow when entering an auxiliary lane from a high-speed through lane. However, long tapers tend to entice some through drivers into the deceleration lane—especially when the taper is on a horizontal curve. Long tapers constrain the lateral movement of a driver desiring to enter the auxiliary lanes. This problem primarily occurs on urban curbed roadways. For urbanized areas, short tapers appear to produce better “targets” for the approaching drivers and to give more positive 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 the 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. This type of design may reduce the likelihood that entry into the auxiliary lane will spill back into the through lane. Recent practice has trended toward using a standard, typically short, taper length instead of a taper ratio. Municipalities and urban counties are increasingly adopting the use of taper lengths such as 100 ft for a single turn lane and 150 ft for a dual turn lane for urban streets. Straight-line tapers are frequently used. The taper rate may be 8:1 (L:T) for design speeds up to 30 mph and 15:1 (L:T) for design speeds of 50 mph. 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 with the resulting potential for a driver losing control. A short curve is desirable at either end of long tapers 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.

45 Width Median widths of 20 ft or more are desirable at intersections with single median lanes, but widths of 16 to 18 ft permit reasonably adequate arrangements. Where two median lanes are used, a median width of at least 28 ft is desirable to permit the installation of two 12-ft lanes and a 4-ft separator. Although not equal in width to a normal travel lane, a 10-ft lane with a 2-ft curbed separator or with traffic buttons or paint lines, or both, separating the median lane from the opposing through lane may be acceptable where speeds are low and the intersection is controlled by traffic signals. Offset Left-Turn Lanes For medians wider than about 18 ft, it is desirable to offset the left-turn lane so that it reduces the width of the divider to 6 to 8 ft immediately before the intersection, rather than to align it exactly parallel with and adjacent to the through lane. This alignment places 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 better visibility of opposing through traffic; decreased possibility of conflict between opposing left-turn movements within the intersection; and more left-turn vehicles served in a given period of time, particularly at a signalized intersection. Parallel offset left-turn lanes may be used at both signalized and unsignalized intersections. An offset between opposing left-turn vehicles can also be achieved with a left-turn lane that diverges from the through lanes and crosses the median at a slight angle. Tapered offset left-turn lanes provide the same advantages as parallel offset left-turn lanes in reducing sight distance obstructions and potential conflicts between opposing left-turn vehicles and in increasing the efficiency of signal operations. Tapered offset left-turn lanes are normally constructed with a 4-ft nose between the left-turn lane and the opposing through lanes. Tapered offset left-turn lanes have been used primarily at signalized intersections. Parallel and tapered offset left-turn lanes should be separated from the adjacent through traffic lanes by painted or raised channelization. STATE MANUALS This section contains an overview of information from the design manuals, standards, and guidelines governing the design of left-turn lanes in individual states. The information was obtained from the online manuals available at the respective websites of the states’ departments of transportation. Not all 50 states are shown in this section; some states did not provide access to the relevant documents through their websites, while others did not have design details in the material contained in the available manuals. Table 33 provides a summary of the design guidelines and standards referenced in this document. The actual text and figures from those manuals are provided in Appendix B.

46 Table 33. Summary of left-turn lane design guidelines. Q ue ue S to ra ge Le ng th En te rin g Ta pe r Le ng th D ec el er at io n Le ng th W id th O ff se t D ua l T ur n La ne s Tw o- W ay L ef t- Tu rn L an e Pe de st ria ns In di re ct T ur n D es ig ns B yp as s L an e Green Book      Alaska    Arizona       California       Colorado    Connecticut     Delaware      Florida   Georgia       Illinois       Indiana       Iowa    Kentucky     Louisiana   Maine     Minnesota      Nebraska      New Jersey     Ohio     Pennsylvania     Rhode Island  Tennessee     Texas       Wisconsin      Queue Storage Length Most states in this review call for a minimum storage length of 50 ft (two passenger cars at 25 ft each), though some have a minimum of 150 ft, which accounts for some taper and deceleration length as well as storage. For additional length greater than the minimum, the state guidelines generally call for sufficient length to store the passenger vehicles expected in a 2-minute period during the peak hour. Adjustments can be made for locations with higher proportions (generally greater than 10 percent) of heavy vehicles. Additionally, a frequently mentioned guideline is that the storage length should provide sufficient space so that neither turning nor through traffic blocks the other.

47 Entering Taper Length Two distinct tapers are defined in the state manuals: approach taper length and bay taper length. The approach taper length is commonly defined as V × W, where V = design speed (mph) and W = turn lane width. Some states subdivide this definition into high speed (≥ 45 mph, V × W) and low speed (≤ 40 mph, W × V2 / 60) conditions. The bay taper length is generally provided as a minimum length between 60 and 120 ft (as short as 50 ft and as long as 200 ft) or a ratio between 8:1 and 15:1; one state defined bay taper length as W × V / 3. Deceleration Length The length of the deceleration portion of a left-turn lane is based on the design speed, speed limit, or operating speed of the facility. In the vast majority of cases, the state manuals either implicitly or explicitly use values from the 2004 Green Book: for design speeds of 30, 40, 45, 50, and 55 mph, the desirable deceleration lengths of the auxiliary lane are 170, 275, 340, 410, and 485 ft, respectively, based on grades of less than 3 percent. Depending on the publication date of the manual, some states use the values from the 2001 Green Book: 230, 330, 430, 550, and 680 ft, respectively. Width Specified turn-lane widths are generally determined relative to the functional classification, urban or rural location, and project scope of work. Widths commonly vary from 10 to 12 ft, with a desirable width of 12 ft. On rural and urban high-speed highways, 11 ft is usually the minimum, while 10 ft is permitted only on urban low-speed roads. In many cases, a shoulder of 1.5 to 2.0 ft is also specified, particularly if the lane is adjacent to a curb. Offset The states that mention offset characteristics all refer to the benefits of improving turning drivers’ line of sight and call for use of offset when wide medians are available. In Nebraska (35), the left-turn lanes in 16-ft raised medians should be designed with a 1-ft offset. Wide striping on the right side of the left-turn lane should be used to encourage traffic to move closer to the median. In Illinois (30), offset left-turn lanes can consist of either a tapered design or a parallel design. Figures provided in their manual illustrate the various designs for offset left-turn lanes. In addition, the designer should consider the following: 1. Tapered offset left-turn lanes. The advantages of the tapered offset design versus a parallel lane design without an offset are that the offset design provides better visibility for the turning motorist to the opposing traffic, decreases the possible conflict between opposing left-turning vehicles, and serves more left-turning vehicles in a given time period. In addition, the designer should consider the following: a. Guidelines. Provide a tapered offset left-turn lane design where at least two of the following are applicable:

48 • The median width is equal to or greater than 40 ft, and only one left-turn lane in each direction on the mainline highway is required for capacity. • The current mainline ADT is 1500 or greater, and the left-turn DHV in each direction from the mainline is greater than 60 veh/hr. Under these conditions, vehicles waiting in opposing left-turn lanes have the probability of obstructing each other’s line of sight. • The intersection will be signalized. b. Median widths. Median widths of 40 to 70 ft are allowed to remain in place on existing expressways or multilane facilities. On new construction or reconstruction projects, use a median width of 50 ft and median slopes of 1V:6H. c. Curb and gutter. Use an M-4 curb and gutter on all corner and channelizing island, unless signals are placed within the island. In this situation, use an M-6 curb and gutter. 2. Parallel offset left-turn lanes. Parallel offset left-turn lanes offer the same advantages as the tapered design. However, they may be used at intersections with medians less than 40 ft but greater than 13 ft. In Indiana (22), on a four-lane facility with a wide median, slotted left-turn lanes are desirable where the median width is equal to or greater than 24 ft. The designer should consider the following: 1. Slot length. The slotted section of the turn lane should be at least 50 ft long with a minimum of 100 ft. The slotted section should not include the required deceleration distance for the turn lane. 2. Nose width. The nose of the slotted lane should be a minimum of 4 ft plus shoulder- or curb-offset width (or return taper) from the opposing through lanes. The nose position should be checked for interference with the turning paths from the cross street. 3. Slot angle. The angle of the slot should not diverge more than 10 degrees from the through mainline alignment. 4. Island. To delineate the slotted portion, the channelized island for the slotted lane should be a raised corrugated island. Raised pavement markers may be used for further delineation. Dual Turn Lanes Dual left-turn lanes are often needed to satisfy high-volume demands. Capacity analysis should be used to identify the need for dual left-turn lanes. Dual left-turn lanes are typically considered at signalized intersections when the peak-hour left-turn volume is 300 vehicles or greater. The decision to use dual left-turn lanes should consider the off-peak periods as well as the peak periods. The off-peak periods may be adversely affected since the use of dual left-turn lanes typically precludes permissive left turns. Georgia guidelines state that if dual left-turn lanes are included in the design, the following design guidelines should be considered: • Because of off tracking and the added difficulty involving two-abreast turns, a minimum 30-ft throat width should be provided through the intersection. • Pavement markings should be provided to guide the path of the turning vehicles.

49 • The design should be checked to ensure that conflicts are minimized between opposing left-turn maneuvers. When dual left-turn lanes are located opposite from an approach that does not have a dual left- turn lane, the design should minimize the lateral offset for vehicles traveling straight through the intersection. This can be accomplished by providing a median or striped-out area opposite the dual left-turn lane. TWLTL Continuous two-way left-turn lanes (TWLTLs) are often used in urban and fringe urban areas to treat the special capacity and safety concerns associated with left-turn demands at high-density strip developments. Two-way left-turn lanes may be used with either two-lane or multilane highways. The lane width should be no less than that of the through traffic lanes. In California, the TWLTL is devised to address the special capacity and safety problems associated with high-density strip development. The minimum width is 12 ft; the preferred width is 14 ft. Wider TWLTLs are occasionally provided to conform to local agency standards. However, TWLTLs wider than 14 ft are not recommended, and in no case should the width of a TWLTL exceed 16 ft. Additional width may encourage drivers in opposite directions to use the TWLTL simultaneously. In New Jersey, lane widths for continuous two-way left-turn median lanes range from 12 ft to 16 ft. The wider pavement width should be used only when raised islands are provided at major intersections with high left-turn demands. A median lane width of 12 ft is desirable where raised islands are not provided at major intersections. In Texas, TWLTL facilities for suburban roadways should minimally be 14 ft and desirably 16 ft in width. The desirable value of 16 ft width should be used on new location projects or on reconstruction projects where widening necessitates the removal of exterior curbs. The “minimum” value of a 14-ft width is appropriate for restrictive right-of-way projects and improvement projects where attaining “desirable” median lane width would necessitate removing and replacing exterior curbing to gain only a small amount of roadway width. Criteria for the potential use of a continuous TWLTL on a suburban roadway are as follows: • Future ADT volume of 3,000 vehicles per day for an existing two-lane suburban roadway, 6000 vehicles per day for an existing four-lane suburban roadway, or 10,000 vehicles per day for an existing six-lane suburban roadway; and • Side road plus driveway density of 10 or more entrances per mile (six or more per kilometer). When both conditions are met, the use of a TWLTL should be considered. For ADT volumes greater than 20,000 vehicles per day, or where development is occurring and volumes are increasing and are anticipated to reach this level, a raised median design should be considered.

50 Pedestrians Several states mention pedestrian considerations in the design of left-turn lanes. Arizona’s guidelines state that when left-turn lanes are placed in raised (curbed) medians, a minimum of 4 ft should remain at the nose for pedestrian refuge and placement of traffic control devices. In Delaware, in urban areas where speeds are low and the intersection is controlled by traffic signals, a 10-ft lane with a 2-ft curbed separator or paint lines, or both, may be acceptable to separate the median lane from the opposing through lane. Where pedestrian use is anticipated, a 6-ft separator should be provided. Deceleration lanes for left turns in Florida should be provided on all high-speed facilities. These turn lanes should not be excessive or continuous since they complicate pedestrian crossings and bicycle/motor vehicle movements. Storage (or deceleration lanes) to protect turning vehicles should be provided, particularly where turning volumes are significant. For median left-turn lanes at Texas intersections, a median width of 16 ft (a 12-ft lane plus a 4-ft divider) is recommended to accommodate a single left-turn lane. For maintenance considerations in preventing recurring damage to the divider, the divider should be at least 2 ft. If pedestrians are expected to cross the divider, then the divider should be a minimum of 5 ft wide in order to accommodate a cut-through landing or refuge area that is at least 5 ft by 5 ft. Seven-lane cross sections should be evaluated for pedestrian crossing capabilities. Wisconsin guidelines state that, for pedestrian accommodation or protection in the median, designers should line up the face of the median nose with the cross-street sidewalk extended. Indirect Turn Designs Indiana mentions the option of indirect left turns in its manual. It states that where operational or safety concerns preclude the use of typical left-turn lanes, the designer may consider the use of indirect left turns or jughandles that cross the mainline or intersect the crossroad at a different location. Because these require special consideration and treatment, they must be developed in consultation with the Bureau of Design and Environment. Bypass Lanes (or Blister Lanes) Four states (Connecticut, Georgia, Indiana, and Minnesota) discussed bypass lanes, also called blister lanes. The illustrations for the typical design for a bypass area are included in Appendix B. PREVIOUS RESEARCH This section contains information from other reference documents and recent research that are national in scope or that have design guidance in addition to that found in state design manuals.

51 Left-Turn Lane Length Kikuchi et al. (60) developed a proposed procedure for determining the appropriate length of dual left-turn lanes (DLTL) in a 2004 study. The procedure surveyed how drivers chose a lane of the DLTL and analyzed the relationship between lane use and the volume of left-turning vehicles. The procedure formulated the probability that all left-turning vehicles would be able to enter the left-turn lanes and derived the adequate lane length such that the probability of vehicles entering the DLTL is greater than a threshold value. Recommended lengths were presented as a function of left-turn and through volumes for practical application, with the purpose of avoiding left-turn lane overflow and blockage of lane entrance. The results were presented as a range of values in terms of number of vehicles and actual length, which were substantially longer than other methods that proposed a basic multiplier of the length of a comparable single left-turn lane. While this method is proposed for signalized intersections, there may be limited application for accommodations in selected locations with short durations of high left-turn demand (e.g., entrances to special event venues that are unsignalized). Gard (61) conducted a study to develop a set of empirical equations to accurately predict maximum queue lengths at unsignalized intersections. Using traffic data from a set of 15 intersections in California, Gard developed a series of regression equations for the turning movements at an unsignalized intersection. His equations for major-street left turns are shown in Table 34. Table 34. Gard (61) regression equations for major-street left-turn queue length at unsignalized intersections. Movement Condition Equation Major- Street Left Turn Approach volume < 100 veh/hr/PHF Max. queue = −2.042 + 1.167 ln (AV) + 0.975 × TS (11) Approach Volume > 100 veh/hr/PHF Max. queue = 4.252 − 1.23 × L + 0.07996 × S + 1.412 × TS – 374.028 / AV + 0.00001144 × AV × CV (12) Where: AV = approach volume, hourly traffic volume divided by peak hour factor (PHF) for subject movement; CV = conflicting volume, hourly traffic volume divided by PHF that conflicts with the subject movement; TS = traffic signal presence, a dummy variable with a value of 1 if a traffic signal is located on the major street within 0.25 miles of the subject intersection and a value of 0 otherwise; L = lanes, the number of through lanes occupied by conflicting traffic; and S = speed, the posted speed limit on the major street (mph). Gard compared these equations to the procedures found in the 2000 Highway Capacity Manual, the monograph in ITE’s 1988 Transportation and Land Development, and the 2-minute arrival methodology in the 2001 AASHTO Green Book. He found that, of the 70 data points for major- street left turns, his method correctly predicted 34 percent of the observations, and 84 percent were predicted within one vehicle. In contrast, the Green Book method predicted 35 percent correctly and 71 percent within one vehicle; the other methods tended to underestimate queues, providing shorter lengths than needed to accommodate queuing traffic.

52 In a comparison of methods similar to Gard’s, Lertworawanich and Elefteriadou (62) also developed a method to estimate storage lengths and compared it to the 2001 Green Book. The authors’ model, based on a Poisson arrival process, considered service times of vehicles arriving at an empty left-turn lane and of vehicles arriving at an occupied left-turn lane, but did not consider the effects of heavy vehicles. They created a series of tables of recommended storage lengths based on a threshold of the probability of overflow, and they compared their results to the Green Book. The authors found that, in comparison to their Poisson model, the Green Book tended to overestimate the necessary storage lengths until the volumes approached capacity, while both the Green Book and Poisson methods underestimated the queue lengths. NCHRP Report 457 (12) developed suggested storage length values using a procedure that was similar to Harmelink’s work regarding storage length of left-turn bays at unsignalized intersections. The storage length equation is a function of movement capacity, which is dependent upon assumed critical gap and follow-up gap. NCHRP Report 457 used a smaller critical gap (4.1 sec as recommended in the Highway Capacity Manual compared to the 5.0 or 6.0 sec used by Harmelink for two-lane and four-lane highways, respectively), which resulted in shorter values than those generated by Harmelink. The Texas Urban Intersection Design Guide (63) states that the length of left-turn lanes depends on three elements: • Deceleration length, • Storage length, and • Entering taper. If insufficient room is available for each of these elements, allowing a moderate amount of deceleration length to be included in the taper section is acceptable. Deceleration length assumes that moderate deceleration will occur in the through traffic lane and the vehicle entering the left- turn lane will clear the through traffic lane at a speed of 10 mph slower than through traffic. Where providing this deceleration length is impractical, it may be acceptable to allow turning vehicles to decelerate more than 10 mph before clearing the through traffic lane. Taper On high-speed highways it is common practice to use a taper rate that is between 8:1 and 15:1 (L:T) (5). Long tapers approximate the path drivers follow when entering a left-turn lane from a high-speed through lane. However, long tapers tend to entice some through drivers into the deceleration lane—especially when the taper is on a horizontal curve. Long tapers also constrain the lateral movement of a driver desiring to enter the turn lanes. For urban areas, short tapers appear to produce better “targets” for the approaching drivers and to give more positive identification of an added left-turn lane. Short tapers are preferred for deceleration lanes at urban intersections because of slow speeds during peak periods. The total length of taper and the 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. This type of design may reduce the likelihood that entry into the left-turn lane may spill back into the through lane.

53 Municipalities and urban counties are increasingly adopting the use of taper lengths such as 100 ft for a single turn lane and 150 ft for a dual turn lane for urban streets (5). If dual left-turn lanes are used, the length required for storage is approximately half that required for single left-turn lanes (5). Flexibility in signalization is provided if the left-turn movements are separated as shown in Figure 10 (dimension m, as described in the note) (63, 64). This separation, if sufficient, can allow concurrent dual left-turn phases. Separate dual left-turn phases eliminate the potential problem of overlapping vehicle paths in the intersection. Source: Fitzpatrick, K., et al., Urban Intersection Design Guide, FHWA/TX-05/04365-P2. Copyright Texas Transportation Institute, The Texas A&M University System, College Station, Texas, 2005. Reproduced with permission of the author. Figure 10. Dual left-turn lane (63, 64). Left-Turn Lane Width The width of auxiliary lanes should preferably match the width of the through lanes although they should be at least 10 ft wide (5). If curbs are present, a curb offset of 1 to 2 ft from the edge of the travel lane to the face of the curb should be used. To accommodate a single left-turn lane, a median width of 18 ft—a 12-ft lane width plus a 6-ft divider—is recommended. The 6-ft divider may provide a refuge for pedestrians, depending on Consider providing special pavement markings (guide lines) to help guide vehicles turning from multiple turn lanes. Optional Markings Adjust throat width to accommodate multiple left and right-turn lanes. Adjust throat width to accommodate multiple left-turn lanes. Optional Markings This dimension applies to the separation of opposing multiple left-turn lanes turning simultaneously. Actual distance may vary based on site conditions. * * m

54 its design; however, it is not sufficient to fully offset the turn lane (discussed below). If dual left- turn lanes are used, the median opening and crossroad should be sufficiently wide to accommodate both incoming lanes; a median width of 28 to 30 ft—11- to 12-ft lanes plus a 6-ft divider—is recommended (63). Offset Left-Turn Lanes Vehicles in opposing left-turn lanes can limit each other’s views of approaching traffic. The restriction on the sight distance is dependent on the amount and direction of the offset between the opposing left-turn lanes. The offset is measured between the left edge of a left-turn lane and the right edge of the opposing left-turn lane as shown in Figure 11 (63). Benefits of positive offset left-turn lanes include: • Better visibility of opposing through traffic, • Improved unprotected left-turn phase, • Decreased possibility of conflict between opposing left-turn movements within the intersection, and • Service for more left-turn vehicles in a given period of time (particularly at signalized intersections). The impact on pedestrian crossings of all roadways should be considered in the design of offset left-turn lanes. Greater right-of-way width is required to offset left-turn lanes, but research has shown that they can provide significantly greater sight distance for left-turn maneuvers, a particularly critical maneuver for older drivers (65). Guidelines were developed for offsetting opposing left-turn lanes at 90-degree intersections on level, tangent sections of divided roadways with 12-ft lanes (see Table 35) (66). The minimum offsets in the table are those required to provide opposing left-turning vehicles with adequate sight distances. They are applicable to left-turning passenger cars opposed by either another passenger car or a truck. The desirable offsets are those that provide opposing left-turning vehicles with unrestricted sight distances, and therefore, they are independent of design speed. The guidelines include minimum and desirable offsets when both vehicles are unpositioned, and the left-turning vehicle is unpositioned and the opposing left- turning vehicle is positioned. Positioned vehicles enter the intersection to obtain a better view of oncoming traffic, while unpositioned vehicles remain behind the stop line while waiting to turn left. A previous study found that 60 percent of older drivers did not position their vehicle. Therefore, in areas with high percentages of older drivers, the guidelines based on both vehicles being unpositioned should be used. Likewise, in areas where there are high percentages of trucks, the guidelines based on the opposing left-turning vehicle being a truck should be used. The guidelines presented in Table 35 typically involve reconstructing the left-turn lanes. Increasing the width of the lane line between the left-turn lane and the adjacent through lanes can also improve the sight distance by encouraging the driver to position the vehicle closer to the median. McCoy et al. (67) developed a methodology for determining the width of the left-turn lane line.

55 Source: Fitzpatrick, K., et al., Urban Intersection Design Guide, FHWA/TX-05/04365-P2. Copyright Texas Transportation Institute, The Texas A&M University System, College Station, Texas, 2005. Reproduced with permission of the author. Figure 11. Examples of offset left-turn lanes (63). Positive Offset Negative Offset

56 Table 35. Tarawneh and McCoy (66) guidelines for offsetting opposing left-turn lanes. Metric Opposing Left-Turn Vehicle Minimum Offset (m) Desirable Offset (m) Design Speed (km/h) Type Location 50 60 70 80 90 100 110 Passenger Car Unpositioned Positioned 1.0 0.2 1.0 0.3 1.1 0.3 1.1 0.4 1.1 0.4 1.2 0.4 1.2 0.4 1.3 0.6 Truck Unpositioned Positioned 1.5 0.8 1.5 0.8 1.5 0.9 1.6 0.9 1.6 0.9 1.6 1.0 1.6 1.0 1.7 1.1 U.S. Customary Opposing Left-Turn Vehicle Minimum Offset (ft) Desirable Offset (ft) Design Speed (mph) Type Location 31 37 43 50 56 62 68 Passenger Car Unpositioned Positioned 3.3 0.7 3.3 1.0 3.6 1.0 3.6 1.3 3.6 1.3 3.9 1.3 3.9 1.3 4.3 2.0 Truck Unpositioned Positioned 4.9 2.6 4.9 2.6 4.9 2.9 5.2 2.9 5.2 2.9 5.2 3.3 5.2 3.3 5.6 3.6 Two types of offset left-turn lanes are typically used: parallel and tapered. Parallel lanes may be used at both signalized and unsignalized intersections, while tapered lanes are usually used only at signalized intersections. An illustration of both types is provided in Figure 12. Tapered offset left-turn lanes are normally constructed with a 4-ft nose between the left-turn and the opposing through lanes. This median nose can be offset from the opposing through traffic by 2 ft or more with a gradual taper, making it less vulnerable to contact by the through traffic (see Figure 12[b]). This type of offset is especially effective for the turning radius allowance where trucks with long rear overhangs, such as logging trucks, are turning from the mainline roadway. This same type of offset geometry may also be used for trucks turning right with long rear overhangs (5). Parallel and tapered offset left-turn lanes should be separated from the adjacent through traffic lanes by painted or raised channelization. Adequate advance signing is essential so that drivers recognize the need to enter the turn lane well in advance of the intersection.

57 Source: A Policy on Geometric Design of Highways and Streets. Copyright American Association of State Highway and Transportation Officials, Washington, D.C., 2004. Reproduced with permission of AASHTO. Figure 12. Parallel and tapered offset left-turn lane (5). Results of a 1996 study by Tarawneh and McCoy (66) indicated that driver performance can be adversely affected by offsets that are much less (i.e., more negative) than −2.95 ft. Such large negative offsets significantly increase the size of the critical gaps of drivers turning left and also seem to increase the likelihood of conflicts between left turns and opposing through traffic. Large negative offsets may be particularly troublesome for older drivers and women drivers, who are less likely to position their vehicles within the intersection to see beyond vehicles in the opposing left-turn lane. The same 1996 study had a somewhat counterintuitive finding. Driver perceptions of the level of comfort were not found to improve with greatly increased offsets. An offset of 5.9 ft was

58 associated with a lower level of comfort and a higher degree of difficulty perceived by drivers than an offset of −2.95 ft, even though the latter provides less sight distance. The study’s authors speculated that this reaction might be because the −2.95-ft offset is more common than the 5.9-ft offset. Intersection Sight Distance Yan and Radwan (68) developed sight distance geometric models for unprotected left-turning vehicles at parallel and taper left-turn lanes. For parallel left-turn lanes, they calculated available sight distance as follows: ܵܦ ൌ ௙ܸ ൅ ܦ ൅ ቀ ಽ೟ మ ା௠ି௡ି௚ି௏ೢ ቁൈሺ௏೑ା஽ሻ ଶ௡ାଶ௚ା௘ା௏ೢ ି௠ (13) Where: SD = available sight distance (ft); Vf = distance from the eye of the driver to the front of the vehicle (ft); D = distance between stop bars of opposing left lanes, which is composed of the width of pedestrian corridors and the width of the minor road (ft); Lt = width of the opposing through lane (ft); m = width of the median (ft); n = width of the median nose (ft); g = distance from the left side of the left-turn vehicle to the left lane line (ft), Vw = width of the opposing left-turn vehicle (ft); and e = distance from the eye of the driver to the left side of the vehicle (ft). The authors produced a table that calculates available sight distance for common dimensions of parallel opposing left-turn lanes, reproduced here as Table 36. They also produced similar equations and tables for parallel lanes with offset and for tapered left-turn lanes. Table 36. Yan and Radwan (68) calculated available sight distance for traditional parallel opposing left-turn lanes. m (ft) n (ft) Lane Width (ft) D (ft) Available Sight Distance (ft) 12 13 14 15 16 17 18 19 20 0 1 2 3 4 5 6 7 8 12 12 12 12 12 12 12 12 12 83 83 83 83 83 83 83 83 83 1729 637 419 325 273 240 217 200 187 The models and related analyses focused only on the unprotected phases of a signalized intersection, but the principles are comparable for sight visibility at unsignalized intersections. The authors caution that the absence of stop bars at unsignalized intersections discouraged a

59 direct application of these models because left-turning vehicles’ positions could be more flexible before crossing the opposing through traffic. Alternate Intersection Designs A number of alternate designs have been proposed and implemented to change the configuration of intersections to improve the efficiency and/or safety of turning movements. One such design is the crossover displaced left-turn (XDL) intersection, also called the continuous flow intersection (CFI). The fundamental design principle of the XDL intersection involves displacement of the left-turn lane to the other side of the opposing through lanes several hundred feet upstream of the intersection. The displaced left-turn lanes are aligned parallel to the through lanes at the intersection. This design results in the simultaneous movement of left-turning traffic with through traffic at the intersection. The key tradeoffs are the need for additional right-of-way to accommodate the displaced lanes and the creation of several smaller ancillary intersections around the primary intersection, which must also be maintained with signing and marking. This design is primarily intended for signalized intersections as an alternative to grade separation, but there may be possible applications for unsignalized intersections at high-speed locations. Jagannathan and Bared (69) modeled the performance of three sample XDL intersections in comparison to conventional intersections and found that average intersection delay, average number of stops, average queue length, and capacity all improved with the XDL. Roundabouts are growing in popularity in the United States, after having been developed in the 1960s in the United Kingdom and used at numerous intersections in other countries, primarily throughout other parts of Europe as well as Australia. Two key characteristics of the modern roundabout include a requirement for entering traffic to yield to circulating traffic and geometric constraints that slow entering vehicles. One result is that traditional left turns are eliminated as all intersection traffic travels around the circulatory roadway in the same direction. A recent NCHRP project (70) examined the safety and operation of roundabouts in the United States, with the purpose of producing a set of operational, safety, and design tools, calibrated to U.S. roundabout field data. The researchers found that, with the exception of conversions from all- way-stop-controlled intersections, where crash experience remains statistically unchanged, roundabouts have improved both overall crash rates and, particularly, injury crash rates in a wide range of settings (urban, suburban, and rural) and previous forms of traffic control (two-way stop and signal). Statistical analysis revealed a 35 percent reduction in crashes for all sites studied. Overall, single-lane roundabouts have better safety performance than multilane roundabouts. The safety performance of multilane roundabouts appears to be especially sensitive to design details, such as lane width. Rodegerdts et al. (70) further concluded that drivers at roundabouts in the United States currently appear to be somewhat tentative, using roundabouts less efficiently than models suggest is the case in other countries around the world. In addition, the number of lanes has a clear effect on the capacity of a roundabout entry; however, the fine details of geometric design—lane width, for example—appear to be secondary and less significant than variations in driver behavior at a given site and between sites. Although the project was unable to establish a strong statistical relationship between speed and safety, the importance of controlling speed in roundabout design is well established internationally. Anecdotal evidence suggests the importance of considering

60 design details in multilane roundabout design, including vehicle path alignment, lane widths, and positive guidance to drivers through the use of lane markings. SUMMARY OF LITERATURE Table 37 presents a summary of the left-turn lane design guidelines from the literature. Table 37. Summary of left-turn lane design guidelines from the literature. Source Queue Storage Length Entering Taper Length Deceler- ation Length Width Offset Dual Turn Lanes Innovative Intersection Design Green Book (5)     Bonneson and Fontaine (12)   Harmelink (1)  Kikuchi et al. (60)    Gard (61)  Lertworawanich and Elefteriadou (62)  Fitzpatrick et al. (63)       Connecticut Department of Transportation (64)  Staplin et al. (65)  Tarawneh and McCoy (66)  McCoy et al. (67)  Yan and Radwan (68)  Jagannathan and Bared (69)  Rodegerdts et al. (70) 