Decision-Making Guide for Traffic Signal Phasing (2020)

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39 6 Phase Sequence Like the selection of left- and right-turn phasing mode, the sequence of phases at an intersection can have impacts on safety and traffic operations. Phase sequence decisions are often closely related to the left- turn phasing mode. 6.1 Common Phase Sequences Phase sequence typically applies to pairs of opposing approaches at an intersection. For example, a certain phase sequence applies to the north and south approaches at an intersection. The same, or a different, phase sequence could be selected for the east and west approaches. Where permissive left-turn phasing mode is used on both opposing approaches, phase sequence is not an issue, because both approaches are served during a single phase. However, where protected left turns are used on either approach, either protected-only mode or protected-permissive mode, a choice must be made about the sequence of the phases. Following are commonly used phase sequences, shown in Figures 11-13: • Leading left-turn sequence, where protected left turns are served prior to the opposing through movement. In this sequence, left turns are considered to “lead” and through movements are considered to “lag”. • Lagging left-turn sequence, where protected left turns are served after opposing through traffic. Left turns are considered to lag and through movements lead. • Lead-lag left-turn sequence, where left turns lead in one direction and lag in the opposite direction. Some in the industry designate phase sequence using the terms “protected-permissive” (to indicate leading left turns) and “permissive-protected” (to indicate lagging left turns). This terminology is not used in the MUTCD or this Guide but is mentioned here because readers may encounter it in other contexts.

40 Figure 11 - Typical Phasing Diagram for Leading Left-Turn Sequence (Adapted from NCHRP 2015) Figure 12 - Typical Phasing Diagram for Lagging Left-Turn Sequence (Adapted from NCHRP 2015)

41 Figure 13 - Typical Phasing Diagram for Lead-Lag Left-Turn Sequence (Adapted from NCHRP 2015) 6.2 Phase Sequence Considerations Some agencies use the same phasing sequence at all (or most) intersections. A 2003 report indicated that 83 percent of signals with protected-permissive left-turn phase mode used leading left-turn sequence, while 11 percent used lagging and 6 percent used lead-lag (NCHRP 2003). These percentages may have changed somewhat since 2003, but the leading left-turn sequence certainly remains the most common. Using a common phase sequence at all intersections is likely not the most efficient, because one phase sequence may be superior on certain approaches and another may be preferable elsewhere. Phase sequence should be considered for each pair of opposing approaches to determine if a certain sequence can provide better operational or safety performance. 6.2.1 Yellow Trap A main goal in selecting phase sequence is avoiding a safety concern known as the Yellow Trap. The Yellow Trap, illustrated below in Figure 14, occurs when drivers making a permissive left turn (along with drivers in the adjacent through lanes) get a circular yellow indication while the opposite direction still has a circular green indication. In this situation, left-turning drivers facing a yellow may incorrectly assume that opposing traffic also has a yellow and hence will be slowing or stopping. This condition can result in left- turning drivers making unwise decisions about when to turn and can lead to opposing left-turn crashes. The MUTCD requires Yellow Traps to be eliminated or mitigated.

42 Figure 14 - Representation of Yellow Trap (Adapted from NCHRP 2003) The Yellow Trap can occur when pairs of opposing approaches have any of the following characteristics. In these cases, a countermeasure is needed to prevent the Yellow Trap: • Opposing approaches use protected-permissive left-turn phasing mode and lagging or lead-lag left-turn sequence • One approach uses protected-permissive left-turn phasing mode with a lagging left turn and the opposing approach uses permissive left-turn phasing mode • One approach uses protected-only left-turn phasing mode with a lagging left turn and the opposing approach uses either protected-permissive or permissive left-turn phasing mode Following are a few countermeasures that, where needed, can prevent the Yellow Trap:

43 • At intersections with a left-turn phase on both opposing approaches, the Yellow Trap can be prevented by ensuring that the through movement phases on opposing approaches end at the same time. This means that if a vehicle actuates one left-turn phase, but the opposing phase is not actuated, the signal must still serve both phases simultaneously. If left-turn volumes are frequently imbalanced on the opposing approaches, this countermeasure could introduce inefficiency by providing green time when no vehicles are available to use it. If the opposing left- turn volumes are balanced (or if the resulting inefficiency is inconsequential), this may be an appropriate measure to eliminate a Yellow Trap. • A flashing yellow arrow (FYA) display, since it uses separate signal indications for the left-turn and parallel through movements. Some resources recommend the FYA as the best option to eliminate the yellow trap (FHWA 2013b). However, some agencies are not comfortable using an FYA display as the sole protection against the Yellow Trap because of concerns that left-turning drivers will still take their cue about when to turn from the signal indications for through traffic rather than the separate FYA display. • Converting the phase sequence to leading left turns. Some agencies use leading left turns as a default phase sequence, mainly as a way to avoid the Yellow Trap (or the negative consequences of other treatments to mitigate the Yellow Trap). • Converting both opposing approaches to protected-only left-turn phasing mode. • Converting both opposing approaches to permissive left-turn phasing mode. • Prohibiting left and U-turns on one or both opposing approaches. The MUTCD allows Yellow Traps to exist if left-turning volume is low and the countermeasures above would cause “significant operational or safety problems.” In these cases, the MUTCD requires one of two warning signs: ONCOMING TRAFFIC HAS EXTENDED GREEN (W25-1) or ONCOMING TRAFFIC MAY HAVE EXTENDED GREEN (W25-2) (FHWA 2009b). The choice of sign depends on whether the Yellow Trap occurs routinely or only occasionally. Although the MUTCD allows this condition, it is preferable to eliminate a Yellow Trap rather than rely on drivers to interpret and understand one of these warning signs. A Yellow Trap can also occur at intersections with a leading left-turn sequence, although it is much less common than with a lagging or lead-lag sequence. For example, at an actuated intersection where opposing mainline approaches have protected-permissive left-turn phasing mode, a Yellow Trap can occur when a signal has no call on either side street approach, no call on one of the mainline left-turn approaches, and a call on the opposite mainline left turn. In this case, the signal would typically serve only one of the two mainline left-turn movements, causing a Yellow Trap in the opposite direction. This type of Yellow Trap can be avoided by preventing the signal controller from serving a mainline left-turn phase unless it directly follows a side-street phase. Most signal controllers have a function to prevent Yellow Traps such as this that are not related to phase sequence. 6.2.2 Gaps in Actuation On approaches with protected-permissive left-turn phasing mode, lagging left turns have an advantage that they are only served if left-turn demand still exists after the permissive left-turn movement is served. If left-turn volume and opposing through volume are relatively light, it is possible that left-turn demand could be served solely by permissive lefts, eliminating the need for a left-turn phase that may have been necessary if a leading left-turn sequence had been used.

44 This advantage only exists with protected-permissive left-turn phasing mode, not with protected-only mode. Furthermore, the advantage in efficiency should be weighed to account for operational inefficiencies due to countermeasures needed to prevent the Yellow Trap. 6.2.3 Overlapping Left-Turn Paths Chapter 4 discussed left-turn phasing mode options for intersections where opposing left-turn movements have turn paths that overlap. This condition is also an important factor in selecting phase sequence. To avoid serving conflicting opposing left-turn movements at the same time, a lead-lag sequence is commonly used so one approach is served prior to opposing through traffic and the opposite left turn is served after. However, lead-lag phasing alone cannot guarantee that opposing lefts are never served simultaneously at an actuated signal if some phases are skipped. The controller needs to be programmed to not allow opposing left turns to operate simultaneously. 6.2.4 Limited Left-Turn Bay Length A left-turn phase is most efficient when a queue of waiting vehicles can discharge quickly while a green arrow is displayed. On approaches with short left-turn storage bays, queued vehicles in the through lanes sometimes block the entrance to the storage bay, preventing left-turning vehicles from entering the bay, as shown in Figure 15a. In this case, a leading left-turn phase sequence may suffer from inefficiency because vehicles are not able to reach the bay in time for the arrow. It may be possible to improve the operations under these conditions using a lagging left-turn sequence, which allows the through movement queue to discharge and left-turning vehicles to enter the turn bay prior to serving the left-turn phase. Conversely, it may be possible that a high volume of left-turning vehicles extends out of a short storage bay and block a lane of through vehicles from reaching the intersection as depicted in Figure 15b. In this case, a leading left-turn sequence would help discharge left-turning vehicles and provide more efficient use of the green time for the through movement.

45 Figure 15 - Limited Left-Turn Bay Storage Lengths (Adapted from NCHRP 2015) Where the left-turn storage bay is relatively short and left-turn volume is high, a reserviced left-turn phase may be beneficial. In a reserviced phase, the left-turn movement is served twice each cycle: both before the opposing through movement and again after. When compared to a single long left-turn phase, two shorter phases lose some efficiency because of the need to provide two yellow change and all-red clearance intervals. However, a longer phase may not serve traffic demand efficiently if the phase is longer than needed to completely serve all vehicles queued in the left-turn storage bay. Allowing the bay to completely fill and then empty twice on every cycle can cause an increase in efficiency that offsets the need for the additional change and clearance intervals (Park 2010). 6.2.5 Corridor Progression On corridors with a series of signalized intersections, it is ideal to develop signal timing that maximizes the corridor progression in both directions. It may be possible to improve two-way corridor progression by varying the phase sequence at individual intersections on the corridor. This is best evaluated using a time-space diagram of the corridor. For instance, if the time-space diagram shows the start of a platoon of through traffic arriving near the start of an opposing left-turn phase, it may be better to switch the opposing left turn from leading to lagging operation so the through platoon can be served nearer its arrival time.

46 Lagging left turns are sometimes used at closely spaced intersections, such as those at a diamond interchange. Lagging left turns may be more efficient when used for downstream left turns, but the specific turning movements most appropriate to lag should also be identified using a time-space diagram. 6.2.6 Other Considerations The following additional issues may be relevant to the choice of phase sequence: • Cross-section panel research conducted as part of developing this Guide showed that phase sequence is associated with significant differences in crashes on major street approaches with protected-only left-turn phasing mode. At these sites, lead-lag phase sequence is associated with fewer correctable left-turn crashes than leading left-turn sequence. The implied left-turn CMF for converting to lead-lag sequence is 0.66, indicating a left-turn crash reduction of 34 percent. No significant results were found for differences in phase sequence on the minor road approaches or for sites with protected-permissive left-turn phasing mode. • Operational research conducted as part of developing this Guide showed minimal impacts of phase sequence on average motorist delay. • Phase sequence can help manage queues at closely spaced intersections. Using lead-lag left-turn phase sequence at both intersections can often provide more green time for through traffic at both intersections. • On approaches with protected-permissive left-turn phasing mode, leading left-turn sequence is not compatible with a LPI. If an LPI is used, a different left-turn phasing mode or sequence is needed. • Where pedestrian volumes are high, drivers may have difficulty finding gaps to make permissive turns (both left turns and right turns). In this case, it may be desirable to terminate the pedestrian phase prior to the concurrent vehicular traffic phase to allow permissive turns without pedestrian conflicts. This technique is compatible with an LPI. • Phase sequence need not be constant at an intersection. For example, conditions such as left-turn bay blocking are often limited to peak periods. It can be operationally more efficient to use a different phase sequence at different times of day where conditions vary. • At very wide intersections, such as those at Single-Point Urban Interchanges (SPUIs), lagging left turns on the cross-street can provide more clearance time for bicyclists traveling along the cross- street.