- Journal of Lightwave Technology
- Elements of detection and signal design / by Charles L. Weber. - Version details - Trove
The objective of the flow chart is to identify the least restrictive left-turn operational mode. A secondary objective is to provide a structured procedure for the evaluation of left-turn phasing for the purpose of promoting consistency in left-turn phase application. The critical left-turn crash counts identified in the figure are based on an underlying average critical crash frequency and recognize the inherent variability of crash data.
Journal of Lightwave Technology
The underlying averages are 1. If the reported crash count for existing permissive operation exceeds the critical value, then it is likely that the subject intersection has an average left-turn crash frequency that exceeds the aforementioned average 5 percent chance of error and a more restrictive operational mode would likely improve the safety of the left-turn maneuver. The flowchart has two alternative paths following the check of opposing traffic speed. One path requires knowledge of left-turn delay; the other requires knowledge of the left-turn and opposing through volumes.
The left-turn delay referred to in the flowchart is the delay incurred when no left-turn phase is provided i. It may be advantageous under certain circumstances to change the sequence in which left turns are served relative to their complementary through movements. This is done by reversing the sequence of a pair of complementary phases, as is shown for phases 1 and 2 in Figure Specifically, Figure shows phases 2 and 6 starting and ending at different times in the cycle.
This independence between the through phases can be desirable under coordinated operations because it can accommodate platoons of traffic arriving from each direction at different times. The most commonly used left-turn phase sequence is the "lead-lead" sequence which has both opposing left-turn phases starting at the same time. If a single ring structure is used, then the two phases also end at the same time. If an actuated dual ring structure is used, then each left-turn phase 14 is assigned to a different ring such that each can end when the left-turn demand is served i.
The advantages of this phasing option are: 1 that drivers react quickly to the leading green arrow indication and 2 it minimizes conflicts between left-turn and through movements on the same approach, especially when the left-turn volume exceeds its available storage length or no left-turn lane is provided. A more detailed discussion of the advantages of leading left-turn phases is provided in Chapter 13 of the Traffic Engineering Handbook This left-turn phase sequence is most commonly used in coordinated systems with closely spaced signals, such as diamond interchanges.
It has both opposing left-turn phases ending at the same time. If it is implemented in a single ring structure, then the two phases also start at the same time. If a dual-ring structure is used, then each left-turn phase is assigned to a different ring such that each can start when the left-turn demand is served i. Lagging left-turn phasing is also recognized to offer operational benefits for the following special situations:. When used with protected phasing, this phase sequence provides a similar operational efficiency as a lead-lead or lead-lag phase sequences.
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However, differences emerge when they are used with protected-permissive mode. One disadvantage of lagging left-turn phases is that drivers tend not to react as quickly to the green arrow indication.
Another disadvantage is that, if a left-turn bay does not exist or is relatively short, then queued left-turn vehicles may block the inside through lane during the initial through movement phase. When lag-lag phasing is used at a four-leg intersection where both phases are used with the protected-permissive mode, then both left-turn phases must start at the same time to avoid the "yellow trap" or left-turn trap problem, illustrated in Figure This problem stems from the potential conflict between left-turning vehicles and oncoming vehicles at the end of the adjacent through phase.
Of the two through movement phases serving the subject street, the trap is associated with the first through movement phase to terminate and occurs during this phase's change period. The left-turn driver seeking a gap in oncoming traffic during the through phase, first sees the yellow ball indication; then incorrectly assumes that the oncoming traffic also sees a yellow indication; and then turns across the oncoming traffic stream without regard to the availability of a safe gap.
In fact, under at least one condition, the second technique can operate more efficiently than dual-ring lead-lead phasing. This condition occurs when the left-turn volume is moderate to heavy and relatively equal on both approaches. Regardless, a detailed operational evaluation should always be used to confirm that lag-lag phasing operates more efficiently than other phasing options. The third technique avoids the yellow trap by using an overlap in the controller and a five-section left-turn signal head.
An overlap is a controller output to the signal head load switch that is associated with two or more phases. In this application, the left-turn green and yellow arrow indications are associated with the subject left-turn phase; and the left-turn green, yellow, and red ball indications are associated with the opposing through movement phase as opposed to those of the adjacent through phase. The flashing yellow arrow is contained within a three-, four-, or five-section head and provides a permissive indication to the driver that operates concurrent with the opposing through movement rather than the adjacent through movement.
This study was conducted over a 7-year period and comprised a very comprehensive research process, including engineering analyses, static and video-based driver comprehension studies, field implementation, video conflict studies, and crash analyses. This study 11 recommended that a flashing yellow arrow be allowed as an alternative to the circular green for permissive left-turn intervals.
The louvered signal head is referred to as the "Dallas Display. This left-turn phase sequence is generally used to accommodate through movement progression in a coordinated signal system. The aforementioned "yellow trap" may occur if the leading left-turn movement operates in the protected-permissive mode and the two through movement phases time concurrently during a portion of the cycle. The "yellow trap" problem can be alleviated by using one of the following techniques:.
The first two techniques will likely have an adverse effect on operations, relative to a dual ring implementation of lead-lag phasing with protected-permissive operation. However, they avoid the potential adverse effect a yellow trap would have on safety. However, in practice, the Dallas Display is used for both the leading and the lagging left-turn signal heads because it improves operational performance Lead-lag phasing is also recognized to offer operational benefits for the following special situations:.
Pedestrian movements are typically served concurrently with the adjacent through movement phase at an intersection. This is done to simplify the operation of the intersection primarily and is largely a legacy issue in our application of signal logic and control.
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Typical application of pedestrian operation puts pedestrians in conflict with right-turning vehicles and left-turning vehicles that operate in a permissive mode, by inviting their movement at the same time. There are specific measures that can be used to mitigate this potential conflict, three common options include:. Two types of right-turn phasing are addressed in this section.
The first type is based on the addition of a phase to the signal cycle that exclusively serves one or more right-turn movements. This type of right-turn phasing is rarely used. If it is being considered, then its operational or safety benefits should be evaluated and shown to outweigh its adverse impact on the efficiency of the other intersection movements. The second type of right-turn phasing is based on the assignment of the right-turn movement to the phase serving the complementary left-turn movement on the crossroad.
The following conditions should be satisfied before using this type of right-turn phasing:. If the aforementioned conditions are satisfied, then the appropriate operational mode can be determined. If the through movement phase for the subject intersection approach serves a pedestrian movement, then the right-turn phasing should operate in the protected-permissive mode.
As shown in Figure , the permissive right-turn operation would occur during the adjacent through movement phase, and the protected right-turn operation would occur during the complementary left-turn phase. If the through movement phase for the subject intersection approach does not serve a pedestrian movement, then the right-turn phasing should operate in the protected only mode during both the adjacent through movement phase and the complementary left-turn phase.
A controller overlap may be used to provide this sequence.
Elements of detection and signal design / by Charles L. Weber. - Version details - Trove
Detectors place calls into the traffic signal controller. The controller uses this information and the signal timing to determine the display provided to the users. Detection for pedestrians is limited in most cases to push buttons as shown in Figure , although accessible 20 pedestrian signal detectors are increasing in their use. There are various forms of vehicle detection technologies, and strengths and weaknesses of each are described in the Traffic Detector Handbook, 3rd edition The detection design for an intersection describes the size, number, location, and functionality of each detector.
Most engineering drawings include the wiring diagram for how detectors are associated to phases. Signal timing settings such as the passage time, delay, extend, and other related parameters are described in more detail in Chapter 5. The size and location of detectors is an important element in traffic signal design. Detectors can consist of one 6-foot-byfoot inductive loop detector, a series of closely spaced 6-foot-byfoot loop detectors may be circular in shape as shown in Figure , one long 6-foot-byfoot loop detector, or alternative detection technology e.
This detection zone can be used to meet the objectives described below. A call can be triggered by an actuation from any detection, vehicular, pedestrian, or other or through a controller function. These parameters are described in Chapter 5. The objective of detection is to detect vehicle presence and identify gaps in vehicle presence that are sufficiently long to warrant terminating the phase. There are many objectives of detection design that can be characterized with the following statements:.
The first and fourth objectives are safety related.