The Traffic Signal Network

Photo Enforcement: Reality

by Arthur J. Dock


The debate over photo enforcement of traffic signal red-light violations has grown into a grand debate. Unfortunately, emotions are running high and objectivity has suffered greatly as a consequence. In this article, I will attempt to explain some very real issues and offer real, practical, solutions.

Who's Responsible

Before looking for conspiracy theories, gross negligence and queuing the spooky music, let's examine who the major players are and what their roles / modus operandi are. Firstly, there are professional transportation people responsible for the design, installation, and operation of signalized intersections. Secondly, there are enforcement personnel who focus on (no surprise) enforcing laws. Finally, there are the capitalists that, by definition, are out to make a profit. For the sake of a shorter article, I will for the most part ignore the related and unrelated peripheral players such as legislative bodies, standards departments/organizations, activists, victims, and so forth.

Part of the job of professionals involved in transportation involves the proper configuration of our roadway systems. Part of that system involves signalized intersections... one of the trickier components of which is the proper setting of yellow (change) and red (clearance) intervals. That last sentence in itself will be considered controversial in some circles due to the parenthetic comments. However, the reality is that there is NOT a true consensus or "cut and dried" method of setting signal timing just as there is no one single solution for all situations. Intersections are like snowflakes - many similarities with a great variety of forms. In any case, to understand the transportation professional's mind, one must remember that safe AND efficient operation is the single goal.

Just as police cars are typically black and white, so are the arguments in court when it comes to traffic violations. When, for instance, the statute states that entering the intersection after the signal is red is a violation, then you WILL be fined regardless of extenuating circumstances. Even the FACT that there exists in Arizona a trap situation regarding the difference between the stop bar and the intersection line is explained to police, they are still more interested in "zero tolerance" and getting on with their primary task: law enforcement. This is by no means a negative statement against law enforcement, but rather it is reality for better or worse. Police are NOT engineers, they do NOT typically operate in a "study the problem and try a solution" mode: they enforce the law. Compromise and balance are not a big part of their reality. Therefore, it is no surprise that photo enforcement operates as it does in practice as it is a given that the police are typically in charge of it.

Despite statements to the contrary, there must be money to be made in this business of photo enforcement or else companies would be going bankrupt left and right (or not getting involved in the first place). Again, this is not an indictment against capitalists: just reality - business is about making money. In the pursuit of money some companies have successfully garnered themselves a position whereby the more tickets they write, the more money they make... whether the intentions of their hearts are good or not, the very system they have created REQUIRES a certain amount of motorist "sin" to keep profits high (and/or keep them in business in many cases). The alternative, given the current typical $x per ticket scheme, would certainly be bankruptcy. Keep in mind that a couple of steps that are often taken prior to implementation of photo enforcement are to 1) video tape intersections to see where cameras would be most profitable, and 2) to include a "no tinkering with the yellow timing" clause in any contracts... hedge your bets anyone?

The combined interests and lack working together between the major players: transportation officials, law enforcement, and the equipment providers have led us to the current inapropriate state of affairs. In the TRUE interest of public safety, there are better ways to do this.

Change And Clearance (Yellow and Red)

One famous statement about yellow times is that they are typically between 3 and 6 seconds (as stated in the Manual on Uniform Traffic Control Devices a.k.a. the MUTCD). Too short a time is not good, and too long a time is not good either (i.e. could lead to abuse). One good way to do things is to set the yellow time as a true change interval (i.e. long enough so an average vehicle at the prevailing speed can recognize, decide, and then either stop or proceed). The addition of an all-red period after this provides clearance time. Guess what, the installation of photo enforcement with "zero tolerance" means that the all-red time is now meaningless and the tickets will fly... who's fault is this? Certainly not the traffic engineers'. And while we're at it, for all of the people who have the solution, don't forget that left turns in reality are typically taken at 1/2 to 1/3 the prevailing speed which LOWERS the necessary left turn yellow time in this mode of thinking (i.e. slow = less stopping time)... and should increase the red clearance time (i.e. more time to clear the intersection width)... and while you are fixing these problems, don't forget that all directions at the intersection (including pedestrians) must be served in a timely fashion or traffic WILL back up (e.g. there is typically a rate of diminishing returns on cycle lengths that begins a somewhere over 100 seconds). The mathematics of this situation are a cruel master and there is no simple solution when all parameters are examined in total.

While we are on the subject, ask yourself this question: do you think that any sane traffic engineer would agree to limit the ability to change yellow times? In most cases, certainly not! However, when the mayor wants left turn enforcement and the police are signing the contract (as happened in Mesa, Arizona for example), these kinds of agreements are made. Yellow and all-red times are tied to critical intersection-specific parameters... when the parameters change, new facts (such as vehicle speed studies) come into view, better standards are implemented, or any other pertinent facts dictate: the timing must be changed - it is dereliction of duty to do otherwise. Just who is in charge here anyway? Maybe that's one big part of the problem.

Distance Dilemma

To reiterate a point I'm keen to make (and have many times), I'm including the information below on the photo "trap" that exists in Arizona (and any other state with a similar law). Until this point is understood, another piece of the puzzle is going unnoticed. Specifically, there is a difference in some states whereby the stop line (stop bar) is NOT the legal intersection definition. Arizona is one such state.

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, used to add an additional 0.3 seconds to this time (until April 1, 2001). Therefore, in most cases for drivers that believe the stop bar is the intersection, they would not have been ticketed because if they crossed the stop bar just before the end of the yellow and were doing around the speed limit, they would have passed into the intersection before the camera started taking pictures. Since the change, even more "honest" folks can be ticked in Mesa due to "zero tolerance."

This problem is compounded at left turn arrow locations which 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 proposed making the stop bar the required location to stop on red - a "solution" that California uses). 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 inapropriate situation.

A Call for Rationality

As stated previously, the real problem is that there is not currently a holistic approach being taken. Below I've pieced together what I feel is a logical approach to this whole situation. Although I don't have time at the moment to do so, this would certainly make for a nice flow chart...

Firstly, logically determine a location for automated enforcement:

  1. Calculate accident rates (i.e. number of accidents per 100,000 intersection entries) for all intersections in the community.
  2. Perform detailed engineering studies on the highest rate intersections (like a Pareto chart - go after the BIG problems first).
  3. Implement appropriate changes (e.g. to geometrics, speed limits, signal timing, signal operation - add left turns - protected left turns, road surface, trees, lighting, and etcetera) at these locations.
  4. Perform follow-up studies of intersections. Examine any that still have a high accident rate and implement more changes if necessary.
  5. Consider enhanced enforcement.

Secondly, after a chronic intersection has been determined (i.e. it didn't respond to changes), determine if enforcement has a chance of helping:

  1. Manually examine accident history (i.e. don't depend on check boxes on accidents - actually read the accident reports) and see if there are certain violations that are commonly occurring (note: if you find a bunch of failure to yield while turning left violations - maybe you messed up on step 3 above and didn't implement protected arrows - try again).
  2. Step up enforcement targeted at those specific violations (human and automated).

Thirdly (if you've even made it this far), properly implement and operate automated enforcement:

  1. Produce a detailed report on the intersection being targeted. The report must include the information about the steps taken to mitigate the problem (including before/after accident information) as well as any other pertinent facts (this is great stuff to have to show you've done the right thing: CYA).
  2. Begin a public education program that will remain in effect throughout the implementation of the automated enforcement. For example, special signs at intersections, media (radio, TV, web, print, etcetera), and any other means deemed effective.
  3. Implement the enforcement (see below)
  4. Evaluate the efficacy of the implementation at each intersection on an annual basis. If the violation and/or accident rates have not been (or do not remain) reduced, the cameras are NOT working and must be removed... you need a new tool.

Finally, Implementation must be in a manner that is above reproach (above all, DO NOT destroy the constitution in the name of convenience and vendor profits):

  1. Do NOT compensate the vendor on a per ticket basis (pay for accident/violation rate reductions is OK).
  2. The vehicle owner (and anyone else for that matter) is NOT automatically guilty.
  3. The vehicle driver shall be responsible - not the registered owner.
  4. Vehicle owner shall NOT be required to identify driver.
  5. Normal legal service procedures (e.g. certified mail or process server) must be used.
  6. Vendors should not be allowed into gray areas (e.g. using a private investigators license to justify their direct access to vehicle registration and other records).


Do not blame any of the players in this drama for a conspiracy. Each has their own focus which collectively result in the mess at hand. As stated previously, the real problem is that there is not currently a holistic approach being taken. As in many things in life, only by working together can great things be accomplished.

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