Page 3 - Radar and Laser Gun Mistakes
Although now thoroughly tested for accuracy by many police agencies, 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.