Speedometer clocks were one of the first methods developed for identifying drivers who exceed the speed limit. Speedometer clocks are still used today, although they are not one of the more popular methods and have gradually been replaced by more technologically advanced methods. However, they are the least expensive way to track the speed of drivers and are usually quite effective.
This method is also referred to as the “pacing method,” a technique requiring police officers to drive behind a vehicle, traveling at the same speed as the one in front of them and maintaining a constant distance. By looking at the speedometer in their cars, police officers can determine how fast a person is driving.
The pacing method is generally used at night, since during the day a driver will easily realize that a police car is following them and slow down before the officer can get an accurate reading.
To obtain an accurate reading, police officers must follow a motorist suspected of speeding for at least 3/16 of a mile. Also, because speedometers are not always accurate, the speedometers in police cars are calibrated regularly to ensure accuracy so that evidence can be used in court. Every car has its own logbook for officers to track and record each time the speedometer is tested and calibrated.
In some cars, drivers must account for as much as a 10% margin of error in terms of speedometer accuracy; tire wear and aftermarket modifications, such as different tire sizes and gearing, can cause erroneous speedometer readings.
Most new speedometers are believed to function with a 5% margin of error. However, the margin of error still causes accused motorists of arguing in court that they were, in fact, not traveling as fast as the reported speed. In 1997, the federal standards in the United States for speedometer readings allowed for a maximum 5% error.
The most important factor in the pacing method is a properly calibrated speedometer. There are several ways that a speedometer can be calibrated. Police speedometers are sometimes calibrated on a rolling road at regular intervals to ensure their accuracy.
Other methods include using a fifth wheel that is added to the rear of a vehicle, clocking the police car over a measured course using a stopwatch that has been tested and certified, or using a device called a dynamometer, which lets the wheels on the police car rotate in place while the device checks the speedometer for discrepancies. The latter is believed to be the best technique, but it is also the most expensive.