Left-Turn Accommodations at Unsignalized Intersections (2013)

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12 Introduction Once the decision has been made to provide a left-turn lane at a particular intersection, and the preliminary plan- ning has been completed to determine the basic elements of the lane relative to the characteristics of the location (e.g., right-of-way, traffic volumes, etc.), the dimensions and other physical characteristics of the lane must be specified based on geometric design principles. Figure 5 shows a typical left-turn lane layout. Selection of Design Speed Speed is one of the most important factors considered by travelers in selecting alternative routes or transportation modes. According to the AASHTO Green Book (5), design speed is a selected speed used to determine the various geo- metric design features of the roadway. The selected design speed should be a logical one with respect to the topography, anticipated operating speed, adjacent land use, and func- tional classification of the highway. In selection of design speed, every effort should be made to attain a desired com- bination of safety, mobility, and efficiency within the con- straints of environmental quality, economics, aesthetics, and social or political impacts. Once the design speed is selected, all of the pertinent highway features should be related to it to obtain a balanced design. As one of the geometric design features of a particular intersection, a left-turn lane should be designed in rela- tion to the main lanes of the adjacent roadway. The selected design speed affects the design of the subsequent elements of the turning lane, and the designer should keep in mind the interaction of vehicles between the main lanes and the turning lane when choosing the design speed. The selected design speed should be consistent with the speeds that drivers are likely to expect on a given highway facility. Therefore, the design speed for a left-turn lane should generally be equal to that of the adjacent roadway, absent constraints imposed by right-of-way or other limiting factors. A single design speed for deceleration lanes may be appro- priate for rural/undeveloped locations where speed is essen- tially constant throughout the day. However, in suburban/ urban (developed) areas, speeds in peak periods are com- monly lower than in off-peak periods, and left-turn queue lengths are typically longer in peak periods. Therefore, it is prudent to consider two speeds (and two corresponding queue storage lengths), one for peak periods and one for off- peak periods. Additionally, conditions in the morning peak period may be different than in the afternoon, most com- monly for commuter routes with a high directional split that reverses from morning to afternoon. When determining the appropriate value for design speed, the value chosen should be that for the conditions that result in the longest required distance for deceleration plus queue storage. When designing other geometric elements based on the design speed, design values that are more generous than the minimum should be used where practical, particularly on high-speed facilities. Some design features, such as taper length and sight distance, are directly related to, and vary appreciably with, design speed. Other features, such as widths of lanes and shoulders, are not directly related to design speed, but they do affect vehicle speeds. Therefore, wider lanes and shoulders should be considered for higher design speeds. Thus, when a change is made in design speed, many elements of the turning lane design change accordingly. Selection of Design Vehicle Key controls in geometric highway design are the physical characteristics and the proportions of vehicles of various sizes using the roadway. Different vehicles have different needs that affect their ability to navigate a left-turn lane. Therefore, it is necessary to select a vehicle, called a design vehicle, with weight, dimensions, and operating characteristics that are C h a p t e r 3 Geometric Design

13 representative of the vehicles likely to most commonly use the left-turn lane. When selecting a design vehicle, the designer must first consider the mix of vehicle types that use the adjacent road- way and then which of those vehicles are expected to use the left-turn lane. AASHTO (5) has established four classes of design vehicles: • Passenger cars, • Buses, • Trucks, and • Recreational vehicles. Within each class, there are multiple unique design vehicles to consider. The AASHTO Green Book (5) provides guidance in selecting the most appropriate design vehicle, including representative turning radii and other characteristics. The TRB Access Management Manual (6) provides sug- gested storage length per vehicle for different percent truck levels (reproduced in Table 4). Desirable and Minimum Lane Widths Left-turn lanes at intersections and interchanges often help to facilitate traffic movements. Such added lanes should be as wide as the through-traffic lanes, but not less than 10 ft. Of course, the width of the turning lane is dependent on the width of the median where it is located. 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 (7). In urban situations with constrained space and low speed, 9-ft lanes have been used, but these are the exceptions rather than the rule. Median widths of 20 ft or more are desirable at intersections with single median lanes, but widths of 16 to 18 ft permit rea- sonably 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 traveled 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. According to draft guidelines for accessible public rights-of-way published by the United States Access Board (8), a median width of at least 6 ft must be provided for storage where pedestrians will be present. Tapers Two distinct tapers are commonly defined in many guidelines: approach taper length and bay taper length. An approach taper provides space for a left-turn lane by moving traffic laterally to the right on a street or highway without a median. The bay taper length is a reversing curve along the left edge of the traveled way that directs traffic into the left- turn lane. Illustrations of the use of these tapers along with how the left-turn lane is added to the roadway are shown in the following figures: • Figure 6 shows a left-turn lane added within a median. • Figure 7 shows a left-turn lane that was added to an un- divided two-lane highway where the through lane on the Figure 5. Typical left-turn lane layout. Reaction Time Deceleration Storage Bay Taper Table 4. Queue storage length per vehicle (6). Trucks (Percent) Assumed Queue Storage Length (ft) per Vehicle in Queue ≤ 5 25 10 30 15 35

14 same approach as the added turn lane was shifted to the right the full width of the turn lane. This condition is known as a full-shadowed left-turn lane. • Figure 8 shows a partially shadowed left-turn lane where both through lanes are shifted to provide the needed space for the turn lane. With partially shadowed left-turn lanes, the offset created by the approach taper does not entirely protect or “shadow” the turn lane (9). • Figure 9 shows the condition when a lane is added to the outside edge of the approach, allowing through vehicles to pass left-turning vehicles on the right. This condition is also known as a bypass lane. The bypass lane minimizes delay to following through vehicles by allowing the vehicle following to pass the left-turning vehicle on the right, and then merge back into the through lane. Some states do not allow informal passing on the right or driving on the shoul- der; therefore, the additional width for through vehicles provides a legal means of passing slowed or stopped left- turning vehicles. Some agencies avoid this layout because of the mixed message to drivers between passing lanes and this condition. For passing or truck climbing lanes, the newly added outside lane is for slower-moving traffic, and the inside existing lane is for faster-moving vehicles. For the configuration shown in Figure 9, the opposite situ- ation is present; the newly added outside lane is for the faster-moving traffic, and the inside existing lane is for the vehicles that are slowing and perhaps stopping while waiting to make the left turn. Tapers for Left Turns (Bay Taper) On high-speed highways it is common practice to use a bay taper rate that is between 8:1 and 15:1 (longitudinal:transverse [L:T]). 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 constrain the lateral movement of a driver desiring to enter the left-turn lanes. This problem primarily occurs on urban curbed roadways (5). Figure 6. Left-turn lane within a median. Bay Taper Deceleration Storage Figure 7. Full-shadowed left-turn lane. Bay Taper Deceleration Storage Approach Taper

15 For urbanized areas, short tapers appear to produce bet- ter visual “targets” for the approaching drivers and to give more positive identification to an added left-turn lane. Short tapers are preferred for deceleration lanes at urban intersec- tions because of slow speeds during peak periods. This results in a longer length of full-width pavement for the left-turn lane. This type of design may reduce the likelihood that entry into the left-turn lane may spill back into the through lane. Municipalities and urban counties are increasingly adopting the use of taper lengths such as 50 to 100 ft for a single-turn lane and 100 to 150 ft for a dual-turn lane for urban streets. Some agencies permit the tapered section of deceleration left-turn lanes to be constructed in a “squared-off” or “shad- owed” section at full paving width and depth, particularly in locations where a very short taper is applied. This configura- tion 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 Green Book provides advice regarding taper design. The recommended straight-line taper rate is 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 left-turn 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. Tapers for Through Traffic (Approach Taper) Though left-turn lanes can be added in such a way that the merge taper guides turning vehicles into the turning lane, certain locations instead use the taper to guide through traffic to the right of the turning lane, as shown in Figure 7 and Figure 8. Such treatments are often used in rural con- ditions where it is beneficial to provide added protection and/or guidance to turning vehicles, particularly at isolated Figure 8. Partially shadowed left-turn lane. Bay Taper Deceleration Storage Approach Taper Figure 9. Direct entry into left-turn lane (also known as bypass lane). Transition Taper Transition Taper Note: Some agencies recommend against using this layout because drivers must change lanes to continue traveling straight; otherwise, the driver would be in the left-turn lane.

16 T-intersections, where there is no median in which to install a shadowed left-turn lane, and/or at locations where right-of- way is limited. At these locations, through traffic is directed to shift its path, while turning traffic can travel straight into the turning lane. The approach taper is commonly estimated by one of the equations shown in Table 5. Comparisons for various speeds and offsets are shown in Table 5. While the design guidelines described in the previous sec- tion pertain to the use of a bay taper for turning vehicles, similar principles apply when the taper is used to shift the path of through traffic. The taper should be short enough to provide sufficient visual clues to the driver that through traffic must shift, but be long enough to allow the shift to take place at the expected or prevailing operating speed of the roadway. Pavement markings and supplemental signs should be used to reinforce the action the driver is expected to take. It is important that appropriate vehicle storage length be provided because, with no median protection, the excess queue will extend into the through travel lane. This is one rea- son why this treatment is more commonly found at isolated intersections with low turning volumes. For similar reasons, it is also important that sufficient deceleration length be pro- vided in the design of the turning lane. 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 decelera- tion to a stop from the full design speed of the highway are shown in Table 6. These approximate lengths are based on grades of less than 3 percent. On many urban facilities, it is not practical to provide the full length of deceleration for a left-turn lane, and in many cases the storage length overrides the deceleration length. In such cases, a part of the deceleration may be accomplished before entering the left-turn lane. Shorter left-turn 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 roadways due to higher levels of driver tolerance for vehicles leaving or entering the roadway due to slow speeds Table 5. Typical length of approach taper to add left-turn lanes. Design Speed (mph) Condition Equation Approach Taper Length (ft) 6-ft Offset 12-ft Offset 20 Typically used for low- speed approaches (e.g., 40 mph and less) L = WS2/60 40 80 30 90 180 40 160 320 50 Typically used for high- speed approaches (e.g., greater than 40 mph) L = WS 300 600 60 360 720 70 420 840 Where: W = width of offset (ft) S = speed (mph) Table 6. Deceleration lengths for left-turn lanes. Design Speed (mph) Deceleration Lengths (ft) from Following Sources: Deceleration Lengths from Other Manuals for Comparison Deceleration Lengths Determined Using 6.0 ft/sec2 Deceleration Rate AASHTO Green Book (5), page 714 TRB Access Management Manual (6), page 172 Florida Department of Transportation 2006 FDOT Design Standards (10) No Speed Reduction in Main Lanes 10-mph Speed Reduction in Main Lanes 30 170 160 170 80 35 145 230 120 40 275 275 155 290 170 45 340 185 370 230 50 410 425 240 (urban) 290 (rural) 460 290 55 485 350 550 370 60 605 405 650 460 65 460 770 550 Blank cells = deceleration length not provided in reference document for the given design speed

17 or high volumes. Therefore, the no-speed-reduction lengths given in Table 6 should be accepted as a desirable goal and should be provided where practical. Vehicle Storage Length The left-turn lane should be sufficiently long to store the num- ber of vehicles likely to accumulate during a critical period; the definition of that critical period can vary depending on the traf- fic conditions at the site. Regardless of the specific critical period, the storage length should be sufficient to avoid the possibility of the left-turning queue spilling over into the through lane. According to the Green Book (5), at unsignalized intersec- tions, the storage length—exclusive of taper—may be based on the number of turning vehicles likely to arrive in an aver- age 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, which the Green Book does not address. For additional infor- mation on storage length, the Green Book refers the reader to the Highway Capacity Manual (3). The equation presented in the TRB Access Management Manual (6) (and reproduced in Table 7) can be used to determine the design length for left- turn storage as described by the Green Book. NCHRP Report 457 (11) developed suggested storage length values using equations identified from Harmelink’s work (12) regarding storage length of left-turn bays at unsignal- ized intersections. The storage length equation is a function of movement capacity, which is dependent upon assumed critical gap and follow-up gap. Critical gap is defined by the Highway Capacity Manual as the minimum time interval in the major street traffic stream that allows intersection entry for one minor-street vehicle. Thus, the driver’s critical gap is the minimum gap that would be acceptable. The time between the departure of one vehicle from the minor street and the departure of the next vehicle using the same major street gap, under a condition of continuous queuing on the minor street, is called the follow-up time. 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 assump- tions made regarding critical gap or follow-up gap and the Table 7. Equations used to determine storage length. Equation in TRB Access Management Manual (1) Where: L = design length for left-turn storage (ft) V = estimated left-turn volume, vehicles per hour (veh/hr) Nc = number of cycles per hour. For the Green Book unsignalized procedure, this would be 30 (V/N is the average number of turning vehicles per cycle). k = factor that is the length of the longest queue (design queue length) divided by average queue length (a value of 2.0 is commonly used for major arterials, and a value of 1.5 to 1.8 might be considered for an approach on a minor street or on a collector where capacity will not be critical). For the Green Book procedure, this would be 1.0. s = average length per vehicle, including the space between vehicles, generally assumed to be 25 ft (adjustments for trucks and buses are available in several documents such as the TRB Access Management Manual) Equations Used in NCHRP Report 457 Equations also used to generate values in Table 8 Where: P(n>N) = probability of bay overflow v = left-turn vehicle volume (veh/hr) N = number of vehicle storage positions c = movement capacity (veh/hr) Vo = major-road volume conflicting with the minor movement, assumed to be equal to one-half of the two-way major-road volume (veh/hr) tc = critical gap (sec) tf = follow-up gap (sec)

18 resulting capacity for the movement used in these procedures can have a significant effect on the calculated storage length recommendations as demonstrated by several researchers (11, 13, 14). It is generally recognized that a storage area should ade- quately store the turn demand a large percentage of the time (e.g., 95 percent or more). A 0.5 percent limit was used for the major road left-turn bay lengths in NCHRP Report 457 based on the recommendation of Harmelink. This smaller limit reflects the greater potential for severe consequences when a bay overflows on an unstopped, major road approach. The critical and follow-up gaps were assumed to equal 4.1 and 2.2 sec, respectively. When the critical gap of 5.0 and 6.25 sec determined in the NCHRP Project 3-91 field studies are used for critical gap (follow-up gap was 2.2 sec), the stor- age lengths shown in Table 8 are generated. A critical gap of 5.0 sec represents the 50th percentile, while the critical gap of 6.25 sec represents the 85th percentile value (which is pre- ferred for design) for the data collected as part of the field studies in this project. Each of the sources on storage length emphasize that the appropriate storage length is dependent on both the vol- ume of turning traffic and the volume of opposing traffic. If volume data are not available, for urban and suburban streets with lower speeds (e.g., less than 40 mph), it is recom- mended that the minimum storage length be at least 50 ft to accommodate two cars; for high speed and rural locations, a minimum storage length of 100 ft is recommended. Some cities use 250-ft storage lanes for left-turn lanes approaching arterial streets, and 150-ft storage lanes for those approach- Left-Turn Volume (veh/hr) Storage Length, Rounded Up to Nearest 25-ft Increment (ft) Storage Lengths from Other Manuals for Comparison Storage Lengths Calculated from Equations b Documented in NCHRP Report 457 Using Revised Critical Gaps and 0.005 Probability of Overflow Green Book Procedure (k=1)a Equation (k=2)a Opposing Volume (veh/hr) 200 400 600 800 1000 Critical Gap = 5.0 sec, Follow-Up Gap = 2.2 sec (Represents the 50th Percentile Critical Gap Found in Field Studies) 40 75 75 50 50 50 50 50 60 50 100 50 50 50 50 50 80 75 150 50 50 50 50 50 100 100 175 50 50 50 50 75 120 100 200 50 50 50 75 75 140 125 250 50 50 50 75 75 160 150 275 50 50 75 75 100 180 150 300 50 50 75 75 100 200 175 350 50 75 75 100 125 220 200 375 50 75 75 100 125 240 200 400 75 75 100 125 150 260 225 450 75 75 100 125 175 280 250 475 75 75 100 125 175 300 250 500 75 100 125 150 200 Critical Gap = 6.25 sec, Follow-Up Gap = 2.2 sec (Represents the 85th Percentile Critical Gap Found in Field Studies, 85th Percentile Preferred for Design) 40 75 75 50 50 50 50 50 60 50 100 50 50 50 50 50 80 75 150 50 50 50 50 75 100 100 175 50 50 50 75 75 120 100 200 50 50 75 75 100 140 125 250 50 50 75 100 125 160 150 275 50 75 75 100 150 180 150 300 50 75 75 125 150 200 175 350 50 75 100 125 200 220 200 375 75 75 100 150 225 240 200 400 75 75 125 150 275 260 225 450 75 100 125 175 325 280 250 475 75 100 125 200 400 0 250 500 75 100 150 225 525 a, b See Table 7 for equations. This table assumes 25 ft per vehicle spacing. Table 4 provides other suggested spacing lengths based on percent trucks. Table 8. Recommended storage lengths for arterials from Access Management Manual equation and NCHRP Report 457 equations with revised critical gap.

19 ing collector streets and most local streets, with a minimum length of 100 ft at local streets and minor driveways. The designer should also consider that if the appropriate design vehicle is a truck or other large vehicle instead of a passenger car, the minimum storage length must be extended accordingly. Sight Distance All locations along a roadway from which vehicles are per- mitted to turn left across opposing traffic, including inter- sections and driveways, should have sufficient sight distance to accommodate the left-turn maneuver. Left-turning driv- ers need sufficient sight distance to decide when it is safe to turn left across the lane(s) used by opposing traffic. Sight distance design should be based on a left turn by a stopped vehicle since a vehicle that turns left without stopping needs less sight distance. Based on that scenario, AASHTO rec- ommends sight distance values for left turns from a major road as shown in Table 9. These distances are for a pas- senger car crossing one lane of opposing traffic; the Green Book provides adjustment factors for other vehicles and lane configurations. If stopping sight distance has been provided continuously along the major road and if sight distance has been provided for each minor-road approach, sight distance will generally be adequate for left turns from the major road. Therefore, no separate check for sight distance for left turns may be needed. However, at three-leg intersections or driveways located on or near a horizontal curve or crest vertical curve on the major road, the availability of adequate sight distance for left turns from the major road should be checked. In addition, the availability of sight distance for left turns from divided high- ways should be checked because of the possibility of sight obstructions in the median. If an intersection is known to have a substantial portion of older drivers, the FHWA Highway Design Handbook for Older Drivers and Pedestrians (15) makes two recommendations for accommodating the slower decision times of older drivers: 1. Where sight-distance requirements incorporate a percep- tion-reaction time (PRT) component, it is recommended that a PRT value of no less than 2.5 sec be used to accom- modate the slower decision times of older drivers. 2. Where a gap model is used to determine sight-distance requirements, it is recommended that a gap of no less than 8.0 sec, plus 0.5 sec for each additional lane crossed by the turning driver, be used to accommodate the slower deci- sion times of older drivers. Median Design Special concern should be given to median width. NCHRP Report 375 (16) has found that most types of undesirable driving behavior in the median areas of divided highway intersections are associated with competition for space by vehicles traveling through the median in the same direction. The potential for such problems is limited where crossroad and U-turn volumes are low, but may increase at higher volumes. In the study, at rural unsignalized intersections, the frequency of undesirable driving behavior and crashes decreased as the median width increased; this implies that medians should be as wide as practical. Also, the frequency of undesirable driving behavior increased as the median open- ing length increased. At intersections where it is necessary to store turning vehi- cles in the median, a width of 12 to 30 ft provides protection for left-turning vehicles. In many cases, the median width at rural unsignalized intersections is a function of the design vehicle selected for turning and crossing maneuvers. Where a median width of 25 ft or more is provided, a passenger car making a turning or crossing maneuver has space to stop in the median area. Medians less than 25 ft wide should be avoided at rural intersections because drivers may be tempted to stop in the median with part of their vehicles unprotected from through traffic. The school bus is often the largest vehi- cle to use the median roadway frequently. The selection of a school bus as the design vehicle results in a median width of 50 ft. Larger design vehicles, including trucks, may be used at intersections where enough turning or crossing trucks are present; median widths of 80 ft or more may be needed to accommodate large tractor-trailer trucks without encroach- ing on the through lanes of a major road. The form of treatment given the end of the narrowed median adjacent to lanes of opposing traffic depends largely on the available width. The narrowed median may be curbed to delineate the lane edge; to separate opposing movements; to Design Speed (mph) Stopping Sight Distance (ft) Intersection Sight Distance Calculated (ft) Design (ft) 15 80 121.3 125 20 115 161.7 165 25 155 202.1 205 30 200 242.6 245 35 250 283.0 285 40 305 323.4 325 45 360 363.8 365 50 425 404.3 405 55 495 444.7 445 60 570 485.1 490 65 645 525.5 530 70 730 566.0 570 75 820 606.4 610 80 910 646.8 650 Table 9. Intersection sight distance—left turn from major road (5).

20 provide space for signs, markers, and luminaire supports; and to protect pedestrians. To serve these purposes satisfactorily, a minimum narrowed median width of no less than 4 ft is recommended and one of 6 to 8 ft wide is preferred. These dimensions can be provided within a median 16 to 18 ft wide using a turning-lane width of 12 ft. Additional information on median design is in the AASHTO Green Book (5) and the Institute of Transportation Engineers (ITE) Urban Street Geometric Design Handbook (17). Channelization and Offset Vehicles in opposing left-turn lanes can limit each other’s views of opposing traffic, and wide medians can affect turn- ing drivers’ ability to accurately judge available gaps in oppos- ing traffic. If large trucks are in one or both directions, there is a substantially higher need to provide offset to allow motor- ists to better see past the stopped truck in the opposite direc- tion. The restriction on the sight distance is dependent on the amount and direction of the offset between the opposing left-turn lanes. For medians wider than about 18 ft, it is desir- able to offset the left-turn lane so that it will reduce 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 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 providing improved visibility of opposing through traffic. The advan- tages 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 (16). 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 10 (7, 17). This left-turn lane configuration is referred to as a parallel offset left-turn lane and is further illustrated in Figure 11A. 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. Figure 11B illustrates a tapered offset left-turn lane of this type. While used primarily at signalized intersections, 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 off- set left-turn lanes are normally constructed with a 4-ft nose between the left-turn lane and the opposing through lanes. This type of offset is especially effective for turning radius allowances where trucks with long rear overhangs, such as logging trucks, are turning from the mainline roadway. Parallel and tapered offset left-turn lanes should be sepa- rated from the adjacent through traffic lanes by painted or raised channelization. A long separation between the left- turn lane and the adjacent through traffic lane results in a “single point” entry (transition) into the left-turn lane. This can result in excessive deceleration in the through traffic lane. An alternative is to provide a parallel left-turn decelera- tion lane adjacent to, and separated from, the through lane marked by striping (see Figure 11B for an illustration) or to use an island (separation) between the left-turn lane and the adjacent through lane in proximity to the intersection to “shift” the turning vehicle to the left and create a positive offset. Bypass Lanes Figure 9 shows a particular type of left-turn lane installation, with direct entry into the turning lane. This alignment allows the through driver to change lanes to avoid the left-turning vehicle (Source: Urban Intersection Design Guide, FHWA/TX-05/04365-P2. Reproduced with permission from author.) Figure 10. Negative and positive offsets of left-turn lanes (7).

21 and continue through the intersection. It is commonly called a bypass lane. This alignment may be used where right-of-way is constrained but a left-turn lane is warranted. The bypass lane alignment has no shadowing for the left-turn lane; thus, the bay taper shown in Figure 7 and Figure 8 is not necessary, which shortens the length of additional right-of-way needed. Though the bay taper length is not needed on a bypass lane, it is still necessary to provide a transition taper to guide through traffic around the left-turn lane. The length of the transition taper for a bypass lane is typically shorter than that used for a shadowed or partially shadowed lane when through traffic is shifted to the right, such as those shown in Table 5. Some states refer to this length as the deceleration transition, because the shorter distance still requires drivers to deceler- ate from the prevailing operating speed. For example, Indi- ana (18) uses 300 ft for a design speed of 50 mph or greater and Minnesota (19) uses 1:15 (180 ft for a 12-ft bypass lane). Depending on the configuration of the intersection, it is also necessary for a bypass lane to have another transition taper as the through traffic returns to its original alignment. This transition may also be referred to as the acceleration transition, and it is commonly the same length as the transi- tion taper on the approach to the intersection (see Figure 12). Agencies may consider the use of bypass lanes at “T” inter- sections in undeveloped areas when left-turn lane warrants are met but the installation of a left-turn lane is not practical. Some states do not allow informal passing on the right or driving on the shoulder; constructing the additional width for through vehicles provides a legal means of passing slowed or stopped left-turning vehicles. Figure 11. Parallel and tapered offset left-turn lanes (5). (Source: A Policy on Geometric Design of Highways and Streets, 2004, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission.) Figure 12. Example of bypass lane with markings. Solid Line Broken Line Dotted Line Transition Taper Transition Taper

22 It is often useful to provide additional guidance to through drivers that they need to change lanes at bypass lanes. This can be accomplished through dotted-line pavement markings that have a much shorter stroke length and shorter spacing than broken lane markings that permit passing. This pave- ment marking is referred to as a dotted line and is illustrated in Figure 12. The pavement marking reinforces the message that through drivers must change lanes but still permits left- turn drivers to travel straight into the turning lane. Pedestrian Storage For urban and suburban locations, as well as rural locations with high pedestrian crossing volumes, adequate storage must be provided for pedestrians in the median adjacent to the left- turn lane. When pedestrian storage is provided, it must also be fully accessible by pedestrians with disabilities. According to the Revised Draft Guidelines for Accessible Public Rights-of- Way (8), medians and pedestrian refuge islands in crosswalks must comply with the dimensions of a pedestrian access route and connect to each crosswalk. A pedestrian access route must have a continuous and unobstructed clear width of at least 4 ft, exclusive of the width of the curb. The cross slope of a pedes- trian access route must be no more than 2 percent. Medians and pedestrian refuge islands must be at least 6.0 ft in length in the direction of pedestrian travel, and they must have detectable warnings at curb ramps and blended transi- tions. Detectable warnings at cut-through islands must be located at the curb line in line with the face of the curb and must be separated by a 2.0-ft minimum length of walkway without detectable warnings. Where the island has no curb, the detectable warning must be located at the edge of the roadway.