Speedometers don’t always match radar feedback signs

Another radar feedback sign has gone up in Everett, this time on Rucker Avenue just north of 19th Street. There are others, such as on Broadway as you enter downtown from I-5, and on Colby Avenue.

The equipment shows drivers how fast they are going, and includes a sign showing the legal speed limit for the roadway.

A handy reminder, but Street Smarts reader Dan Brown noted that none of the signs seem to match his own speedometers in three different vehicles.

How accurate are these signs? And should they be calibrated or updated to keep them accurate?

City traffic engineer Tim Miller said the signs are for informational purposes to encourage people to follow the law. The calibration is set at the factory and left as-is.

But Miller also added some interesting math and science to explain why our speedometers often differ from other speed-measuring devices, including newer GPS units.

“Most car speedometers are accurate to only plus or minus 2.5 mph which may explain some of the variation Mr. Brown reports,” Miller wrote. “Another aspect is when tires are replaced if they differ in effective rolling circumference from the original equipment, that can also introduce speedometer error. Finally, as a car gets nearer to the radar speed feedback sign, the speed reported will drop slightly due to cosine of the angle between the car and the sign.”

The radar equipment that local police use, by the way, is far more accurate than these feedback signs.

The sign uses a narrow beam width, which means it sometimes picks up more than one vehicle within its radar beam. Police equipment have “pinpoint accuracy” to focus on only one vehicle at a time, as well as a much farther range than the signs, Miller wrote.

Everett Police officer Aaron Snell added that the police equipment is certified and calibrated.

Have a question? Email us at streetsmarts@heraldnet.com. Please include your name and city of residence. Look for updates on our Street Smarts blog.

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