The use of police traffic radar is so widespread that we naturally assume the technology is reliable. However, radar makes mistakes, and many of them. Some experts estimate that 10-20% of all radar-backed speeding tickets are issued in error; and in the case of radar that is operated from a moving police vehicle the number of inaccurate tickets may be as high as 30%.¹

The Texas Department of Public Safety produced a comprehensive manual based on Federal tests. It cautions operators, "The radar operator [officer] must be familiar with situations that can produce 'error' readings." If the officer is not aware of these situations, a ticket can be wrongfully issued.

Here are the radar "errors" detailed by the Texas manual:²

1. Antenna Positioning Error

The radar beam travels in a straight line. It doesn’t bend around curves or follow the contour of hilly terrain. If the antenna is not properly positioned, it may seem to clock an approaching car when, in fact, it's clocking another car in the background (see illustration below).

2. Look-Past Error

Even if the officer aims his antenna properly, radar is still subject to the "look-past" error. This is caused by the radar looking past a small reflection in the foreground to read a larger reflection behind.
Texas instructors warn, "It is a widely held misconception that the reflected target signal received by the radar antenna will always be that of the closest vehicle to the antenna. There are times, due to traffic conditions, that the closest vehicle is not returning the strongest signal."
Evidence of the potential size of this error appeared in the October 1979 issue of Car and Driver magazine. The author measured the effective range of a Kustom Signals KR11 traffic radar against various vehicles. The typical small sedan did not show up on the radar until it was less than 1200 feet away from the antenna, but the same radar unit locked onto a Ford 9000 semi at 7600 feet. This shows how various vehicles reflect radar microwaves differently.
The Texas instructors confirm this problem, stating, "It is not unfair to say that the reading you register could be a larger, better target three-quarters of a mile down the road (see illustration below)."

3. Vehicle Interference Error

Vehicle Interference Error occurs when the police car is using radar while moving in traffic. For example, traffic ahead can confuse the radar's estimate of the officer’s patrol car speed. Moving radar calculates target speed by subtracting patrol car speed from the closing speed of the target. Therefore, anything that produces a low evaluation of patrol car speed will automatically result in a higher speed reading of target.
Texas tells its radar operators that this "...situation becomes more critical if the difference in patrol speed and interference-vehicle speed is 5-10 mph. A target vehicle moving 61 mph may be recorded at 66-71. These borderline speeds are more difficult to detect with the eye (see illustration below)."

4. Cosine Error

Cosine Error produces a result similar to Interference Error except no moving traffic needs to be present. A stationary object near the road, such as a building, road machinery or even a sign, makes a more efficient reflector than the horizontal pavement. Therefore the radar uses that reflection of the stationary object as the basis for the patrol car speed.
If this reflector were positioned straight ahead on a collision path, the patrol car speed estimate would be close enough. However, the further the object is located off a direct line of the target, the lower will be the estimate of the patrol car speed. This is a simple trigonometry problem relating to the cosine of the angle between the target and the ground reflector, hence the name Cosine Error. Since Cosine Error always makes the patrol car speed seems lower than it actually is, it always acts to raise the reading of target car speed (see illustration below).

5. Double-Bounce Error

Radar microwaves are easily reflected. That's what makes radar possible. However, the officer must be aware of the difference between an ordinary reflection and a “bad” bounce. Big objects such as trucks are very efficient reflectors and it's possible for the radar beam to bounce off several moving trucks at once, producing erroneous readings (see illustration below).

6. Beam-Reflection Error

Because radar microwaves are so readily reflected, Texas instructors recommend caution, even in mounting the antenna within the patrol car. They say it's possible that a reflective path can be set up through the rearview mirror that will produce radar readings on vehicles behind the patrol car when the radar is aimed forward. The vehicles behind can be either coming or going, since radar does not distinguish directions (see illustration below).

7. Road-Sign Error

The ready reflectivity of radar microwaves means that road signs are also a source of errors (see illustration below).

8. Radio-Interference Error

According to the Texas course, "UHF radio now in use can force radar to read various numbers when you transmit or just key the mike. Citizens band radio transmissions from within the patrol vehicle can cause ghosting (false readings)." It recommends that no radio transmissions be made while clocking target vehicles.
Radio or Microwave Interference can come in a variety of forms, both natural and man-made, but they have one thing in common - they produce a false or incorrect reading on the radar unit's display. Common sources of electromagnetic interference include airport radar; microwave transmissions; transmissions of CB, ham, VHF/UHF, and cellular two-way radio/telephones, including police and business radios; faulty spark plug wires; mercury vapor and neon lights; high-tension power lines; and high voltage power substations. The radio energy from these sources can overload or confuse the sensitive circuits in a radar gun.³

9. Fan-Interference Error

When the antenna is mounted inside the patrol car, the Texas course says, "Radar will have a tendency to read the pulse of the fan motor (air conditioner, heater or defroster)." The instructors go on to say that the fan reading will disappear when a target comes into range, and that the fan will not distort the speed reading of the target car.
However, in the case of moving radar they say, "Sometimes a steady fan speed will override patrol car speed reflected from the roadway." When this happens, the false speed reading produced by the fan will be substituted for patrol car speed in the moving radar's calculation of the target car speed. Since the calculation consists of subtracting the patrol car speed from the target speed, if the fan reading is less than the patrol car speed, then the speed displayed for the target will be incorrectly high. The Texas course offers no safeguard for this type of error.

10. Panning

Panning occurs when the radar beam accidentally sweeps past the counting/computing unit. this can happen only to a two piece radar unit. The radio energy from the antenna portion overloads or confuses the counting/computing circuitry.

In conclusion, the Texas Department of Public Safety notes "Radar cannot identify [the] speeding vehicle: [the] officer must do that."
There are four types of radar: military, air traffic control, weather, and traffic. Of the four, traffic radar is the most limited because of the following:

  1. Lowest cost: municipalities typically procure the lower cost units which means they lack many of the more complex features and are less powerful, thereby making them less effective

  2. Does not have sweep: it uses a stationary beam that shines either forward or backward but not both

  3. Does not have a modulated beam: it is unable to distinguish between multiple moving objects

  4. Does not have a radar screen: it only has a single digital readout thereby relying on the officer to determine which moving vehicle is being tracked

To this date there are no industry standards in effect and no government performance requirements for traffic radar. Each radar manufacturer adheres to their own non-published standards.⁴
You might now have a better understanding of how a Miami television station demonstrated a radar gun clocking a palm tree at 86 mph and a house at 28 mph. In the first instance the reading was caused by panning the radar antenna, and in the second the radar unit was measuring the fan motor in the patrol car.⁵

¹ Blind Trust published by RADAR (Radio Association Defending Airwave Rights, Inc.)
² The Texas Department of Public Safety and http://www.escortradar.com/errors.php
³ Blind Trust published by RADAR (Radio Association Defending Airwave Rights, Inc.)
⁴ www.escortradar.com/rdrprobs.php
⁵ Blind Trust published by RADAR (Radio Association Defending Airwave Rights, Inc.)