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by Paul Orosz

I got a photo radar ticket in Commerce City, Colorado and decided to pursue it in court. I spent a lot of time researching this issue, and have concluded that there are some technical issues that call this technology into question.

As you probably know, photo radar is deployed at an angle to the flow of traffic. When any radar device is deployed at such an angle, the measured speed is less than the actual target speed. This is known as “cosine error” and can be calculated:

Measured Speed = Target Speed (cos t)

where t is the radar beam’s angle to the direction of travel.

Commerce City has contracted with American Traffic Systems who has deployed their Autopatrol™ PR-100. According to a PR-100 spec sheeet that I obtained from ATS’s attorney, the PR-100 is to be deployed at a 22.5 degree angle to the flow of traffic.

This angle is specified because the system internally compensates for this angle when calculating the “actual” speed. So if this angle is not exact, the measured speed will be incorrect.

The setup procedure used by the ATS operators instructs them to park parallel to the curb, and use marks on the radar unit’s mountings to set the 22.5 degree angle.

The place where I got the ticket, the road narrows from three lanes to 2, so the curb is not parallel to the flow of traffic. Plus it was on a curve, so one could argue that the angle was not properly set.

How incorrect? A mile or two off is not at all out of the question.

Is a mile per hour off a big deal? Sure is. The National Highway Traffic Safety Administration’s (NHTSA) standards for traffic radar(1) specify that stationary traffic radar units be accurate to within one mile per hour. I imagine that in the lab or on the test track the PR-100 is; but in real life it is not.

P.S. When my case came to trial, the prosecutor moved for dismissal before it even started.

1. “Performance Standards for Speed Measuring Radar Devices,”
Federal Register, vol. 46, pp. 2097-2120, January 8, 1981.