Decision-Making Guide for Traffic Signal Phasing (2020)

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17 4 Left-Turn Phasing Mode Left-turn phasing mode is a key component of the design and operation of a traffic signal, with impacts on safety and delay for all users of an intersection. This chapter of the Guide is intended to address accommodations for left turns from two-way streets as well as U-turns to the left. For left turns from one-way streets, the right-turn phasing chapter of the Guide (Chapter 5) may provide more relevant guidance. 4.1 Left-Turn Phasing Mode Left turns can operate using one of the following phasing modes, as shown in Figures 7 through 10: • Permissive-Only, where left turns are usually served by a circular green or flashing yellow left arrow signal indication. Permissive left turns may only be made after yielding to opposing traffic and conflicting pedestrians. During a permissive left-turn movement, a pedestrian phase can be served across the left-turn movement’s departing lanes. Left-turning drivers are required to yield to pedestrians in the parallel crosswalk when making a permissive left turn. A flashing red left arrow may also be used to indicate permissive left turns. Such a display communicates to motorists that they must stop prior to making a permissive left turn, unlike a circular green or flashing yellow arrow, which require motorists to yield. • Protected-Only, where left turns can be made only on a green left arrow indication. During a protected left-turn movement, no conflicting traffic or pedestrian phases may be served, and no phases may be served that use the same departing lanes (unless sufficiently channelized). • Protected-Permissive, where both modes may be used on the same approach during the same signal cycle. Split Phasing is a special case of protected-only phasing mode in which the green indications for all movements on a single approach always start and end at the same time. On approaches where split phasing is used, a yellow left arrow is not required as it is in all other protected-only or protected- permissive configurations. The circular yellow indication serves all movements on the approach. The left-turn mode may change by time of day or in response to changing traffic conditions.

18 Figure 7 - Typical Phasing Diagram for Permissive-Only Phasing (Adapted from NCHRP 2015) Figure 8 - Typical Phasing Diagram for Protected-Only Phasing (Adapted from NCHRP 2015)

19 Figure 9 - Typical Phasing Diagram for Protected-Permissive Phasing (Adapted from NCHRP 2015) Figure 10 - Typical Phasing Diagram for Split Phasing on the Minor Street (Adapted from NCHRP 2015)

20 4.1.1 Signal Faces Left-turn movements are controlled by one of two types of signal face: • Shared signal face, where the same signal face controls the left-turn movement and another movement (usually the through movement) on the same approach. • Separate signal face, where a signal face controls only the left-turn movement. Table 1 illustrates how the two types of signal displays are typically used to communicate each of the three possible left-turn phasing modes. While the displays in the table are typical, other configurations are possible. Table 1 - Left-Turn Signal Faces for Each Left-Turn Phasing Mode Left-turn phasing mode Example shared signal face Example separate signal face Permissive-Only *Flashing Protected-Only A shared face may only be used where the circular green and green left arrow always start and end together. Protected-Permissive *Flashing

21 4.2 Left-Turn Phasing Mode Considerations Typically, protected-only phasing mode is operationally less efficient than permissive-only and protected- permissive modes in most cases because motorists cannot use available gaps to turn left. Rather, green time must be allocated to the left-turn movement, which reduces the time available for other movements, increasing delay for all movements. Protected-only phasing mode often requires a longer cycle length, compounding the inefficiency. Because of this inherent inefficiency, protected-only mode should be used only where permissive left turns are determined to be inappropriate. A two-step process is recommended to select a left-turn phasing mode: 1. Determine if permissive left turns can be allowed for safety reasons. 2. If so, determine if protected left turns are needed for operational reasons. The steps should be repeated for every left-turn movement at a signalized intersection. This Guide considers each of these two steps separately. Since the second step may not be necessary depending on the outcome of the first step, it is important to consider the steps in this order. 4.2.1 Safety Impacts of Permissive Left Turns A designer should consider the following features of an intersection approach to determine whether permissive left turns are appropriate for safety reasons: • Sight Distance • Lane Configuration • Crash History If any of these factors indicate that permissive left turns are not appropriate, protected-only phasing mode is often recommended, and further evaluation of that approach may not be necessary. Sight Distance Si gh t D ist an ce To allow permissive left turns, left-turning drivers must have a clear view of opposing traffic, including bicyclists. Insufficient line of sight between left- turning drivers and opposing vehicles, as discussed below Protected-Only recommended To allow permissive left turns, left-turning drivers must have a clear view of pedestrians in the conflicting parallel crosswalk. Blocked line of sight between drivers and pedestrians Protected-Only recommended Intersection sight distance should be determined according to A Policy on Geometric Design of Highways and Streets, also known as the Green Book (AASHTO 2018). The Green Book assumes that drivers of passenger cars crossing one opposing lane need a gap in opposing traffic 5.5 seconds long to make a permissive left turn. As such, drivers must be able to see oncoming traffic at least 5.5 seconds away. For

22 each additional opposing lane, passenger car drivers need an additional 0.5 second time gap.1 Sight distance can be calculated using the following formula: 𝐼𝑆𝐷 = 1.47 ∗ 𝑆 ∗ 𝑡 where: ISD = Minimum required intersection sight distance (feet) S85 = Speed (mph) (More information about the appropriate speed to use in this formula follows) tgap = Time gap (seconds) Table 2 shows minimum required left-turn sight distance from the Green Book based on a 5.5-second time gap for crossing one opposing through lane. The Green Book provides more information to determine minimum required sight distance for situations such as multiple opposing lanes or a design vehicle other than a passenger car. Table 2 - Minimum Required Sight Distance for One Opposing Through Lane 85th percentile speed (mph) Minimum required sight distance to opposing through vehicles (ft) 25 205 30 245 35 285 40 325 45 365 50 405 55 445 60 490 The following factors should be considered: • Speed. The Green Book methodology uses a roadway’s design speed as an input value. However, design speed is often unknown, and even where known, it may not be a good indication of actual speeds. The 85th percentile speed, where known, should be used in the Green Book formula. The 85th percentile speed is already used as an input value at signalized intersections, as it is the speed recommended for timing yellow change and red clearance intervals (NCHRP 2012). Where the 85th percentile speed is unknown, it is recommended to use the speed limit plus 7 mph, since NCHRP 731 determined that the 85th percentile speed averages about 7 mph higher than the speed limit. Engineering judgment may be necessary to determine the appropriate speed for roadways that have not yet been constructed or do not have posted speed limits, such as private streets. • Measurement. Sight distance should be measured from the perspective of the eye of the driver waiting to make a left turn in a vehicle stopped behind the stop line2 . Another measurement

23 point may also be desirable if drivers tend to wait at a different point, but the left-turn signal face should be visible from any point where a measurement is made. For adequate sight distance to exist, an opposing vehicle must be continuously visible in any approach lane for the distance specified in the Green Book. For intersections not yet constructed, it may be necessary to estimate sight distance using design plans. • Opposing Left Turns. Vehicles waiting to make a left turn in opposite directions sometimes obstruct each other’s sight distance. Opposing left-turning vehicles should be considered where they are a persistent visibility obstruction on an approach. Because protected-only phasing mode is almost always less efficient than allowing permissive left turns, agencies should evaluate intersections lacking sufficient sight distance to determine if the line of sight could be improved by measures such as the following: • Trimming or removing vegetation • Providing a positive geometric offset or improving the geometric offset of opposing left-turn lanes Lane Configuration La ne C on fig ur at io n On an approach where left turns are permitted from more than one lane, most agencies use protected-only phasing mode.3 Some agencies allow permissive left turns from dual left-turn lanes, but these applications are uncommon. Protected-Only recommended On an approach where left turns and through traffic share a lane, it is undesirable to give green indications to the two movements at different times of the signal cycle. However, permissive left turns may be acceptable where one of the two movements has very low volume or conflicting opposing traffic is very low. The MUTCD prohibits protected-only mode on approaches without an exclusive left-turn lane. or Protected-Only not permitted4 Split Phasing recommended Where the paths of opposing left-turning vehicles overlap, these left turns are required not to be served at the same time. If both directions of through traffic are served concurrently, then permissive-only phasing must not be used on at least one of the conflicting left-turn paths. Overlapping opposing left-turn paths Protected-Only required for at least one of the two overlapping paths

24 La ne C on fig ur at io n (C on tin ue d) The roadway used by vehicles departing the left-turn movement may impact the choice of left-turn phasing mode. It may be desirable to eliminate permissive-only phasing where the departing roadway has a characteristic that limits a vehicle’s ability to travel at an otherwise expected speed. Examples of such characteristics include a sharp driveway apron or valley gutter, narrow lanes, poor surface condition, or driveway entrance immediately downstream of the intersection. Departing lanes limit expected turning speed Protected-Only recommended Where a left-turn movement must cross four or more opposing lanes, protected-only phasing is used by many agencies. 4 or more opposing lanes, as discussed below Protected-Only recommended The following factors should be considered when counting the number of opposing lanes crossed by left- turning traffic: • Through Lanes. Any lane in which through traffic is permitted should be counted, even if turns are also permitted from that lane. • Left-Turn Lanes. Opposing exclusive left-turn lanes should usually not be counted, because typically opposing left turns do not conflict with each other. • Right-Turn Lanes. It is acceptable to exclude opposing right-turn lanes. Omitting right-turn lanes is particularly appropriate where the right-turn movement is physically channelized from opposing through lanes and not under signal control. It may be desirable to include right-turn lanes in the count of opposing lanes where right-turn volume is heavy or where conflicts with left turns are unusually high; however, the accompanying research did not find a significant difference in left-turn driver gap acceptance when an opposing right-turn lane was present. Crash History Cr as h Hi st or y Protected-only phasing mode should be considered on approaches with correctable left-turn crashes that exceed the critical count in the table below. Correctable left-turn crashes include those where a left-turning driver failed to yield to opposing traffic or pedestrians or bicyclists in a parallel crosswalk. Crashes exceed critical count, as discussed below Protected-Only recommended Table 3 indicates the number of correctable left-turn crashes above which it may be desirable to prohibit permissive left turns (FHWA 2009d; NCHRP 2015). Some agencies have different crash reporting thresholds than others. For instance, some agencies do not report crashes involving only property damage, so in these cases the critical count needs to be adjusted accordingly. This table shows the crash

25 count from published industry state of the practice. Chapter 11 provides more information about combining the impacts of safety and operations at individual intersections. Table 3 - Critical Correctable Left-Turn Crash Counts (Adapted from FHWA 2009d; NCHRP 2015) Number of left-turn movements on subject road Period during which crashes are considered (years) Critical correctable left-turn crash count One 1 6 2 11 3 14 Both 1 11 2 18 3 26 The cross-sectional research conducted as part of this Guide’s development determined that protected- permissive phasing mode was not associated with significantly fewer left-turn crashes than permissive- only mode. However, significantly fewer left-turn crashes were observed at intersections using protected- only mode. This study’s research shows that adding protected-only phasing mode to opposing major road approaches is associated with a left-turn CMF of 0.507, indicating a crash reduction of 49.3 percent, and adding protected-only to the minor road approaches has a CMF of 0.702, a crash reduction of 29.8 percent. More details about this safety research can be found in Chapter 9. A safety review to determine left-turn phasing mode usually focuses on correctable left-turn crashes. However, at congested intersections, particularly where left-turn bays are short, a different safety problem may emerge if a queue of left-turning vehicles extends out of the left-turn bay and blocks a through travel lane. In this case, the presence of stopped vehicles in a through lane may result in rear-end crashes. If so, protected-only left-turn phasing mode may aggravate the problem because of the less efficient operation. In these circumstances, the frequency and severity of rear-end or sideswipe crashes under protected-only conditions should be weighed against the frequency and severity of correctable left- turn crashes under permissive left-turn conditions. Chapters 9 - 11 provide additional information on comparing safety and operational impacts of phasing alternatives. For intersections without a crash history, such as those under design or recently built, it may be possible to use a SPF to evaluate the likelihood of crashes. Chapter 9 provides more information on using SPFs to estimate expected safety performance. 4.2.2 Operational Impacts of Permissive Left Turns If any of the previous factors recommend against the use of permissive left turns, it is usually not necessary to evaluate the operational impacts. However, where it has been determined that permissive left turns can operate safely, the following factors should be evaluated to determine if permissive-only phasing mode is sufficient or if protected-permissive phasing mode should be used for operational reasons: • Left-Turn Volume • Left-Turn Delay

26 Left-Turn Volume Le ft- Tu rn V ol um e Permissive-only phasing mode is efficient when the number of sufficient gaps available to make a left turn exceeds the number of vehicles making the turn. In this context, a gap is usually measured in units of seconds. The acceptable gap duration may vary by intersection, but the Green Book assumes a gap of 5.5 seconds for passenger cars crossing a single lane plus an additional 0.5 second for each additional lane crossed (AASHTO 2018). It is possible to measure the number of gaps, but the approach is labor intensive. Other measures (below) may be used where detailed gap information is not available. Left-turn volume per hour > gaps per hour Protected- Permissive recommended Left-turn volume per hour <= gaps per hour Permissive-Only recommended Protected-Permissive phasing mode should be considered on approaches where the average number of left-turning vehicles per cycle (VC) is more than two during the peak hour. For intersections with a fixed cycle length, VC can be determined as follows: 𝑉 = 𝑉 ∗ 𝐶3600 where: VLT = number of vehicles turning left during the intersection’s peak hour C = intersection cycle length, seconds In many cases, two vehicles can turn left during the yellow change interval on every cycle even if no other gaps are available to make permissive left turns. Left-turn volume per cycle > 2 vehicles Protected- Permissive recommended Left-turn volume per cycle <= 2 vehicles Permissive-Only recommended Protected-Permissive phasing mode should be considered on approaches where the left-turn cross product (LCP) exceeds 50,000 (FHWA 2009d; NCHRP 2015) per opposing lane during any hour of a typical day. LCP can be determined as follows: LCP > 50,000 Protected- Permissive recommended

27 Le ft- Tu rn V ol um e (C on tin ue d) 𝐿 = 𝑉 ∗ 𝑉𝑁 where: VLT = number of vehicles turning left during the intersection’s peak hour VOT = number of opposing through vehicles during the same hour NL = number of opposing through lanes It may be desirable to include opposing right-turn volume in VOT and opposing right-turn lanes in NL where right-turn volume is heavy or where conflicts with left turns are unusually high, such as where dual right-turn lanes exist. LCP <= 50,000 Permissive-Only recommended

28 Left-Turn Delay Le ft- Tu rn D el ay Permissive-only phasing mode may result in excessive delay for left-turning vehicles even where the volume criteria above indicate that permissive- only mode provides acceptable operations. This is particularly true when left-turning vehicles arrive nonuniformly during the peak hour. Where both of the following delay measures are met, protected-permissive phasing mode should be considered: • Average peak-hour left-turn delay exceeds 35 seconds per vehicle (FHWA 2009d; NCHRP 2015). • Peak-hour total left-turn delay exceeds 2 vehicle-hours (FHWA 2009d; NCHRP 2015). Left-turn delay can be determined using the methodology in the Highway Capacity Manual (HCM) or software that implements the HCM methodology, such as Synchro. Peak-hour total left- turn delay (DTOT) can be computed (in vehicle-hours) as follows: 𝐷 = 𝑉 ∗ 𝐷3600 where: VLT = number of vehicles turning left during the intersection’s peak hour DVEH = average delay per left-turning vehicle during the same hour (seconds) Left-turn delay > 35 seconds per vehicle AND peak-hour left-turn delay > 2 veh-hrs Protected- Permissive recommended Left-turn delay <= 35 seconds per vehicle OR peak-hour left-turn delay <= 2 veh-hrs Permissive-Only recommended

29 4.2.3 Other Considerations At intersections with a high volume of pedestrians, conflicts between permissive left turns and crossing pedestrians may make protected-only left-turn phasing mode desirable. Some intersection configurations may require protected-only phasing mode for reasons not already indicated here. Examples of these conditions include the following: • Streets with center-running or left-side exclusive transitways. At intersections on these streets, transit vehicles pass to the left of left-turning vehicles. Left-turning drivers cannot be expected to see and yield to transit vehicles approaching from behind, so in these cases protected-only left- turn phasing mode is normally used. • Streets with median or left-side two-way separated bike lanes. Where a left turn conflicts with a two-way separated bike lane, left-turning motorists may not expect through cyclists coming from behind, so protected-only left-turn phasing mode, or another method of serving bicycle traffic and conflicting left turns at different times, can be appropriate. Some agencies require the same left-turn phasing mode to be used on opposing approaches. In practice, this often results in use of protected-only phasing mode where permissive turns would otherwise be allowed. No known research indicates any safety or operational benefit to matching the left-turn phasing mode on opposing approaches. The impacts of an intersection’s left-turn phasing mode should be reviewed for possible adverse effects at adjacent intersections. One phasing mode may be best for a signal in isolation, but if it causes queues to spill back to surrounding intersections, its left-turn phasing mode may need to be reviewed to mitigate negative network impacts. For example, the closely spaced intersections at a diamond interchange may require careful review to ensure that their phasing modes contribute to effective overall operation of the interchange as a whole. Phasing should not contribute to excessive queues formed at one intersection that extend into a nearby intersection. At unsignalized intersections, all uncontrolled left turns are permissive. If permissive left turns are operating safely and effectively at unsignalized intersections along a corridor, drivers may not understand the rationale if permissive left turns are disallowed at one or more signalized intersections with similar characteristics on the same corridor. However, unsignalized intersections may operate differently because, for instance, nearby signals may create gaps that facilitate left turns from the major street. When permissive left turns are not appropriate because of a temporary condition, it may be desirable to consider using a variable left-turn phasing mode. A variable mode can change by time of day, week, or year, such as when opposing left-turning vehicles frequently obstruct sight distance during peak hours, but not during off-peak hours. A variable mode can also change dynamically in response to intersection conditions, such as using protected-only phasing mode only when a parallel pedestrian crossing has been actuated. Following are two examples where variable phasing mode may be appropriate:

30 • An approach where a significant portion of crashes occurs at a certain time of day. During high- crash times, protected-only phasing mode may be appropriate even though protected-permissive mode is used at other times. • An approach where a conflict between crossing pedestrians and permissive left-turning vehicles leads to crashes or near-misses. An actuated pedestrian phase can be used to call for protected- only phasing mode only on cycles with pedestrian actuations. This can be particularly helpful at crossings with vision-impaired or elderly pedestrians. A variable left-turn phasing mode can be implemented using a four-section5(FHWA 2014) flashing yellow arrow display. It should be noted that this Guide does not recommend a threshold based on travel speed or speed limit for recommending protected-only phasing mode. This study’s survey of practitioners found that many agencies do include speed as a decision-making criterion. However, research conducted as part of development of this Guide did not identify a speed threshold that would preclude permissive left turns. Chapter 11 of this Guide provides a methodology to consider the balance of safety and traffic operations in selecting a left-turn phasing mode. This methodology may provide an additional factor to aid in decision-making. 4.2.4 Alternatives to Left-Turn Phasing Allocating green time to serve left-turn phases takes time away from through movements that may carry more traffic. At some intersections, it is possible that delay experienced by through movements is too high to support conventional left-turn phases. Several alternatives are available to avoid the use of left- turn phases, including the following: • Prohibiting left turns may be feasible where left-turn volume is low. It may be desirable to identify an alternative route for impacted left-turn movements that passes through less congested parts of the street network, but an alternative route may not be necessary where drivers have several other options to avoid the impacted left turn. Alternatively, a left-turn prohibition may be established only during certain hours of the day where congestion is limited in duration. • Median U-turn (MUT) configurations reroute major street left turns from a main intersection to nearby U-turn movements. Median U-turns can improve traffic operations because the main intersection operates with just two signal phases, minimizing delay. When properly coordinated, the U-turn movements do not degrade major street progression because they can be timed to be served at the same time as the side street, when major street traffic is already stopped. • Restricted Crossing U-turn (RCUT) configurations reroute minor street left turns and through movements from a main intersection to nearby U-turn movements. RCUT intersections are also known as Superstreet intersections. They offer benefits similar to MUT configurations. • Displaced left turn configurations route left-turning traffic across opposing travel lanes upstream of an intersection. This configuration can also reduce the number of signal phases at an intersection because the crossover movement can occur at the same time as the side street is served and left turns and through traffic can be served together at the main intersection. This

31 configuration is more intuitive to drivers than an MUT because left turns are served from their typical position in the left lane. • Jughandle configurations typically route left-turning vehicles onto the side street, where they join the stream of side street through traffic. A jughandle helps eliminate signal phases and can improve safety by avoiding left-turn crash conflicts. More information about these and other alternative intersection treatments, including potential disadvantages, can be found FHWA’s Alternative Intersections/Interchanges Informational Report (FHWA 2009a). 1 The Green Book suggests that wide medians “should be converted to an equivalent number of lanes; for example, an 18-ft median would be equivalent to one and a half lanes.” 2 On an approach without a stop line, the measurement can be made from the near-side crosswalk line or drivers’ normal stopping point, which may need to be determined from a field assessment of driver behavior. 3 The research conducted as part of this Guide’s development was not able to model impacts on crashes where varied left-turn phasing modes are used on approaches with multiple left-turn lanes. 4 On an approach without an exclusive left-turn lane, Protected-Only left-turn phasing mode is only acceptable when the left-turn movement begins and ends at the same time as the adjacent through movement (i.e., split phasing), per MUTCD Sec. 4D.17, para. 06. 5 Interim Approval IA-17 from the Federal Highway Administration (2014) allows optional use of a three-section flashing yellow arrow signal face where the flashing and solid yellow arrows are combined in a single section. Agencies using this option must comply with the terms of IA-17.