The Traffic Signal Network
by Arthur J. Dock
Another term to be redefined... this time the term means the misunderstanding of signal timing, yellow in particular, on a national scale as misreported in the press and by others. Please look at the article Photo Enforcement: Reality which covers this issue in more depth.
Below is repeated an article posted on trafficsignal.net in January calling for a correction to a very large and very real part of the problem that continues to go unnoticed by almost everyone... enjoy while noting that without much fanfare on April 1, 2001 Lockheed Martin (the contractor responsible for running the City of Mesa, Arizona's cameras) reduced the pre-picture delay to its minimum (0.1 second which is as low as the equipment allows) with the proud announcement from Mesa's PD of "zero tolerance." Also, note that HB 2277 did NOT pass (thankfully).
Red Arrow Photo "Trap"
Lately the Arizona press has been running stories related to photo enforcement and persons who feel that they have been wrongly ticketed. Unfortunately, the focus has been on the signal timing and not the real culprit: intersection geometry and the way the photo enforcement system is setup. The problem is described in detail below.
This problem has caught many drivers. Although legally they do not have a leg to stand on (i.e. they did cross into the intersection AFTER the signal was red), the reality is that their expectations were violated. Many technically correct but seriously flawed tickets have been issued leading to many Arizonans trapped and abused by the implementation of automated enforcement.
At 45 MPH a vehicle travels at 66 feet per second. As currently defined, the intersection is the prolongation of the lateral curb lines (extend the curb lines along each side of the street through the intersection to draw a box) and the point at which vehicles are supposed to stop at on a red signal indication. The crosswalk and stop bar are typically behind this point. The distance from the intersection line to the stop bar is in the neighborhood of 20 feet on a typical through movement. At 66 feet per second this takes around 0.3 seconds to traverse. A red-light camera typically provides a 0.1 second minimum delay from when the signal turns red until it begins snapping photos. The City of Mesa, for example, adds an additional 0.3 seconds to this time. Therefore, in most cases for drivers that believe the stop bar is the intersection, they will not be ticketed because if they cross the stop bar just before the end of the yellow and are doing around the speed limit, they will pass into the intersection before the camera starts taking pictures.
Left turn arrow locations are very different. Quite often the stop bar will be significantly back from the intersection line for the left turn lane. Intersection geometry and dual-left turns make this a necessary reality so that vehicles don't collide (i.e. if vehicles were waiting at the intersection line cross street left turning vehicles would not be able to clear them. This distance can easily be in the 35 foot range at many locations (e.g. dual left turn locations). At a typical left turn average approach velocity of 15 to 30 MPH (22 to 44 feet per second) this distance takes 0.5 to 1.6 seconds to traverse. This means that the left turning driver that thought the stop bar was the intersection line will be ticketed.
Even if a driver knows that the stop bar is not the intersection still has a problem. Due to the geometry of the intersection it is quite reasonable to use the stop bar as a guide for stopping. To stop at the intersection line may be legal, but it would still expose the driver to cross street left turners. Options are scant: 1) back up (which is illegal) 2) stay in place and hope the cross street drivers clear, or 3) proceed through (and be ticketed if automated enforcement is in place - this decision is mostly made as the driver approaches the yellow indication - think fast).
Please realize that changing the yellow time will NOT fix this problem! Regardless of the length of the yellow any vehicle passing over the stop bar at the end of the yellow will have insufficient time to clear the photo enforcement detectors (i.e. will cross them after RED) and WILL be ticketed
The City of Mesa (and possibly other locales as well) currently violate driver expectations when cameras do not take into account the realities of intersection geometry. This could easily be fixed without legislation (HB2277 proposes making the stop bar the required location to stop on red - a "solution" that has other problems to be detailed in an upcoming report on this site next week). Rather than set the camera to a fixed 0.4 second delay, vary the delay according to the distance from the stop bar to the intersection as necessary on a location-specific basis. This will assure that only those that really were intentionally running the light will be ticketed and will clear up the current innapropriate situation.
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