Clearance times

Change and clearance interval are what traffic engineers call the yellow and all-red times.

There are a lot of different thoughts about how to set the yellow and all red times. This posting only addresses how they are done. There is a lot of controversy about yellow and all-red times with regard to red light cameras.

Most states adopt the Manual of Uniform Traffic Control Devices (MUTCD) as the standard for signs, markings, signals, and so forth. Most states also have laws that define how the requirements of the MUTCD are modifies for that state. The 2003 MUTCD states the following regarding the yellow and all red.

      Section 4D.10 Yellow Change and Red Clearance Intervals
 
     Standard:

     A yellow signal indication shall be displayed following every CIRCULAR
     GREEN or GREEN ARROW signal indication.

     The exclusive function of the yellow change interval shall be to warn traffic
     of an impending change in the right-of-way assignment.

     The duration of a yellow change interval shall be predetermined.

     Guidance:

     A yellow change interval should have a duration of approximately 3 to 6 seconds. The
     longer intervals should be reserved for use on approaches with higher speeds.

     Option:

     The yellow change interval may be followed by a red clearance interval to provide
     additional time before conflicting traffic movements, including pedestrians, are released.

     Standard:

     The duration of a red clearance interval shall be predetermined.

     Guidance:

     A red clearance interval should have a duration not exceeding 6 seconds.

This leaves a lot of open area for interpretation.

Yellow Change Interval

Some agencies have a set the yellow times as a fixed time, say 3 seconds. Some agencies do some field adjustments on the settings.

Many engineers follow the Institute of Transportation Engineers kinematic model. This considers the driver's perception-reaction time, prevailing speed of traffic, grade of the road and other factors, and calculates a yellow time. The kinematic model can mathematically calculate the yellow at less than 3.0 seconds.

Most traffic signal controllers have a default setting that will not allow less than 3.0 seconds of programmed yellow time. Most conflict monitors (and malfunction management units) will send the signal into flash if the signal provides less than 2.7 seconds of yellow time. Some controllers and CMU/MMU's can be programmed to defeat this minimum setting for the yellow. I have worked on well over 100 different traffic signals, and have never had a reason to reduce the yellow below 3.0 seconds.

All Red Clearance Interval

All red rimes can vary widely by agency. Some agencies use zero seconds everywhere by policy. Some agencies use one second everywhere by policy. Some agencies use a calculated value.

The ITE kinematic model calculates the all red time with respect to the width of the intersection, speed of traffic, and the average length of a vehicle. In many states, it is legal to enter the intersection during the yellow. The red time calculation assumes that an average length car (typically 20 feet) enters in the last instance of the yellow time, and calculates the time required to have the rear bumper clear the conflict area at the posted or prevailing speed.

This may be excessive when the signal has timed out, and the signal has no one in the intersection during the all-red time, but it is near impossible to have fail safe detection, where the lack of vehicles can be sensed.

There are a couple of controllers that have the ability to provide variable all-red time. These make assumptions as to how the controller has terminated the phase (Gap Out vs. Max / Force Off) and applies an extra amount of all red. The problem with this is it is unpredictable and will not work while in coordination.

There is also a method using external logic circuits (and, or, set / reset flip flop cards) to have special vehicle detection apply an Omit All-Red by Ring. This can work in coordination, however if the detector fails, the signal will default to never provide any amount of all-red time. The applications of these that I have REMOVED from service have been buggy.

So why put in yellow and all-red time?

It is done for safety. It may be aggravating to wait 4, 5, or 6 extra seconds, but they are provided to let the drivers know that the signal is changing and they have to decide to stop, or to go through the signal, and then clear the signal before the next phase is given a green.

Yellow and all-red times can provide an interesting view into the human mind. The 5 or 6 seconds that you wait seem like an absolute eternity when you are waiting. On the flip side, the same time may seem really short if you are rolling through the intersection. The same 5 or 6 seconds will seem like an instant flash of time when you are doing something enjoyable.