Measuring Vehicle Speed with Radar

Despite the fact that it is older technology, radar is still frequently used by police to measure vehicle speeds today. When used properly, it is an accurate method of determining how fast a vehicle is traveling. The courts also accept qualified radar evidence of speed during a trial as commonplace.

When I was trained to use radar to measure traffic speed it was a one day long course. We were taught the basic theory of operation including an explanation of the Doppler Effect which is the basis for the device. A written test followed to insure we understood what had been taught. Finally, we all went to the side of the road where we were given a chance to make some measurements under the watchful eye of an experienced officer.

I typically started my traffic enforcement shift by testing my radar and recording the results of the test in my notebook. These tests vary a little depending on the manufacturer and type of radar in use, but it usually consists of a power on self test or an internal test initiated by pushing a button, a phase where all indicators and display segments were lit simultaneously to show they were functioning and a tuning fork test.

Tuning forks substituted for the moving vehicle. The fork was struck to make it vibrate and then held in front of the radar antenna. This would produce a specific reading on the radar display.

If and only if all of these tests were passed was the radar considered ready for use. If there was a failure the unit was taken out of service and sent for repair.

During some 28 years of operating traffic radar I can only recall one instance when the radar failed to operate correctly and it was immediately apparent to me.

A typical investigation involving radar to measure vehicle speed begins not with the instrument, but with the officer's eyes. A visual observation of the target is made and a speed estimation developed. Some officers become quite accurate in making this estimation after years of practice with the instrument.

Following the estimate, a measurement of the vehicle speed is made with the radar. The officer compares the estimate with the measurement to insure that the two reasonably coincide. If they do, the offending driver is stopped and ticketed. If they don't, further observation and measurement is required.

Should the visual estimate and radar measurement never reasonably compare, a ticket based on the radar evidence cannot be written.

A radar beam is similar to a flashlight beam. It begins relatively narrow but widens as you move away from the antenna. Ideally, only the target vehicle should be in the radar beam at the time of the speed measurement, but this is not always possible. In this case, careful observation and measurement may still result in an accurate measurement and confidence in which vehicle is producing the speed reading.

Radar measurements also suffer from what is known as cosine error. If the vehicle being measured is moving directly toward the antenna, a true speed will be detected. If the vehicle is moving at an angle to the beam, a lower than true speed will be read depending on the cosine of the angle.

The benefit goes to the driver with stationary radar operations.

The cosine error is critical with moving radar as it affects the patrol vehicle speed reading which is used to calculate the violator's speed from the closing rate of speed. The officer must compare the patrol vehicle speed to the speedometer when making a measurement. If the two are not the same, a higher than true speed will be displayed.

If all of this adds up, the speed investigation is complete and the officer can decide on what, if any, action to take.

The final step in my daily patrol after parking in the detachment lot was to test the radar again and record the results in my notebook.