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The previous two chapters were devoted primarily to signal equipment and traffic control devices related to safety, although there are certainly operational aspects of traffic control devices. Chapter 5 is devoted to traffic operational practices that promote, or have the potential to promote, improvements in intersection safety. This chapter discusses operational practices that are not directly related to specific hardware. Rather, they are items that largely fall under the domain of traffic operations engineers in state and local governments.
A signal display known as the Dallas Left-Turn Display for Left-Turn Lead-Lag Signal Phasing, or “Dallas Phasing” has been used in the area for well over 15 years, and is not considered innovative. However, many others around the United States involved in signal operations do not know of it. For this reason, the topic of Dallas Phasing is included in this report. Essentially, the objective of the left-turn display is to more safely accommodate left turning drivers at intersections using protected-permitted lead-lag left-turn sequencing. Consider a left turn phasing scheme in which the northbound drivers get a leading green left arrow and green ball indications for the through traffic. Then, the protected portion is terminated and left-turning drivers would normally receive a green ball indication in which they are permitted to turn left in the absence of opposing vehicles. Left turning drivers may think that when this phase ends for their approach, it is also ending for the opposing through movement, when in reality the opposing through traffic receives a continuous green ball indication because of the lag left on that approach.
The Dallas Display was conceived to avoid the risk that left-turning drivers would see the signal indication for through traffic on their own approach and make an incorrect assumption about the signal indication being presented to through traffic on the opposing approach. By displaying a separate signal indication to left turn traffic that is seen only by them and not drivers in adjacent through lanes, a green ball indication can be shown to drivers in the left lane while a yellow and then red indication can be displayed to adjacent through traffic, as shown in figure 58. Left turning drivers are still permitted to turn left in the absence of opposing vehicles, and they will not expect that the opposing through traffic is receiving yellow and then all red when the adjacent through traffic is receiving those indications. The left-turn display is shielded from the view of the adjacent through traffic lanes by placing louvers in the red ball, yellow ball and green ball signal hoods. In the left-turn signal head, the red, yellow and green ball indications are activated using overlaps. A Kittle & Associates Web page provides an animation of the Dallas Phasing signal indications and sequence at http://projects.kittelson.com/pplt/displays/dallas_horiz_lead.htm. Intersections requiring lead-lag operation in all four directions require a 16 bay load-switch cabinet to accommodate eight vehicle phases, four pedestrian phases, and four Dallas left-turn display overlaps.
Figure 58. View from left-turn pocket where a Dallas (city in the background) phasing is in operation.
Although not yet approved for inclusion in the MUTCD, flashing yellow arrow displays are currently being evaluated as experimental devices. Intuitively, the flashing yellow left arrow may offer potential safety benefits to agencies that now utilize flashing red balls, such as the Michigan DOT or the Dallas Phasing, on one or more approaches. Consequently, this is included in the scan report.
Preliminary studies have indicated that public reaction to flashing yellow arrows is positive. In addition, studies suggest that flashing yellow arrow signals offer greater degrees of safety compared to other signal displays used for permitted-protected phasing.
During the scan, the team visited an intersection in Livingston County, Michigan, where the flashing yellow left arrow had been approved for experimental use and was in operation. In Michigan, the standard practice is to provide permitted-protected left-turn phasing (i.e., where the left green arrow lags) and not protected-permitted left-turn phasing (i.e., where the left turn green arrow leads). In other words, the protected left-turn green arrow follows a signal interval in which the signal indication displayed to drivers in the left turn lane is a flashing red ball. Drivers can turn left during the presence of a flashing red ball in the absence of oncoming opposing vehicles or when there is a sufficient gap to safely do so.
Figure 59a displays a view of the signal displays when the yellow arrow, which is in the third section down from the top of the signal head, is flashing. During this first photo, a set of green balls and left green arrow is being displayed to the opposing oncoming traffic on the opposite approach (which has a leading protected-only left turn phase). After the opposing left turn phase has maxed or gapped out, then the display shown in Figure 59b is presented. The flashing left arrow is on during the same time that the adjacent signal heads display solid green ball indications. During this interval, solid green balls are being displayed to the opposing through approach and a solid red left arrow is being displayed to the opposing left turning traffic. Figure 59c presents the next sequence when the solid left green arrow is displayed concurrently while the solid green balls are displayed on the adjacent heads that serve the through lanes and right turn maneuver. During this interval, steady red balls and a steady red left arrow indication are displayed to the opposing approach. After the phase has gapped or maxed out and the signal indications have gone through their clearance intervals, then the next displays are presented in Figure 59d, in which a steady left red arrow is displayed to the left turning traffic and steady red ball signal indications are displayed to the through traffic.
Figure 59a. First set of signal indications when flashing yellow left arrow is displayed.
Figure 59b. Second set of signal indications when flashing yellow left arrow is displayed.
Figure 59c. Third set of signal indications when steady left green arrow is displayed.
Figure 59d. Fourth set of signal indications when steady left red arrow is displayed.
Livingston County has received approval to operate four additional intersections with flashing yellow arrows, and they intend to install flashing yellow arrows in the future on all signals in which they wish to show a separate signal head for “permitted” use.
For nearly 20 years, Portland, Oregon, has used vehicle detection to control when the yellow is displayed. The detector furthest from the stop bar is set at the safe stopping distance for the approach. One or two intermediate loops are installed on the approach and the gap time is set so that the yellow is displayed just as the driver arrives at the stop bar. When the detection is used, the city has documented a two-thirds reduction in rear-end crashes in which the driver disregarded the signal indication. The city has also documented a two-thirds reduction in the frequency of drivers entering an intersection during a red display when the display of yellow is controlled by the detection.
The transportation officials in Portland and Dallas indicated that they have implemented, attempted to implement, or considered delaying the onset of the pedestrian walk interval by one or more seconds as an additional safety measure to ensure vehicles that entered from conflicting approaches have cleared the intersection. The delay in the onset of the walk was thought to have a safety benefit, although this could not be substantiated by available documentation.
One treatment that has been implemented by Dallas was both relatively simple and very creative. The treatment included programming the signals such that if a pedestrian pushed the pedestrian push button by less than five seconds, then the “normal” pre-programmed times for pedestrian walk and pedestrian clearance (i.e., FLASHING DON’T WALK) intervals are called into service. However, should a pedestrian press the push button continuously for longer than five seconds, a second set of pre-programmed times for pedestrian walk and pedestrian clearance phase is called into service. By analogy, it can be considered similar to a Max I green and a Max II green in which the Max II green interval is called into service during specific times, or in response to certain volume conditions. Dallas officials have implemented this operational treatment at several intersections, as shown in figure 60, and have spent time to instruct elderly pedestrians in the area on how to depress the button for five seconds or more. Moreover, the Dallas transportation officials have worked with individuals to set the most applicable pedestrian clearance intervals that would be based on the walking speed and behavior of local citizens who cross at that intersection.
Figure 60. Intersection in Dallas' Central Business District where longer walk and “FLASHING DON’T WALK” intervals can be put into service by depressing the push button for five or more seconds.
In a manner similar to the situation described, Dallas also has developed a software routine in which audible pedestrian signals can be activated after a pedestrian has depressed a pedestrian push for a longer time (e.g., five or more seconds). The audible signal speaker is mounted on the far side of the crosswalk and serves as an audible indication for sight-impaired pedestrians. The sounds correspond to the walk display (solid sound) and flashing don’t walk display (beeping sound). The controller logic was programmed in response to complaints by nearby residences and businesses about the frequency of the audible sound when activated by all pedestrians or continuously activated during push button malfunctions. The controller logic only activates the audible signal if and when a pedestrian pushed the button for an extended (i.e., five or more seconds) period.
Portland tested the concept of extending the pedestrian clearance interval, i.e., when the pedestrian signal displays a flashing “DON’T WALK” (FDW) indication. The pedestrian clearance interval is extended when microwave detectors sense the presence of a pedestrian still in the crosswalk. This was implemented at an intersection where elderly pedestrians cross frequently to a meal site. The “normal” FDW interval was 20 seconds, but the city made provision to allow the interval to be extended up to 27 seconds when pedestrians were still in the crosswalk at the end of the 20 seconds. The results show that the FDW interval was extended for approximately one-third of the signal cycles, but less than three percent of the times were the FDW interval extended to the maximum 27 seconds.
Portland had recently implemented a treatment that seemed both relatively simple and potentially promising. By placing loops beyond the stop line at an intersection, the Portland signal personnel have devised a way to extend the red clearance interval for vehicles that cross the loops late in the yellow interval or during the red clearance interval. While they have yet to complete a rigorous controlled evaluation of the effects, the raw crash counts and anecdotal experience suggests that this treatment does have a true positive safety effect. Figure 61 shows the intersection approach in Portland where loops have been installed beyond the stop line to provide a variable extension of the red clearance interval.
Figure 61. Intersection approach in Portland where loops beyond the stop line are used to delay the onset of the yellow interval.
Taking a slightly different approach compared to Portland, the city of Richardson worked with a local software company to develop an application of a “smart” signal controller that can be programmed to react to speed trajectories of arriving vehicles and determine the need to responsively extend the all-red interval to reduce the possibility of red light running crashes. This was done on a trial basis and the results were encouraging, although the costs are likely to be prohibitive for wide scale or even experimental applications. Richardson’s experimental concept relied on the logic presented in figure 62. There was a significant concern about the effect of holding the all-red interval on signal coordination and progression of flow.
Figure 62. Speed prediction algorithm for Richardson experimental red clearance interval hold.
(Courtesy of the City of Richardson).
At a joint meeting with the Michigan DOT, the Michigan State Police, and the Federal Highway Administration’s Michigan Division Office, the question was raised on what is the true effect on safety of implementing or improving coordinated signal timings. Signal coordination is the process whereby the signal controllers at intersections that are in close proximity are operated as a system. The start and end of selected signal phases are “synchronized.” For example, the controllers can all operate on the same cycle length and have pre-established maximum phase intervals when the synchronized phase(s) are terminated. The information provided at the meeting was that for a county-wide signal coordination and re-timing project done in Oakland County, the initial research report found that traffic delay was reduced throughout the corridor, as expected, but also there was a substantial reduction in crashes. Funding was sought by Michigan DOT to provide a more comprehensive before and after crash analysis.
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