Why Correct Signal Timing is so Important
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At signalized intersections, the period between conflicting green phases is known as the change interval. It is composed of a yellow signal phase plus an optional all-red interval. The purpose of the yellow phase is to alert drivers that their right-of-way is about to end and, depending upon their relative proximity to the intersection, to permit them to come to a safe stop or allow them time to clear the limit line prior to the onset of the red phase. The purpose of the all-red interval is to allow vehicles which have lega1ly entered the intersection prior to the onset of the red signal to clear any potential points of conflict before the release of opposing vehicles or pedestrians.
Transportation officials and engineers know that the proper timing of the change interval is essential to intersection safety. If the yellow signal phase is too short, a “dilemma zone” is created. Motorists who are unfortunate enough to be within this section of the roadway when the yellow signal illuminates may neither be able to stop safely nor legally enter the intersection before the onset of the red phase. Dilemma zones virtually assure that some percentage of drivers will be forced to brake suddenly or violate the red, thereby creating the potential for rear end collisions or vehicle conflicts within the intersection. If the all-red interval is insufficient, vehicles from the previous green phase may not have completely cleared the intersection prior to the release of cross traffic. Again, this generates the potential for vehicle conflicts including highly dangerous broadside collisions within the intersection.
When both the yellow and all-red phases are too short, a significant safety hazard is created. Some drivers will be forced to violate the red due to the too-short yellow and cross traffic will be released before they completely clear the intersection. Intersections which exhibit a higher than average number of red-light related collisions most likely have either a yellow phase or all-red phase which is too short. Often times, both problems exist at the same intersection. Correcting the signal timing will most likely eliminate the vast majority of collisions at these locations.
How the Yellow Phase is Determined
A mathematical formula established by the Institute of Transportation Engineers (ITE) is used to calculate the minimum safe time for the yellow signal and is primarily based on the speed of the traffic approaching the intersection and the reaction time of the average driver.
The ITE specifies the 85th percentile speed of free flow traffic be used as the approach speed to ensure that the yellow phase provides sufficient stopping time for at least 85% of the vehicles approaching the intersection. California law requires the use of the 85th percentile speed or the posted speed limit. However, if the posted speed limit is used and the 85th percentile speed is substantially greater, then the yellow phase will be under-timed and a safety hazard will exist as a significant number of drivers will encounter a dilemma zone. For example, if the yellow signal is timed for 35 mph, the yellow light will be set at 3.6 seconds, but if the 85th percentile speed of traffic is 45 mph the yellow signal should be 4.3 seconds, too short by almost 1 full second. While this difference may not seem like much, it will often make the difference between whether or not a violation occurs.
Driver perception-reaction time, is the time between the onset of the yellow phase and when the vehicle’s brakes engage. During this time, the driver must see the light change from green to yellow, perceive the meaning of the change, decide whether they are able to safely stop or if they should continue through the intersection, and if they decide to stop, move their foot off the accelerator and apply the brakes. The ITE suggests that a value of 1 second be used for this variable. However, a number of studies suggest that this figure may not adequately account for driver perception-reaction times, especially in an urban environment such as Los Angeles due to “cognitive overload” where the reaction time may actually be closer to 1.5 seconds.
Maximizing Safety through Longer Yellow Timing
Safety dictates that the yellow signal time be calculated using the 85th percentile speed of free flow traffic approaching the intersection based on a current traffic survey. When a survey is unavailable, the 85th percentile speed should be assumed to be at least 10 mph above the posted speed limit. Likewise, in the interest of safety, driver reaction time should be assumed to be at least 1.4 seconds. Together, this change would likely increase the yellow time at many intersections by about 1 second and significantly improve safety.
The safety benefit of longer yellow signal times has been proven in a number of studies. A Texas DOT study found that when the yellow interval is set one second longer than the minimum time, violations decreased by 53% and crashes decreased by 40%.
The chart below shows a similar 30% to 55% reduction in violations achieved at San Diego red-light camera sites when the yellow interval times were increased.
Likewise, the following two figures show how Fairfax County, VA achieved a significant, sustained reduction in violations when the yellow timing was increased
by ½ second. Note also that red-light cameras were present during the entire review period, but a dramatic reduction in violations was seen only after the yellow timing was increased.
As for concerns that drivers will modify their behavior to account for the longer yellow light and still run the red, the data shows that such an adjustment does not happen provided the yellow time does not exceed about 5.5 seconds.
How the All-Red Clearance Phase is Determined
The ITE uses another formula to calculate the appropriate all-red phase which takes into consideration the width of the intersection and the speed of traffic. In contrast to the formula for the yellow signal timing this formula uses the speed of the slowest 15% of drivers to ensure that these vehicles have sufficient time to clear the intersection prior to the release of cross traffic. Generally the 15th percentile speed can be assumed to be 5 – 10 mph below the posted speed limit.
Maximizing Safety through a Longer All-Red Phase
As discussed previously, the purpose of the all-red phase is to allow vehicles that enter the intersection in the last moments of the yellow phase to clear the intersection prior to release of cross traffic or pedestrians. Wider intersections necessarily require longer all-red phases. A vehicle traveling at 35 mph will require just over 2 seconds to clear an intersection 90 feet wide and 2.83 seconds to completely clear an intersection 125 feet wide. Slower vehicles would require more time to clear those intersections. These clearance intervals provide a sufficient all-red phase to clear any vehicles that cross the limit line just as the signal changes from yellow to red. However, in real world situations, some drivers will enter the intersection just after the signal turns to red. This is often due to yellow times that are too short, drivers who simply miscalculate the time it will take them to reach the limit line before the yellow phase ends, or drivers who hesitate or slow momentarily before deciding they have sufficient time to make it into the intersection before the red illuminates. The 2004 Texas DOT study determined that when a red light violation occurs, about 60 percent of the time it occurs within the first half second of the red phase. The distribution of red light violations as a function of time-into-red appears in the figure below.
In order to minimize the chance that collisions will occur at signalized intersections, the all-red clearance interval must be long enough to allow all vehicles to clear the intersection before cross traffic is released. This interval should be extended slightly to account for vehicles that enter the intersection just after the red phase begins. For example, a vehicle traveling at 35 mph would require approximately 2 seconds to clear an 80 foot wide intersection. However, if the driver miscalculates or hesitates and enters the intersection ½ second after the light turns red, his vehicle will not completely clear the intersection until 2.5 seconds after the red phase begins. In this situation, the 2 second all-red clearance interval dictated by the ITE formula would not be sufficient to prevent a possible vehicle conflict within the intersection. While it may not be desirable to increase the all-red phase to account for every vehicle that might enter the intersection after the red phase begins, extending the all-red phase ½ second beyond the time calculated using the ITE formula would prevent collisions that might be caused by the 60% of violations that occur immediately after the onset of red. This could add an additional measure of safety without unduly reducing traffic flow.
Existing Conditions at Los Angeles Intersections
As part of an ongoing study of traffic safety in Los Angeles, Safer Streets L.A. identified intersections within the city with a higher than average number of red-light related collisions. Without exception, we found that either the yellow time, the all-red phase or both are currently below the ITE minimums based on the actual speed of traffic at these locations and therefore insufficient to provide the margin of safety necessary to protect the motoring public. In some cases the yellow signal time is 1 full second too short and the all-red phase is a much as 2 seconds too short.
Unquestionably, at intersections where red-light related accidents are over represented, an engineering deficiency exists. As explained above, drivers in an urban environment such as L.A. may need additional reaction time to respond to the yellow signal and routinely travel at 10 mph or more beyond the posted speed limit. Therefore, it would be prudent to extend the yellow signal time at least 1 second and extend the all-red clearance interval at least 0.5 seconds beyond the ITE minimums. Additional engineering improvements may be necessary at some intersections to be determined based on a comprehensive engineering survey.
The City of Los Angeles now has an economical option available that would achieve a greater safety benefit than the proposed multi-million dollar red-light camera program. The cost to change signal timing is insignificant. The City can swiftly achieve a reduction in accidents as well as violations at numerous signalized intersections simply by extending the yellow and all-red signal phases. There is no reason not to immediately implement these strategies at problem intersections. Failure to do so not only puts the motoring public at risk, it leaves the City open to the possibility of litigation should injuries or fatalities occur due to deficient signal timing.
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