By Jordan W. Charness; photo courtesy the City of Burlington, Massachussetts

“I really don’t know why I got ticketed!” exclaimed Peter as he came barging into my office. Now, not a lot of people get to barge into my office, but Peter is a special case: he’s only allowed to do it when I do not have a client with me, and besides, he provides me with a lot of good stories to share with you. I can always tell if it’s going to be a good one by how much of a look of righteous indignation is on Peter’s face as he comes through my door. This one looked to be about an eight out of 10.

“I got a speeding ticket, and this time I’m absolutely certain that I could not possibly have been going as fast as the officer said I was! In fact I’m pretty sure that the police officer mistook my car for the little truck that blew by me at around the same time I got pulled over!” he explained.

The rest of the story turned out to be that he was pulled over by a police officer for speeding just as he was coming down a small hill. He knows enough to ease up on the gas when going downhill, because claiming that you were going downhill is never a valid excuse for exceeding the speed limit. He was sure that he did not speed down that hill at any where near the 125 km/h the police say they clocked him at.

He claims that it was possible that he might have been going 105 km/h, but certainly no faster than that and in any case there was a truck that passed him at a much quicker speed that he was going. He tried to explain that to the police officer who told him that he had been caught on radar and that he could explain it to a judge if he felt the urge.

As soon as he said the word “radar” my ears perked up. Many police forces these days use extremely effective and efficient laser targeting systems for catching speeders. In order to properly use these devices a police officer actually “paints” a red dot of laser light on the exact car that he or she wants to verify. Right after pressing the trigger there is an almost instant return of the exact speed of that vehicle.

Doppler radar, however, is a completely different system. Although it has fallen out of favour with most police forces, there are still several of the old Doppler units around and in use. I took a look at his ticket and indeed it indicated that he had been caught on Doppler radar.

This system does not allow the precise targeting of a single vehicle. Instead of painting a laser light dot, it emits a radar “wave” that spreads out from the antenna and then reads the bounce back signal from what ever object is in its path. Although it is fairly well defined, it is not always possible to know for a certainty which vehicle was painted by the radar.

To make matters even more complicated, the Doppler has a tendency to send back a reading on only the vehicle with the highest mass if the two vehicles were side by side when the radar wave hit. A police officer would not really know which vehicle was being targeted in that particular scenario.

In Peter’s case it is entirely possible that the radar painted the truck that was moving faster than Peter was and sent back that reading to the police officer. The cop may not have noticed the truck, since he was concentrating on the radar screen. It could be that he only noticed Peter’s car and assumed that Peter was speeding and had been caught on radar.

Given all this, Peter has a fighting chance to convince a judge that there was indeed a reasonable doubt that the vehicle caught by Doppler radar was not his and his story might be the truth.

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