Truck Priority Traffic Signal Operation – a potential opportunity

This post is based on the reader understanding basic and advanced traffic signal controller overlap operations.  These are outlined at:

http://ntcip-unleashed.blogspot.com/2012/02/basic-overlap-information.html

and

http://ntcip-unleashed.blogspot.com/2012/02/advanced-overlap-applications.html

Trucks decelerate and accelerate slower than other vehicles.  The operation described here provides a method to allow the signal to operate such that when a truck approaches a signal at a point where the signal may normally gap out, or max out, the truck will have a better chance of getting through the signal on a green, reducing the frequency of truck stops.

 

In this example, the truck is approaching the traffic signal on a continuous 10% uphill grade.  The continuous 10% uphill grade breaks to relatively flat right at the signalized intersection.  Any large vehicle stopping would have increased difficulty in getting moving on a green indication due to the grade of the street.

 

This is not a sales document.  There are a variety of methods that can be used to determine the classification of the vehicle.  This method uses a Wavetronix HD Count station, located approximately 400 feet from the stopbar down the hill from the traffic signal.  The HD Count Station uses RS-485 communications from the count station to the traffic signal cabinet.  The processing is done within the traffic signal cabinet.

 

More information about the HD Count Station can be found here:

 

http://www.wavetronix.com/en/products/smartsensor/hd

 

In the cabinet, there is a Wavetronix backplane, which provides the power bus and the RS-485 com for the Click! Devices on the back plane.  This backplane also provides power and communications for the Wavetronix Matrix stopbar detection system.

 

One key componenet is the Wavetronix Click! 512 vehicle alert module.  This module was developed for overspeed conditions, not necessarily for truck priority.  The Click! 512 module allows the user to program 4 unique channels of output, where the combination of the HD Count Station and the 512 module will identify a large vehicle, and make different actions based on the speed of that large vehicle.

 

More information about the Click! 512 Vehicle Alert module can be found at:

 

http://www.wavetronix.com/en/support/product_lines/2/products/53

 

When a large vehicle is progressing towards the signal, and passes the HD Count Station, the Click! 512 module determines the speed of the large vehicle, and then places a contact closure output to a Wavetronix 114 detector card.  The relation of the speed range, and which input to the controller is shown on the table below.  These are starting points, based on the HD Count Station being 400 feet from the stopbar.
 

Controller Det Input	Speed Range	Lower Speed	Ph 12 Extension Timing
17	More than 40 mph	60 fps	7 seconds
18	30-40 mph	45 fps	9 seconds
19	20-29 mph	30 fps	13 seconds
20	15-19 mph	22 fps	18 seconds

The extension timing is the controller’s extension timing, such that the controller detection input will call and extend for the time listed, based on one or more truck priority calls.

 

The traffic signal controller operates in USER mode (a modification from the standard 8-phase dual quad operation).  The USER mode is configured such that phase 4 and 12 are sequential, running to overlap 12, through load switch 4.  Phases 4 and 8, and phases 8 and 12 will terminate simultaneously.  This means that when the signal is getting close to gapping out, or maxing out, and a truck approaches the count station, the signal will transition from phase 4 to phase 12, and hold the green for the approach with the truck, while also holding the green for the opposing thru movement. 

 

The phase sequence diagram looks as follows:

If additional trucks arrive while the signal is running in phase 8 and 12, the signal will continue to hold the green until either phase 12 gaps out, or the phase 12 max timer is met.

 

If there is no traffic in the opposing left turn, or on the side street, then the controller may cycle between 4 and 12, based on the actual traffic on the main street.  Even though the signal is cycling between phase 4 and 12, the drivers will not see a change in the state of the indications, since 4 and 12 are run on an overlap through the load switch for phase 4.

 

It is important to keep the min green time for phase 12 relatively short.  The detection inputs for phase 12 are set to call and extend.  If the signal is operating in phase 4 and 8, and ready to gap out, and a truck approaches the intersection via the count station, the count station system places a call to the controller on the specific detection channel associated with the approach speed.  The following will occur within the controller while displays attached to load switch 4 stays green.

 

• Phase 4 times the yellow interval (typically 3 to 4 seconds, depending on what the engineer sets it to)
• Phase 4 times the red interval (0 to 2 seconds, depending on what the engineer sets it to)
• Phase 12 times the min green interval
• The remainder of the phase 2 extension timing occurs

 

If the min green time is excessive, the signal will hold for a longer time than necessary.  For example, if the truck is going 40 mph uphill, the call and extend time on phase 12 is 7 seconds.  This should get the truck to the stopbar at 40 mph.  If the signal is ready to end phases 4 and 8 and move on to 2 and 6, the controller will stay green for load switches 4 and 8, while the call / extension for phase 12 causes the signal to hold that green through the timing of phase 4 yellow and red plus the phase 12 min green time.  If phase 4 yellow is 3.2 seconds, and phase 4 all-red time is 1.8 seconds and the phase 12 min green time is 5 seconds (10 seconds total), the signal will hold green 3 seconds longer than necessary for the truck.  However, if the truck is traveling at 20 mph with the same settings, the extension timing will hold the green for 8 seconds longer than the sum of phase 4 yellow and all-red plus phase 12 min green.

 

There needs to be a balance of the timing that will need to be set by field calibration.

 

If the signal is to be in coordination, Phase 12 will need to be given adequate time in the split divisions to function within the coord plan. 

 

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.