Radar has been used to monitor vehicle speed since the end of World War II. During the mid-1950s, radar became widely used with the building of the Interstate Highway system.  The best radar detectors today are designed to detect these radar guns.

Radar is an electromagnetic pulse generated by a transmitter, which sends the RF pulse down the highway to reflect off a moving vehicle. The reflection induces a very slight frequency shift, called Doppler Shift. The frequency shift can be analyzed to determine the true speed of a moving vehicle. The first oil embargo from the Arabian Peninsula hit the USA in the early 1970s. I remember as an Aviation Marine in 1974 standing in gas lines 20-30 cars long. We have not seen problems like that for 35 years. The result of that embargo was a national speed limit of 55mph, where nearly every state implemented this speed limit to conserve fuel. It was maddening to drive down a wide super highway, but limit your speed to 55mph. That killed off many of the high performance sports cars, for example the Mustang turned into a pathetic, less than desirable, passenger car.

Radar Guns became a standard use by police to monitor highway speed. X band at 10.525GHz was the first radar frequency used in the mid 1960s and this did not begin to change with any momentum until 1972, when radar started to move towards K band, 24.150GHz. Ka radar came into existence with Applied Concepts of Texas (of Stalker renown) with one radar frequency of 34.7GHz. Ka has since expanded to 33.7, 34.4, 34.6, 34.7, 35.5 GHz.

In the early 1990s the National Bureau of Standards (now called NIST - National Institute of Science Standards) looked at police radar guns and discovered speed detection accuracy and performance was inconsistent and more often than not, incorrect. 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. More recently, the International Association of Chief's of Police (IACP) took over responsibility to test and approve police radar guns to ensure accuracy. While today many radar guns are produced with superb design, there are still mistakes and operating errors. Perhaps 20% all radar tickets issued today are in error, either due to mechanical issues or operator errors.

Errors induced by technical anomalies are Shadowing and RFI Interference; while operators cause Cosine Error and weather itself also causes speed errors. Shadow Error occurs when the radar gun is aimed at a smaller moving vehicle (passenger car), which is in the same line of sight as a larger moving vehicle (a commercial freight truck or 18 wheeler). The two Doppler signals can merge into one and induce an error in the speed read out.

New technology using complicated math analysis can now differentiate these two speeds. Prior to 1995, that was not possible. Another form of Shadowing caused by a moving police car has been eliminated by connecting the police radar gun to the police vehicle speed. The Doppler signal is now more accurate because the speed of the police vehicle is used as a reference. RFI, Radio Frequency Interference is caused by using poor noise shielding in the radar gun design. Noise can be induced by a CB radio transmission or a police radio. In effect the poor design is permitting the radar gun to be jammed by false noise. For you CB guys out there, it does not work today! Neither do fish finders, spinning windmills, etc.

Radar Gun operators cause Cosine Error when they stand off to the side of the road; i.e, they are not in the direct path of the oncoming vehicle. The true speed is measured in the direct path. Thus, it is smart to get off to the side of the road or get your butt ran over for the sake of accuracy. It is a “No Brainer”. This cosine error is always in the favor of the driver, but having said that, the radar gun operator would have to be at an angle greater than 15 degrees to have any significant effect. At 10 degrees, the error would be less than 1%; at 60mph and a cosine angle of 10 degrees, the speed reading would be about 59mph. It is fair to say that a policeman standing on the side of the road, who waited until the last possible moment to get a vehicle speed, the angle would be significant and to your advantage.

So, police will almost always trigger the radar gun at 400 to 1000 feet to keep the Cosine angle as near zero as possible. At a Speed Measurement Labs test in 2004, we threw rocks into the air, and the radar guns detected a false speed.

Another version of this is Shadowing induced by a truck or car driving by a large roadside, metal sign. As the wind pressure causes the sign to vibrate (rapidly wobble in the wind), any radar gun transmitting to get the speed of the truck as it passes a roadside metal sign, would have a false Doppler induced by the RF reflecting off the metal sign vibrating rapidly in the wind turbulence. That is an argument any driver can use effectively, as long as the Judge will listen to a technical discourse. Listening is not necessarily one of their interests.