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Lasers: In the Legal Crosshairs

Overcoming ‘sweep error’ and meeting the legal standard, police and scientists made a strong case for the speed laser.

February 01, 2002  |  by Craig Peterson

But up to that time, no U.S. department had conducted extensive tests of any speed laser under real-world conditions. And Stanton was the first judge in the nation to request such data. Stanton declined to admit the results from several international tests, including that of the stringent German PTB.

Reluctantly, Stanton decided in favor of the defendant, but not before encouraging the prosecution to mount an effort to put together an operational field-testing program and return to court with the results.

Help wasn't hard to find. Both the New Jersey State Police and the New Jersey Department of Transportation quickly signed on to assist. It was decided to compare the laser to the standard issue State Police radar as well as to the DOT's weigh-in-motion (WIM) system used to weigh heavy trucks on the fly, itself calibrated using Doppler radar.

The goal was to answer two fundamental questions: Did the laser tend to produce the same average level of target speeds as the control methods, and on any individual measurement, how much variance was there between the laser and the others' target speeds?

To find out, the NJSP clocked a total of 1,908 targets using all three types of speed-measuring hardware. The conclusions that landed on Judge Stanton's desk included:

"...on the average, laser speed measurements tend to be lower than the control radar measurements by less than one-quarter mile per hour. Although this is statistically significant, a difference this small probably would not be regarded as practically significant from the standpoint of the intended use of this [laser] equipment."

And in comparison to the WIM: "... similar to the radar data, the difference [in target speeds] is relatively small (average of -0.418 mph, or less than one-half mile per hour) and in the favor of the motorist (negative average value, meaning the laser device tends to read lower than the WIM measurements based on Doppler radar). In this case, 14 out of the total 799 laser measurements exceeded the WIM measurements by more than 1.0 mph."

Armed with the report, the prosecution approached a receptive Judge Stanton. After reviewing it the judge issued his opinion. "I end up being impressed by the fact that when we combine the results for the comparisons with both the WIM system and radar, we have only 16 cases out of 1,908 in which the speed measurements produced by the laser speed detector exceeded the measure produced by the comparison device of more than one mile per hour. That amounts to 0.8 percent. I also note that the speed measurement produced by the laser speed detector never exceeded by more than one mile per hour the measurements produced by the track timer, the PEEK 241 or the fifth wheel.

"...I am satisfied from the totality of the evidence presented to me that the laser speed detector produces reasonably uniform and reasonably reliable measurements of the speed of motor vehicles under conditions likely to be present on New Jersey highways when the detector is used for law enforcement purposes," he concluded.

Case closed.

Or was it? Even if the judge was satisfied, to the state cops and DOT the issue of sweep error remained unresolved. They devised an exhaustive series of nine different tests and headed back out into the field.

And did they notice any sweep error? Nyet, nada, not even close. Satisfied, they folded their tents and went home, later issuing a report nearly twice as lengthy as their first effort.

The landmark New Jersey case was laser's first big win in court and the resulting field tests were the first public acknowledgement of the speed laser's accuracy. And despite the passage of five years and countless court cases, no similar tests have been requested by judges.

Craig Peterson has lent his expertise on vehicles to POLICE in the past and joins us again after a two-year absence from our pages. 

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