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.

Basic traffic signal operation Pretimed Operation

Pretimed operation is the most basic type of operation. In this mode of operation, the signal will alternate the green, yellow and all-red time without regard to the traffic flow.

A signal may have one, some, or all phases on pre timed mode. For many downtown grids, most of the signals are on pre timed mode, and in many cases, the signals are in pedestrian recall.

Some signals are temporarily essentially set to pre timed mode if the vehicle detection fails.

The amount of time for each movement may be programmed to change by time of day.

The minimum amount of time for each phase should equal the sum of WALK, Flashing DONT WALK, Yellow and Al-Red. In some cases, some agencies allow the flashing DONT WALK to extend through the Yellow time. This can shave a few seconds of time off the cycle length of the signal.


Pretimed operation with the signals in pedestrian recall can be a very effective way of dealing with a grid network of signals. Many agencies don't put pedestrian pushbuttons in downtown grid signal networks, as the typical pedestrian volumes would have the signals constantly serving pedestrians anyway.

A pretimed signal operates on a user specified amount of time for each specific movement. For instance, if the signal is a 2-phase signal (2 specific signalized movements), like at the intersection of two one-way streets, and is operating in pretimed mode, the signal will serve one movement for a specified amount of time, followed by the second movement.

In this example, the first movement may be programmed for 20 seconds of green, followed by 3 seconds of yellow, followed by 3 seconds of all-red. The second movement would then commence, programmed for 30 seconds of green, then 3 seconds of yellow and 3 seconds of all-red. The total cycle time would be 62 seconds (20+3+3+30+3+3=62).

There are advantages, and disadvantages to this type of operation. The advantage is, it is pretty straightforward to set up, and the specific amount of green assigned to each phase (unique signalized movement) can be varied by time of day.

The variation by time of day allows for the cycle length to be adjusted to lengthen when necessary, and shorten when necessary.

The downside to this type of operation is that the signal is essentially a dumb box, without any way for it to modify operations by what is actually happening with the traffic. If the second movement has no cars, why serve it? If the 2nd movement has only 1 car waiting, why serve it for the entire time?

The fact of the matter is that with some situations, pretimed works pretty well, like in a downtown central business district grid. It also is a lot cheaper than installing vehicle or pedestrian detection systems in the intersection. It is also a pretty good way to deal with failed detection, as a temporary measure until the detection can be fixed.

It is possible that specific movements are timed, and other movements have vehicle detection, which would make that type of a signal semi-actuated.

Terminology

All professions have technical jargon. Traffic engineering is no exception.

This post will be a running commentary with an attempt to define some of the technobabble that will be in this blog. I will attempt to keep it in some sort of order... Maybe grouped in some way, maybe alphabetical... No promises.

DEFINITIONS

MAX (Max1, Max2, Max3) - The max time is the maximum amount of time that the signal will serve a phase after an opposing call is served.  For example, if a signal has served the MIN green time and is resting green on the main street, and there is constant traffic on the main street, the MAX timer starts timing after the first conflicting movement has a valid vehicle call.  If the traffic on the main street reduces, and the signal gaps out before the MAX timer times down to zero, then the signal will gap out and serve the side street.  This is similar, but different than forcing off in coordinated mode.
MAX (Dynamic) - NTCIP controllers have a dynamic max ability.  This allows the controller to be set with a short max (20 or 30 seconds) for each movement, but also with a higher dynamic max time and a step interval.  Essentially, if the traffic is low volumes, the signal will gap out, and the normal MAX time will apply.  As the traffic volumes increase, some of the phases of the signal will begin maxing out, and the signal will increase each phase's MAX time by the step interval up to the Dynamic Max time.  As traffic volumes decrease, the Dynamic Max time will reduce automatically.  This means that the main street could have a 20 second MAX time, but a 70 second Dynamic Max Time with a 10 second step interval.  This allows the signal to breathe for the uncoordinated cycle time based on the past 2 to 10 minutes of actual use.
Overlap - an overlap is a special type of signal operation that is allowed to operate across one or more vehicle phases. There are a lot of uses for overlaps to be described later
Phase - a phase is a distinct movement at a signal. Generally, the through movements are each given a phase number that is even. Generally, the protected lefts are given a phase number that is odd.
Recall - in general, all of the recall types can be assigned to any or all of the vehicle phases. When a signal has one or more phases in some form of recall, the signal will cycle between the movements with the recalls. Many signals are programmed to have the signal rest on the main street by putting the main street thru movements in min recall.
Recall (max) - this is where the phase is constantly being serviced for the maximum amount of time, either MAX1, MAX2, MAX3, Dynamic Max, the coordinated operation split time, or for a coordinated phase, the currently available bonus time plus split division time.
Recall (min) - this is where the signal will serve the minimum green time programmed, plus the variable initial green time.
Recall (pedestrian) - where the signal automatically will serve the pedestrians each cycle. This may be for any, or all phases.
Recall (soft) - Soft recall is not used very often. Essentially, this mode of recall allows the signal to cycle to different movements when there are no active vehicle calls on the movements in soft recall. This can be useful for low volume operations, to allow the signal to cycle between some movements before going back to the main street. The signal needs to have good stop bar detection in order to make this work. Care needs to be taken to make sure that the signal can not create a yellow trap with the soft recall settings..

Welcome

Welcome to my blog. I will be discussing how traffic signals work, why they do what they do, and why you may be getting more reds than greens. 

This blog will also provide information about how I am implementing NTCIP signals, using the new and fancy operations to improve traffic flow.  The general concepts are shown here, but much of the details are left to the imagination of the reader...  No sense in giving all of the secrets away.  Feel free to ask questions... Most of what I am describing here is no secret.  I have presented most of these topics at ITE and IMSA regional and national meetings over the last few years.

Traffic signals can be very complicated devices. Traffic signals may be pre timed, semi actuated, fully actuated, coordinated, uncoordinated or running with an adaptive algorithm. The vehicles can be undetected, Or detected by a variety of methods. Vehicle detection can be accomplished via pressure plates (very rare now), or detected by a variety of inductive loop wires, video detection and radar detection.

There are a variety of brands, makes, models, software versions and standards. I have extensive experience with Naztec Apogee, Econolite ASC/3, ASC/2 and 2/S and 8000, Traconex J-9, CJ and CJ-32, Peek 3000 and 9200, and LMD 8000 controllers. I also have worked on Eagle Epac, Wapiti 170's and VS-Plus controllers. These range between NTCIP, NEMA TS-1, NEMA TS-2, and CalTrans TEES Control environment.

What does all this mean? Well, the devil is in the details.

New blog, some old stuff

My kids had some fun with my Google account.  I spent about 3 hours trying to undo what they did, then decided to can it, and start over.  I have posted the old info, and will post more as time goes on.