Key Takeaways from NHTSA Radar Manual: Cosine Effect
Written By: Benjamin Goldman, Esq.
Radar devices will only measure the exact speed of a target vehicle when it is traveling directly at or straight from the radar device. Obviously, there are some situations where it would be unsafe for a law enforcement vehicle to be driving straight at you. For safety reasons, you will usually see law enforcement vehicles parked on the side of the roadway. Depending on the angle of their vehicle, there may very well be a cosine effect present. A cosine effect can contribute to an inaccurate reading of a vehicle’s speed.
If an angle exists, this effect can occur. The cosine effect starts at around 10 degrees, and increases as the angle gets larger. At a 90-degree angle, the radar device will not see a target vehicle speed because the motion of the target is not moving towards or away from the device. In fact, a car driving in circles around a radar device would not be picked up at all. For this reason, operators are taught to position their cars around 10 feet from the edge of the roadway for every 100 feet that a target vehicle is down the roadway. The antenna needs to be positioned as straight as possible. If you have a commute, you will probably see law enforcement officers set up at the same spots over and over again. These spots are usually long, flat stretches of road that allow for the best technical readings.
The cosine effect can be further seen in situational settings such as an overpass, or curved and windy roads. A curve in the road could cause the target vehicle to approach the law enforcement vehicle at a significant angle resulting in an inaccurate vehicle speed. Overpasses can interfere with the radar beams. An improper target speed can also result if the signal is received from some roadside object that is at an angle to the patrol vehicle’s path of travel. This can interfere with vehicle speed because the radar incorrectly interprets the patrol vehicle’s speed. If there are any situational settings that you feel your attorney should be aware of, feel free to let them know.