Yellow light seemed short (Yellow Light Intervals)
The California Department of Transportation (Caltrans) provides uniform standards and specifications for all traffic control devices in the state of California. These standards can be found in the California Manual on Uniform Traffic Control Devices. Caltrans received contributions for the California Manual on Uniform Traffic Control Devices from the Federal Highway Administration, California Traffic Control Devices Committee, and others in determining the standards enclosed in the manual. The California Manual on Uniform Traffic Control Devices provides standards regarding design of traffic control devices, responsibility for traffic control devices, placement of traffic control devices, excessive use of signs, application of regulatory signs, application of guide signs, pavement and curb markings, and traffic control signs among other things. One type of traffic control standard that comes into question frequently is yellow light intervals. Many people catch themselves in a situation where they feel that a particular yellow light seemed shorter than normal. When a green light changes to yellow you quickly have to make a decision if you should proceed and attempt to safely cross the intersection or stop your vehicle. However, the yellow light interval is an important factor that you contemplate in those split seconds when you are making your decision.
The Yellow Change and Red Clearance Intervals can be found under Section 4D.26 of the California Manual on Uniform Traffic Control Devices. The manual states, “the purpose of the yellow change interval is to warn traffic of an impending change in the right-of-way assignment. According to the manual, the duration of the yellow change interval shall be determined using engineering practices. Those practices state that the minimum yellow change interval for through traffic movement shall be determined by using the 85th percentile speed of free-flow traffic rounded up to the next 5mph increment. Where the posted or prima facie speed limit is higher than the rounded value, the posted or prima facie speed limit should be used for determination of the minimum yellow change interval for the through traffic movement”. Basically, what that means is the yellow change interval is based on the speed limit and numerous other factors. Those factors are including, but not limited to, field observation of traffic behavior, intersection geometrics, downhill grade, perception-reaction time of drivers in the area, and actually driving the protected left-turn or protected right-turn movements to assess the need for longer yellow change intervals. Particular attention should be paid where setting minimum yellow change interval timing when exclusive turn lane exceeds 150 feet in length excluding the transition.
However, the manual does provide a general approach based on these engineering practices. A yellow change interval should have a minimum duration of 3 seconds and a maximum duration of 6 seconds. The longer intervals should be reserved for use on approaches with higher speeds. For a speed of 25 mph or less the minimum yellow interval should be 3.0 seconds. For speed of 30 mph the minimum yellow interval should be 3.2 seconds. For speed of 35 mph the minimum yellow interval should be 3.5 seconds. For the speed of 40 mph the minimum yellow interval should be 3.9 seconds. For a speed of 45 mph it should be 4.3 seconds. For a speed of 50 mph the minimum yellow interval should be 4.7 seconds. For 55 mph it should be 5.0 seconds. For a speed of 60 mph the minimum yellow interval should be 5.4 seconds. For the speed of 65 mph the minimum yellow interval should be 5.8 seconds.
The manual further explains, “the duration of a yellow change interval shall not vary on a cycle-by-cycle basis within the same signal timing plan. However, the duration of a yellow change interval or a red clearance interval may be different in different signal timing plans for the same controller unit”. This means that the duration of a yellow change interval shall remain consistent on a cycle-by-cycle basis. A cycle is the amount of time required for the lights to go through all phases for each direction of an intersection before returning to the starting point. However, the yellow change interval can change when the signal timing plan times due to different factor such as time of day or traffic volumes.
It turns out the California Manual on Uniform Traffic Control Devices has very specific standards and guidelines for how yellow light intervals work and how long they need to be. As a result, not every light is the same and the interval can change when a light’s timing plan changes. However, they have specific guidelines to keep them as consistent as possible so you know what to expect when deciding if you should proceed through the intersection or come to stop. It is important to remember the purpose of the yellow light is to alert drivers that the red signal is about to appear, and you should stop your vehicle if it is safe to do so. However, when making this decision you can keep in mind that depending on the factors discussed in this article you have between 3 and 6 seconds before the light turns red.
http://www.foresitegroup.net/a-beginners-guide-to-signal-timing/
https://driversed.com/driving-information/signs-signals-and-markings/traffic-signals.aspx
