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Traffic Tickets- How Radar Detectors Get it Wrong

In the early 1970’s, the national maximum speed limit was reduced due to oil and gas shortages. Sometime thereafter, the first radar detector was invented. Since then, the battle between police an speedsters has led to technological advances.

In January of 1994, The National Highway Safety Administration (NHTSA) issued revised standards for radar gun accuracy and officer training. The “Model Minimum Performance Specifications of Police Traffic Radar Devices” standard was issued after the National Bureau of Standards looked at police radar guns and found dismal accuracy and performance results. The International Association of Chief’s of Police (IACP) now approve and test police radar and laser guns to insure accuracy.

We handle traffic tickets for our clients, largely as a courtesy for a minimal fee. Staying up on the technology is tough.

Here is a little insight from

Radar and laser guns and their operators still make mistakes. It is estimated that over 25% of all radar tickets are in error. The most noticeable and common mistakes include shadowing, RFI interference, cosine error and mechanical interference.

Shadow Error happens when the moving radar’s “Low Doppler” incorrectly locks onto a large metal object like an 18 wheeler in front of the patrol car and adds the speed differential to the opposite lane target vehicle’s speed. Low droppler is used to determine the patrol vehicle’s speed. Shadowing has and is being eliminated by interfacing the police radar gun into the vehicle’s speed sensor. This is known as VSS or Vehicle Speed Sensor interface. Now that the patrol car’s speed is obtained by the vehicle’s own speed sensor, the low droppler signal from the police radar gun can be compared and accuracy increased.

RFI stands for Radio Frequency Interference. Many poorly shielded radar gun’s speed readings go blank when a commercial radio or police radar is keyed up.

Cosine error is standard with both radar and laser guns. The greater the transmission angle of the gun to the target vehicle, the greater the error. However, the angle is always to the advantage of the driver. It always shows a speed less then the actual speed. An example would be a speed radar gun transmitting at a 10′ degree angle from the approaching target vehicle. The target vehicle’s actual speed is 60 mph but the radar gun shows 59 amps.

The third error is most troubling. In 2004 the Pennsylvania State Police purchased hundreds of new radar guns. They were clocking rocks at 70 mph. This is an example of mechanical interference as the police car’s heater/air conditioner fan was producing the erroneous speed reading. The fact remains, radar and laser guns still make mistakes. New Jersey Superior Court Judge Reginald Stanton issued a landmark ruling in 1996 stating that laser guns may not target vehicles past 1,000 feet due to the gun’s one millidadian beam divergence of 36″ at that distance might incorrectly target an adjacent vehicle.

When / Where are radar detectors illegal?

The use of a radar detector in a passenger vehicle is legal in all states with the exception of Virginia, Washington DC. and on military bases. Radar detectors are illegal in all commercial vehicles weighing over 10,000 pounds. In Canada, the only three provinces that allow their use are Alberta, British Columbia and Saskatchewan. Many overseas countries also ban their use, such as Portugal, Finland and Spain. In areas that radar detectors are illegal, law enforcement use a device called the Spectre that is able to detect the use of radar detector in your car or truck.

Some companies cell radar blockers. These Laser Jammers and Radar Jammers are illegal in the states of Nebraska, Minnesota, Utah, California, Oklahoma, Virginia, Colorado, Illinois and Washington DC.


Now you know the law and development of speed monitoring devices. The best bet is to slow down and be careful, but sometimes things happen. If that happens to you, call or email us. We will help fight or at least try and prevent points from being assessed.

John M. Phillips
Law Office of John Phillips