Canton Red Light Cameras

The update for today deals with an actual case study from Chillicothe, Ohio. The City of Chillicothe entered into a contract with Redflex to install red light cameras. The City played the same tricks as many other cities to try to increase ticket revenue with their new red light cameras system. The issue we will discuss now is shortened yellow lights at red light camera intersections.

From the Traffic Engineering Manual:

Substitute House Bill Number 30, passed by the 127th General Assembly, took effect September 12, 2008. This law imposes certain responsibilities on local authorities who choose to use traffic law photo monitoring devices. Section 201-14 addresses the local authority’s duties, which include assuring that the yellow change interval exceeds by 1 second the yellow change interval determined in accordance with Section 403-2.

You see that this is a law, not a recommendation or suggestion, or “best practice.”  To know what is required of this law, you need to know what is stated in Section 403-2, but you already know that if an intersection is monitored by a red light camera, it is REQUIRED to have a 1 second longer yellow light than would be required if no red light camera were installed.

Section 403-2 states:

The vehicle change interval (or phase change interval) described in OMUTCD Section 4D.10
consists of the yellow change interval and the all-red clearance interval. The exclusive function of the yellow change interval is to warn traffic of an impending change in the right-of-way assignment. For ODOT-maintained signals, the yellow change interval should be followed by a red clearance interval (all-red phase) of sufficient duration to permit traffic to clear the intersection before conflicting traffic movements are released. For more efficient operations, start-up time for the conflicting movements may be considered when setting the length of the all-red.

The length of the phase change interval can be determined using the following equation, in which
[(W+L)/ V] represents the all-red interval:

Y + AR = t + V/( 2a + 64.4g ) + [( W + L ) / V] (for English units)

Where: Y + AR = Sum of the yellow and all-red,

t = perception/reaction time of driver, ( typically assumed to be 1 second )

V = approach speed, ft/s (m/s)

a = deceleration rate, ft/s2 (m/s2)
(typically assumed to be 10 ft/s2 (3.0 m/s2))

W = width of intersection, ft (m)
(measured from the near side Stop Line to the far edge of the conflicting traffic lane, along the actual vehicular path)

L = length of vehicle, ft (m)
(typically assumed to be 20 feet (6.0 meters))

g = approach grade, percent of grade divided by 100 (plus for upgrade, minus for downgrade)

Yellow change intervals typically are in the range of three to six seconds, and the typical maximum all-red interval is two seconds.

Now, You see that the typical yellow light range is three to six seconds, and they are REQUIRED to add one second if a red light camera is installed at the intersection.  The City of Chillicothe would not respond to Public Information Act requests (made by BigKid of http://www.myrosscounty.com ) for the widths of intersections where they had shortened yellow lights, but you can see that the minimum yellow light duration for a photo enforced intersection should be four seconds.

The following figure, shows an intersection that has a properly set yellow light duration.  This diagram was crated by BigKid (W. Loucks).  You can see that a car traveling at the speed limit requires a certain amount of time from the light turning yellow to be able to stop in time for the intersection.  During that time, the car travels a certain distance.  The distance traveled by the car in that time can be measured back from the stop line and is can be called the critical point.  If the car is not yet past this critical point, it stops safely before the light turns yellow.  If the car is past the critical point, it should proceed through the yellow light, safely making it before the red light.

A legally set yellow light time allows for drivers to make a decision whether to proceed through the intersection or stop.

When a city reduces the yellow light time, as Chillicothe did at the red light camera intersection of Paint and Main, they create an artificial “critical point” in addition to the real “critical point.”  The artificially created critical point is closer to the intersection than the real critical point.  If you remember, once a car has passed the “critical point” it needs to proceed through the intersection when the light changes, as it cannot safely stop.  The problem with a shorter yellow light at red light camera intersections is that the light is STILL GREEN when the car passes the true critical point.  Since the car is past that point, it cannot possibly stop safely when the light does change to yellow, but since the yellow light is shorter than the law allows, it cannot proceed through the intersection either.  There is no good choice for a driver that has passed the true critical point, but not yet passed the artificial critical point.  This area between the two critical points has come to be known as the “Dilemma Zone.”  

In the Dilemma Zone, a driver has three choices to make, and none of them are safe.  The driver can proceed through the intersection at the speed limit, but if the driver makes that choice the light will be red prior to the driver’s arrival at the intersection resulting in a red light camera ticket.  The driver could “punch it” or speed up to beat the red light, thereby speeding through the intersection.  In this case the driver would not be running a red light, but would be speeding and also runs the risk of a ticket.  The third choice, and one being made by many people at red light camera intersections, is to make a panic stop.  In the Dilemma Zone, braking has to be far greater than the 10ft/sec² in the Traffic Engineering Manual (TEM).  The Person who decides to stop must do so with the risk of being rear ended, as drivers behind them expect them to proceed through the light as they are past the true critical point.  There is another illustration below to show the Dilemma Zone.

You can see in the illustration that a driver has no good choice to make when short yellow lights are employed to increase violations at red light camera intersections.

As you can see, safety is not the priority in this situation.  The City is thrilled with the end result any way you look at it.  If a driver speeds up, they get a photo speeding ticket.  If the driver stops and gets rear ended, a citation is issued to the driver that rear ends the first car.  If the driver maintains the speed limit, they run the red light and get a red light camera ticket.  The only winner is the city budget.  Drivers lose because they are being set up for failure.  No matter what a driver chooses to do, it will be wrong.

These are not “What if” situations.  I will post video of a 2.8 second yellow light at the intersection discussed above in another post.  The intersection has since been changed to a four second yellow light, because the entire red light program is in trouble, being challenged in court because of tactics like this and violations of constitutional rights.