Who taught YOU to drive?

How reliable are radar guns?

A police radar gun clocks a car at 33 miles per hour in a 25 mph zone in Medway. Angles can affect readings. A police radar gun clocks a car at 33 miles per hour in a 25 mph zone in Medway. Angles can affect readings. (Wendy Maeda/Globe Staff)
By Peter DeMarco
August 17, 2008
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How reliable are the radar guns police use? It's a question probably everyone who's ever gotten a speeding ticket might ask. Reader Diane McNamara of Boston sure has.

"I was recently stopped on the Riverway in Jamaica Plain and - despite the trooper's allegations - am absolutely positive I wasn't going 46 in a 25-miles-per-hour zone," McNamara wrote to me last winter.

"My car shakes frantically when I hit 35 miles or so. Sad, but true - and my mechanic can attest to this," she continued. "How accurate is this process when they record your speed at 347 feet away (that's what my ticket says)? Is there any way they can be wrong - and what do you do if you know - without a doubt - you weren't going that fast?"

I got to wondering myself recently about radar guns, too. Driving on a lonely road in Bridgton, Maine, a few weeks ago, I blew past an officer going the opposite direction I was. I was clearly going faster than the posted 30 mile-per-hour limit, so much so that I didn't even wait for the officer to put on his blue lights. I simply pulled over and waited for him to arrive.

He let me off, possibly because of my unprecedented cooperativeness. But what surprised me was that he said he clocked me going 47. How did he know my exact speed if we were going in opposite directions, I wondered? Don't radar guns need to be stationary to produce accurate readings?

Decatur Electronics, of Illinois, is the nation's oldest radar-gun producer, and still one of the largest. So I called them up to ask how speed guns work. As for appealing speeding tickets, alas, I can't offer you much encouragement. But I'll give you the pointers I have.

How guns work
Radar, short for radio detection and ranging, came into its own just before World War II, with police radar traps arriving in the 1950s. The guns in use today are greatly advanced, of course, giving readings down to 1/100th of a mile. In other words, you're clocked not just at 51 miles per hour, but at 51.48 miles per hour.

Before we get to the question of accuracy, a bit about how radar works, as explained by Kevin Morrison, a radar expert and public safety product specialist with Decatur, and the reputable

When an officer presses the button on a radar gun, the gun emits a radio frequency toward your car. The frequency hits your car and bounces back to his radar gun.

Frequencies are measured in "cycles per second." If your car is approaching the radar gun, the number of cycles per second will increase when the frequency bounces back. The faster your car is moving, the greater the increase in the number of cycles. A computer in the radar gun records the change, plugs it into a mathematical formula, and comes out with your speed in miles per hour.

Pointed straight
To get the best bounced signal, though, the radar gun should be pointed directly at your car. Since an officer can't stand in the middle of the road, hugging the roadside has to do, which translates into a 10 or 15 percent angle. If you're a speeder, having the gun pointed at such an angle actually helps you.

"If the police officer is parked on the corner of a curve, and the car is coming around the curve, it will actually register a lower speed," said Jake Tolbert, spokesman for Decatur.

A police officer positioned along the left side of a multilane highway will get a slightly slower reading for a car in the far right lane than it will for a car in the far left lane because the gun has to be held at a slightly wider angle. (The difference is only a mile or two, but it's there.)

A radar gun can pick up your car's speed from a considerable distance away, but most of the time an officer will position himself or herself about 1,000 feet down the road. Why? As Morrison explains, when a speeding ticket is questioned in court, the officer can't simply rely on the number that popped up on the radar gun. He must also testify that he visually estimated you going 65 or 75 or 105.

"When a police officer is sitting on the ground, roughly 1,000 feet is as far as they can see to judge a target," Morrison said. "If I can get on a plane or a hill looking down, I can stretch that out to 1,500 feet."

Radar beams can't go over hills or around curves (or at least, can't with any accuracy), so police tend to set up speed traps where the road is long and straight, as you've probably noticed.

As for my example of the officer clocking me while he was moving, well, that's pretty normal, Tolbert said.

"The radar just calculates the difference between the patrol speed and what the target's speed is," he said.

In part two of speed guns next week, we'll address Diane McNamara's question of whether speed guns are foolproof.

What drives you nuts? Is there a traffic rule you've always wondered about, or a pet peeve that never fails to annoy you? Tell City Weekly about it at and we'll check it out.

What drives you nuts? Is there a traffic rule you've always wondered about, or a pet peeve that never fails to annoy you? Tell City Weekly about it at and we'll check it out.

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