Training is key to proper radar gun use

Law officers are given training in how to use, and how to test, the accuracy of radar guns to avoid ticketing the wrong driver. (George Rizer/Globe Staff/File 2003)

Legends abound about police radar guns picking up mailboxes going 50 miles per hour. But does that really happen? It might, if the officer using the gun isn't properly trained on signal interference.

"I can point a radar detector directly at my air conditioner in my car and get a reading" from the fan, said Kevin Morrison, a public-safety product specialist with Decatur Electronics, the country's oldest maker of radar guns.

Interference

"Weather can cut down on the radar's range because rain obscures some of the radar signal," Morrison said. As a result, you probably won't encounter many speed traps in a downpour.

As mentioned, seemingly inanimate objects, such as your car's fan, can screw up the machine, too.

But police are (or should be) trained to watch out for such problems. The easiest way to check for interference is by listening to the high-pitched whistling sound the radar gun makes, Morrison said. If the sound, known as an "audio Doppler tone," rises and falls smoothly, there's no interference. "If it's broken and raspy, it's not a clear return. It's not a good signal coming in," he said. The officer should be able to testify in court about the clarity of the gun's sound.

But how does he or she know that you're the one speeding, as opposed to the car in front of you or next to you? Morrison said this is another reason why the officer must see you speeding, as the gun, even when pointed in your direction, may be registering someone else's speed.

"It depends on the reflectivity of the target. The gun gets all these signals at a time, and it picks the strongest signal," he said.

"I've had reporters ask, 'Doesn't that mean a semi gives a better return signal, and is more likely to be picked up by the gun?' Yep, it does. But they shouldn't be speeding, either."

An officer, given sufficient experience, should be able to visually estimate your car's speed within 5 miles per hour. If the gun reading isn't close to the visual estimate, the officer should know that the gun was picking up someone else's car, Morrison said.

Testing, testing

Diane McNamara, a Boston reader who groused about a speeding ticket, asked how often radar guns malfunction. The answer is, very infrequently. Police know this because they typically test a gun's accuracy every time they use it.

"Per State Police policy, troopers check the calibration on the units at the beginning and end of every shift, and periodically if needed," said David Procopio, director of media communications for the Massachusetts State Police. "If the calibration check determines that the unit is not working properly, policy mandates that the unit be taken out of service immediately until it is repaired."

The easiest way to test a machine, Morrison said, is with a simple tuning fork. Decatur (and other manufacturers) sell tuning forks to police departments that vibrate at a predetermined number of cycles per second. For example, a tuning fork that vibrates 4,320 times per second should produce a reading of 60 miles per hour when the radar gun is pointed at it. If it doesn't, something's off.

The State Police, in addition, has all of its speed guns calibrated annually by an outside company, Procopio said. (Local police departments have their own maintenance schedules.)

Morrison said that his company's radar guns, as well as those made by reputable competitors, are approved by the International Association of Chiefs of Police, which conducts testing on various models. The federal government won't provide any police department with funds to purchase guns that aren't IACP approved, he said.

What drives you crazy about local drivers? Send us a message about it: ciweek@globe.com.