Left-Turn Accommodations at Unsignalized Intersections (2013)

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23 Introduction Traffic controls are important complements to the geo- metric design of unsignalized intersections. Signs, signals, and markings should conform to guidance set forth in the Manual on Uniform Traffic Control Devices (MUTCD) (20). State and local standards for roadway and intersection illumi- nation should reflect AASHTO and Illuminating Engineering Society of North America (IESNA) guidelines (21). Consis- tency in the type, size, shape, messages, and placement of devices is essential. In this chapter, there are three categories of devices considered: • Signs, which include regulatory, warning, and information signs; • Pavement markings, which include centerlines, lane lines, stop bars, and crosswalk striping; and • Illumination, which includes intersection and approach lighting. Signs Traffic signs are used to convey essential information along streets and highways and at intersections. Sign types, shapes, sizes, symbols, colors, messages, illumination, reflectoriza- tion, and placement should be consistent with guidelines set forth in the MUTCD (20) and state and local require- ments. Additional guidance is given in the ITE Traffic Con- trol Devices Handbook (22) and the ITE Traffic Engineering Handbook (23). Signs should be used only where warranted based on engi- neering judgment or studies. Signs should be coordinated with the geometric design and with the associated pavement mark- ings. Excessive signing should be avoided. However, where the population of older drivers is significant, some redundant signing may be applicable. Sign Types Traffic signs are classified as regulatory, warning, or guide signs: • Regulatory signs give notice of traffic laws and regulations. Common signs used at unsignalized intersections include STOP and YIELD signs. Where special turn lanes are pro- vided, lane-use signs are normally installed. ONE WAY and DO NOT ENTER signs are used where some streets operate one way. • Warning signs give notice of situations that might not be readily apparent. The Intersection Ahead sign may be used on approaches to intersections. • Guide signs show route designations, destinations, dis- tances, and services. Street Name signs are guide signs that are commonly installed at intersections. Sign Design Sign size, design, and location are governed by Part 2 of the MUTCD (20). Signs must be sufficiently large, legible, read- able, and properly placed for both day and night visibility. The general principles of sign size, shape, color, and messages are discussed in Chapter 2A of the MUTCD (20). Larger signs may be used where increased visibility is essential. FHWA’s Standard Highway Signs manual (24) provides detailed sign layouts for standard signs contained in the MUTCD. Key features of commonly used intersection and intersection- related signs are listed in Table 10. The table gives the sign type, MUTCD number, size, shape, and color. Advance traffic control signs, such as STOP Ahead or YIELD Ahead, may be used on approaches to intersections that are controlled by these devices. The MUTCD requires the use of these signs where the primary intersection traffic control devices are not visible for a sufficient distance. C h a p t e r 4 Traffic Controls and Illumination

24 Word messages should be as brief as possible. Abbrevia- tions should be kept to a minimum. Punctuation (e.g., periods, apostrophes, ampersands) should not be used; word messages should contain only characters that are letters, numerals, or hyphens unless other characters are necessary to avoid confu- sion. Letter sizes should be large enough to be readable at appro- priate distances in advance of sign locations. The MUTCD uses a guideline of 30 ft of distance for each 1 inch of letter height. The Highway Design Handbook for Older Drivers and Pedestrians (15) recommends a ratio of 33 ft to 1 inch of letter height for enhanced visibility. Increases above standard sizes should be used where greater legibility or emphasis is needed. Wherever practical, the overall sign dimensions should be increased in 6-inch increments (15). Most standard highway signs use all capital letters for the sign legend. However, a combination of uppercase and lower- case letters provides greater legibility for destination names on guide signs. Allowing drivers to read the unique footprint of the words displayed in uppercase and lowercase letters increases accuracy, viewing distance, and reaction time com- pared to words in all capital letters. When a mixed-case legend is used, the height of the lowercase letters shall be three-fourths of the height of the initial uppercase letter. Placement Sign location and placement should reflect several basic objectives: • Location and placement should be consistent along high- ways and at intersections. • Signs should be located within the approaching driver’s cone of vision. • Signs should not be located within the traveled way (an exception is some pedestrian crossing signs) or pedestrian areas. • Signs should not interfere with drivers’ visibility of haz- ards, especially intersections or driveways. The 2009 MUTCD contains guidelines for advance place- ment of warning signs in Table 2C-4 (20). Longitudinal Placement Signs should be placed at locations where there are no sight obstructions between the sign location and the intended point of observation. Locations that should be avoided include dips and hillcrests, trees and foliage, other signs, and parked vehicles. Sign placement should not create an obstruction to pedestrians, bicyclists, or drivers’ visibility of hazards (near intersections or driveways), and should not be vulnerable to roadside splatter or snow-plowing operations. Using an excessive number of signs should be avoided. Regulatory and warning signs should be used conservatively because these signs, if used to excess, tend to lose their effec- tiveness. If used, route signs and directional guide signs should Table 10. Characteristics of signs commonly used at unsignalized intersections and approaches (adapted from 20). Type Sign MUTCDNumber Shape Typical Size (Inches) Background Color Text Color Regulatory STOP R1-1 Octagon 30×30 or 36×36 Red White YIELD R1-2 Equilateral triangle 36×36×36 Red White No Turns R3-1 through R3-4 Rectangular 24×24 White Black Lane Controls R3-5 through R3-8 Rectangular 30×36 White Black DO NOT ENTER R5-1 Circle on square plate 30×30 Red White ONE WAY R6-1R6-2 Rectangular 36×12 24×30 White Black Warning Stop Ahead W3-1a Rectangular 24×30 Yellow Black Yield Ahead W3-2 Rectangular 24×30 Yellow Black Crossroad W2-1 Rectangular 24×30 Yellow Black Intersection Ahead W2-1 Rectangular 24×30 Yellow Black Guide Street Name D-3 Rectangular Varies Green White Advance Street Name W16-8P, W16-8aP Rectangular Varies Green White

25 be used frequently because their use promotes efficient opera- tions by keeping road users informed of their location. Signs should not interfere with each other and should be placed where they are not obstructed by poles or trees. The longitudinal placement depends on the type of sign, nature of the message, and desired motorist response, but some general guidelines are as follows: • Regulatory signs should be placed at the locations where the regulation begins. • Warning signs should be placed sufficiently in advance of the hazard, where drivers can take appropriate corrective actions. • Information signs should be placed near the location where they apply. Specific locations vary with the type of message offered. • Successive signs on approaches to intersections should be spaced as widely as possible. Signs requiring separate decisions by the road user should be spaced sufficiently far apart for the appropriate decisions to be made. One of the factors considered when determining the appropriate spacing is the posted or 85th percentile speed. Signs should be individually installed on separate posts or mountings except where: • One sign supplements another; • Route or directional signs are grouped to clarify informa- tion to motorists; • Regulatory signs that do not conflict with each other are grouped, such as turn prohibition signs posted with ONE WAY signs or a parking regulation sign posted with a speed limit sign; or • Street Name signs are posted with a STOP or YIELD sign. Lateral Clearance Signs are generally located as far as practical from the trav- eled way while still providing good visibility. They should not protrude into pedestrian areas. Some guidelines include: • Signs on rural roadways should be placed at least 6 ft beyond the shoulder or at least 12 ft from the edge of the traveled way, whichever is greater. • On streets and roadways with curbs, signs should be at least 2 ft beyond the face of the curb. A clearance of 1 ft is permissible where sidewalk width is limited. Vertical Clearance and Mounting Height. The vertical clearance (of the bottom of the sign) must be at least 7 ft above the surface in urban areas to provide visibility above parked cars and not pose a hazard to pedestrians. The minimum height is 5 ft in rural areas. The vertical clearance on overhead- mounted signs is at least 17 ft above the edge of pavement. Both span-wire and mast-arm cantilevered mountings are permissible. Street Name signs may be placed above a regula- tory or STOP sign with no vertical separation. Application Guidelines The application of specific signs should be consistent with requirements set forth in the MUTCD and state/local require- ments. Relevant application guidelines for regulatory, warn- ing, and information signs follow: • Regulatory signs should be installed at or near where the regulations apply. They inform users of traffic laws, regula- tions, or restrictions that are applicable for the given inter- section and approaches. • Warning signs for intersection approaches are commonly used upstream of an intersection to advise motorists of upcoming intersections. They include signs advising motor- ists of the Intersection Ahead and of a STOP Ahead or YIELD Ahead. These signs are placed a sufficient distance ahead to enable motorists to take appropriate actions. Typi- cal placement of advance traffic control signs is shown in the MUTCD. Various signs are used to give direction to pedes- trians and give warning to motorists. A common applica- tion is to provide Pedestrian Crossing signs along roadways where pedestrian movements might occur. • Guide signs give road users information in the most sim- ple and direct manner possible. They: – Direct road users along streets and highways and inform them of intersecting routes; – Direct road users to cities, towns, villages, and other important destinations; and – Identify nearby rivers and streams, parks, forests, and historical sites. Conventional guide signing gives highway class, number, and cardinal direction. This information is provided at key intersections. Pedestrian information, usually in advance of intersections, is sometimes repeated at intersections. Street Name signs should be installed at all intersections in urban and suburban areas. They should be installed in rural areas to identify important roads not otherwise signed. In residential areas, at least one Street Name sign should be mounted at each intersection; their faces should be mounted parallel to the streets they name. Signs may also be mounted overhead on the far side of an inter- section. Advance Street Name signs using white letters on a green background may be installed near exclusive turn lanes, as well as on approaches to major highways. In busi- ness districts and on principal arterials, Street Name signs should at least be placed on diagonally opposite corners so

26 they will be located on the far right side of the intersection along the major street. Example applications are available in the MUTCD (20). The Texas Department of Transportation Sign Crew Field Book (25) provides specific guidance for intersection signing developed to provide field sign personnel with information beyond that contained in the Texas MUTCD (26) to improve statewide uniformity in the placement of traffic signs. Pavement Markings Overview Pavement markings are a key element in roadway design and operations. They are an effective and efficient way to improve clarity and safety. They are located directly in the driver’s cone of vision, they provide continuous information, and they are an important complement to signs. They have a dual purpose of guiding users and optimizing roadway efficiency. They are nor- mally used to supplement signs and signals and to convey mes- sages that might not otherwise be understood. Typical markings are attached to the pavement surface and convey information by color, line design, words, letters, sym- bols, and patterns that regulate, warn, or guide users. Some examples of the uses of pavement markings include: • No passing zones (indicate regulations), • Stop bars (supplement or reinforce the message of other devices), • Lane lines (guide road users), and • Text markings such as SIGNAL AHEAD or RR XING (warn users). In addition to markings, post-mounted delineators, object markers, and colored pavements can also be considered part of the marking system. Markings define edge lines, travel lanes, stop lines, and cross- walks. They are sometimes used in channelizing intersections and as painted islands that separate opposing traffic streams. Markings, like other traffic controls, should comply with the MUTCD and state manuals. Uniformity and clarity are essential to convey a consistent message to drivers that meets their expectations and effectively communicates the neces- sary information. Types of Markings For the purposes of these guidelines, pavement markings are classified as longitudinal (along the roadway), transverse (perpendicular to the roadway), and on-pavement messages. • Longitudinal markings include edge lines, lane lines, cen- terlines (or painted median islands), lane transition lines, no passing zones, and curb delineations. • Transverse markings include stop lines and crosswalks. • Messages on pavement include markings such as arrows, bus or light-rail transit lane symbols, high-occupancy vehicle (HOV) symbols, or text. • White is used for most pavement markings. In the United States, centerlines that separate opposing directions of travel are yellow. • Other colors (e.g., red, blue, or purple) are sometimes used to indicate restrictions or special conditions, but they are not common at unsignalized intersections. • Black may be used in combination with white or light- colored markings to improve the contrast with light- colored pavements. The basic types of markings, sample applications, and their colors are set forth in Table 11. Applications The applications of pavement markings at unsignalized inter- sections are generally straightforward. They include the delin- eation of through and turning lanes, provision of crosswalks, and possible provision of broken lines (commonly known as “skip stripes” or “cat tracks”) to guide left turning. The MUTCD (20) shows examples of typical markings at intersections. Type Application Color Markings Longitudinal Edge lines White Solid Lane lines White Usually dotted Center lines Yellow Solid or dotted Lane transition lines White Dotted No passing zones Yellow Solid Transverse Stop lines White Solid Pedestrian crosswalks White Various patterns Messages on Pavement Directional arrows (optionally accompanied by text) White Symbol (and optional text) HOV or bus symbols White Symbol PED XING White Text Table 11. Overview of pavement markings (7).

27 Pavement markings should be integrated into the physical design. Key elements include: • Through and left-turn lane widths, • Required left-turn lane storage lengths, and • Transition markings for lanes guiding through traffic around left turns and shadowing the left-turn lane. Specific pavement markings depend upon how the left- turn lanes are provided. Following are three example cases: • Where the left-turn lanes can be provided by utilizing a central point (or raised island) median, there is no align- ment change for the through travel lanes, as shown in Figure 6. • In most cases, it is necessary to widen roadways or elimi- nate on-street parking to provide space for left-turn lanes. The example in Figure 7 diverts the through travel lanes around a fully “shadowed” left-turn lane. This concept has application where there are high approach speeds. • The example in Figure 9 offsets the through travel lane and allows direct entry into the left-turn lane. This concept is commonly used in many cities (e.g., Chicago, New Haven, Toronto). Illumination Overview Roadway and intersection lighting is an important com- plement to traffic controls in improving both vehicle and pedestrian operations and safety. They provide for the visual needs of motorists, bicyclists, and pedestrians. Dark condi- tions make it difficult for drivers to make correct decisions with adequate time. Where adequate lighting is provided, sudden braking and swerving are reduced, and visibility of pavement markings and signing is enhanced. This section gives a broad overview of intersection light- ing. It contains discussion of the types of systems, how lighting can be designed, and where roadway and intersec- tion lighting should be placed. Further details are set forth in the ITE Traffic Engineering Handbook (23), the AASHTO Roadway Lighting Design Guide (27), and the IESNA Lighting Handbook (21). Types of Illumination Street and roadway lighting units comprise the lamp, bal- last, and luminaire. The following factors influence their selection (21): • Lamp lumen output (lamp size), • Lamp life, • Lamp lumen depreciation, • Ambient temperature range in the area, • Cost (lamp and luminaire), • Lamp restrike time, • Luminaire light distribution, • Physical size (lamp and luminaire), • Physical durability (lamp and luminaire), • Lamp color, and • Energy consumption. Low-pressure sodium (LPS), high-intensity discharge (HID) (commonly known as mercury), metal halide, and high- pressure sodium (HPS) light sources are frequently used in today’s roadway lighting systems. Other types include incandes- cent and fluorescent. Light-emitting diodes (LEDs) are mainly used in traffic signals and are under field tests for street lighting. Homburger et al. (28) described a number of characteristics of these lamps (listed in Table 12). • The incandescent or filament lamp was the most com- monly used for many years. It was inexpensive, simple, and easy to install. It produced pleasing color rendition, and its small size permitted good light control with a reasonably sized fixture. However, its low efficacy and short-rated life have made it undesirable for new installations. Table 12. Roadway lighting lamp characteristics (28). Type of Lamp Initial Light Output (1000 Lumens) Approximate Efficacy (Lumen/Watt)a Approximate Lamp Life (1,000 Hours)b Incandescent 0.6–15 9.7–17.4 2–6 Fluorescent 6.8–14 61–72 10 Clear Mercury 3.7–57 37–57 18–28 Phosphor-Coated Mercury 4.0–63 40–63 18–28 Metal Halide 34.0–100 85–100 10–15 High-Pressure Sodium 9.5–140 95–140 15–28 Low-Pressure Sodium 1.8–33 100–183 10–18 a Except for incandescent, these values exclude wattage losses due to ballast. b Number of hours for a group of lamps where 50 percent will remain in operation; based on 10 hours of operation per start, except 3 hours per start for fluorescent lamps.

28 • The fluorescent lamp is no longer used for new roadway lighting installations but is still utilized for tunnel and sign lighting. Its large size makes it difficult to obtain good light control in a reasonably sized luminaire. The lamp requires ballast, and its light output is affected by low temperature more than other lamps. An advantage is the broad light patterns that it provides on wet streets. • The mercury lamp, developed in the 1930s, replaced the incandescent lamp in popularity. Its initial cost is higher, and it requires ballast, but its higher efficacy and long life make it more attractive than the incandescent lamp. The blue-white color of the clear lamp is generally acceptable, and the arc tube size provides a light source that is small enough to permit good light control. • The metal halide lamp, a type of mercury lamp, has arc tubes that also contain certain metal halides without the use of a phosphor-coated bulk. The lamp produces a white light bulb that is pleasing to motorists and pedestrians. The light source is that of the arc tube, permitting good light control in the same feature used for clear mercury lamps. They work well in high-mast lighting. The lamps have a wide selection of sizes, good lamp life, and a compact size; they are easily optically controlled. • The HPS lamp is characterized by a golden-white color out- put. HPS lamps are normally operated with special ballasts that provide the necessary voltage to start the lamp. Some of the newer HPS lamps include improved color rendition, internal starting devices that operate with mercury or metal halide lamp ballasts, dual arc tube or “standby” lamps that provide light as soon as power is restored after a momen- tary power interruption, and a rated life of 40,000 hours. • The LPS lamp is characterized by a monochromatic bright yellow color output. It requires special ballasts and increases materially in size as the wattage increases; the 185-W lamp is 1120 mm long. This large size makes it difficult to obtain good light control in a reasonably sized fixture. The poor color rendition of the LPS lamp previ- ously made it unpopular for use in other than industrial or security applications. However, the potential benefits of energy conservation produced by the high efficacy of the lamp have resulted in its increasing acceptance for lighting both commercial and residential areas. • LEDs have the advantages of good light distribution and long service life (100,000 hours). They are used for traffic signal displays and offer promise, but are not yet widely used for street and roadway lighting. Good heat manage- ment is essential. • Ballast is required for HID and fluorescent lamps. The bal- last provides the proper starting and operating voltage and wave form, and limits the operating current to the proper value; for certain types of lamps it supplies the neces- sary cathode heater voltage. Ballast may be located in the luminaire housing, in the pole base, or in an underground pull box, the most popular location being in the luminaire housing. Intensity Roadway and intersection lighting design and placement should consider the visual capabilities of motorists and pedes- trians, roadway geometry, traffic volumes, operating speeds, energy consumption, and environmental features. The principal steps in the lighting design process are sum- marized as follows [further details are contained in the ITE Traffic Engineering Handbook (23)]: 1. Survey and evaluate existing roadway, traffic, and environ- mental conditions. 2. Classify roadway types, pavement reflections, and the sur- rounding environmental features. 3. Select the required minimum illumination levels for the roadway or intersection under consideration. 4. Acquire photometric and related information for the lamps and luminaires to be used. This information includes watts, lumens, light distribution, and lumen depreciation factors. 5. Estimate light loss factors for luminaire dirt depreciation, effects of ambient temperature, possible field adjustments to laboratory tests, operating voltage variations, and lamp/ ballast combined luminaire depreciation factors. 6. Establish desired luminaire mounting heights. This nor- mally depends on the lumens provided: • ≤ 20,000 lumens ≤ 35 ft • 30,000–45,000 35–45 ft • 45,000–90,000 45–60 ft 7. Compute the desired luminaire spacing for the required illumination levels. 8. Compute the uniformity ratio (the ratio of the average maintained intensity to the minimum). Repeat steps 4 through 7 as needed. 9. Also, transitional lighting at the beginning and end points of continuous lighting systems should be considered. This may not be necessary when illumination is being provided for an isolated location. Location Most urban and suburban street intersections are illumi- nated. In rural areas, however, intersection illumination is often lacking. Accordingly, many states have developed guide- lines for when intersection lighting is warranted. Improved nighttime safety is usually a main consideration, but the prac- titioner should consult the guidance documents that are in effect for that jurisdiction to learn about requirements and guidelines that are specific to the state, county, or city in which the intersection is located.

29 The following factors should be considered in assessing intersections for lighting: • Intersection location (urban, suburban, or rural); • Functional classification of the approach roads; • Intersection visibility, especially nighttime approach visibility; • Street geometry, including merging lanes, curves, and grades; • Intersection channelization, including the presence of physical medians; • Presence of lighting on approaches; • Traffic volumes and speeds; • Pedestrian and bicycle volumes; and • Intersection safety (night-to-day crash ratios). Based on current practices in the United States, and a review of existing guidance documents and previous research, several general guidelines are established regarding where lighting should be considered for rural intersections. An intersection approach with one or more of the following characteristics is a good candidate for lighting: • The intersection geometry is complex and includes raised channelization. • The intersection sight distance is less than ideal (e.g., a roadway located on a curve). • Nighttime crash rates are high over a multiyear period, and the nighttime-to-daytime crash rate ratios are high. • One or more approaches to the intersection are already illuminated. Provision for Future Signalization Though the material in this Design Guide is focused on unsignalized intersections, it is prudent to make provisions to add traffic signals in the future, particularly in areas of expected growth and development on suburban fringes and in areas described by a city’s comprehensive development plan. This section discusses a selection of key items to con- sider when planning for future signalization. Power The need to provide electrical power to the intersection must be a primary consideration. Typically there are proce- dures in place for extending the power grid to new locations, and it is just a matter of paying for the cost of new power lines and, depending on the location, conduits or poles to carry the lines to the intersection. If lighting is already included in the left-turn lane installation, then those procedures will be fol- lowed, and power will already be available at the intersection when signals are added. Location of Signal Poles and Controller Cabinet This provision dovetails with that of providing power. It is common to install roadway lighting on signal poles. Con- versely, if the lighting is installed first at a location where signals are planned, the luminaire poles can be placed such that they can be used for pole-mounted or mast-arm signal heads when the traffic signals are installed. When lighting is not installed, it is important to maintain the necessary clear space within the right-of-way (or acquire additional right- of-way) to accommodate the future installation of the signal poles and controller cabinet. Vehicle Detection It is common to use video cameras to detect vehicles awaiting a green indication at a signalized intersection, and these cameras can be placed on the signal pole or mast arm. However, if it is anticipated that other types of detection will be used, particularly some form of in-pavement sensor, it may be beneficial to install much of the hardware for that detection system while the intersection is under construc- tion. This will prevent the need to go back into the new pavement at a later date when volumes are likely higher and the impact on operations at the intersection will be higher. Similarly, an important consideration in addition to the sen- sors themselves is the installation of conduit and junction boxes. If conduit of the proper size to accommodate a future signal is not installed with the project, a future project will be required to create a trench for the conduit at a later date, which will likely eradicate any in-pavement sensors previ- ously installed. Access Management At unsignalized intersections of lower volume, adjacent developments may install (or request installation of) access points near the intersection. These points of access may func- tion adequately under initial traffic conditions, but as vol- umes rise, operations and level of service will likely suffer, and safety problems may develop. If future signalization is anticipated, it is prudent to prohibit access points in close proximity to the intersection. This will eliminate the later need to close or move a driveway or to accommodate access during future growth or construction.