Fighting a Speeding Ticket When the Cop Used Radar

Contesting a speeding ticket when police measured the driver’s speed with a speed detection device.

Police use different types of speed-measuring devices (radar, LIDAR, and VASCAR) to enforce speeding laws. But even if a cop clocks you going over the speed limit with one of these devices, you might still have some defenses. Improper maintenance and user error can affect the accuracy of an officer's speed measurement. This article provides an overview of common speed-measuring devices and ways to challenge their accuracy.

Types of Speed-Measuring Devices

Generally, a speeding ticket involves the use of one of three types of speed-measuring devices: radar, LIDAR, or VASCAR. To challenge a speeding ticket, the motorist or attorney needs to know which device the officer used to measure speed and the basics of how that device works.


Radar ("radio detection and ranging") sends radio waves that reflect off the target vehicle and return to the unit. The device calculates the speed of the vehicle based on the length of time it takes the radio waves to return to the unit from the target vehicle.

Radar devices can be handheld, portable units. But most radar devices are mounted inside the patrol car. Radar can measure the speeds of drivers while the officer's patrol car is moving or stationary.


LIDAR ("light detection and ranging") devices emit pulses of infrared laser light that reflect off of the target vehicle and return to a receiver in the unit. The device measures the distance between the LIDAR and target vehicle based on the time it takes the light to travel back to the device. To determine the speed of the target vehicle, the device calculates the change in distance.

Lidar devices are often referred to as "laser guns." Using the sight, the officer aims the device at the target vehicle and pulls the trigger to measure its speed. Lidar devices can't be used in a moving patrol car.


A VASCAR ("visual average speed computer and recorder") device is an electronic stopwatch that's connected to the speedometer of a patrol car. It measures the speed of a vehicle based on the time it takes the vehicle to travel between two points.

The officer measures the distance between two preselected points. When the target vehicle passes the first point, the officer starts the stopwatch. The officer presses the stop button when the target vehicle passes the second point. To calculate the target vehicle's speed, the device divides the distance between the two points by the time it took the target vehicle to travel that distance.

Contesting the Accuracy of a Speed-Measuring Device

Speeds measured by radar, LIDAR, and VASCAR devices are generally more accurate than an officer's visual estimation of speed. However, speed-measuring devices aren't infallible, and a driver can make various challenges to their accuracy.

Incorrect Calibration or Lack of Maintenance

Speed-measuring devices generally require regular calibration and maintenance to provide accurate measurements.

Jurisdictions vary regarding how frequently speed-measuring devices must be calibrated. Generally, a certified technician must calibrate the device every 60 days to three years. Some states also require officers to test the speed-measuring device at the beginning and end of each shift. When a speeding ticket is based on radar, LIDAR, or VASCAR, a driver might try to show that the device produced an inaccurate reading because it wasn't properly calibrated.

Other maintenance issues can also lead to inaccurate measurements. For example, incorrect speed calculations can occur if the lens of a LIDAR device is dirty or scratched or the sighting system is misaligned. Because the laser beam is invisible, there's no way of knowing whether the device is aiming accurately at the target vehicle, unless the officer frequently performs a sight alignment test.

Operator Errors

All speed-measuring devices rely on an officer's input to some extent—so, human error is a possible cause of erroneous speed measurements.

Lack of training. To produce accurate information, officers must know how to operate the speed-measuring device correctly. In most jurisdictions, officers must receive training and certification to operate radar, LIDAR, and VASCAR devices. If an officer hasn't received the required training or certification, a driver might be able to successfully argue that an elevated reading occurred as a result of the officer's error.

Wrong identification of target vehicle. The most common radar operator error is misidentifying the target vehicle. In other words, the officer inadvertently clocks the speed of a vehicle that's next to the target vehicle. As the radar beam travels, it quickly increases in width, encompassing multiple lanes of highways and adjacent roadways. If other vehicles are within the radar's beam, the device can't identify the specific vehicle that's responsible for the speed calculation.

The likelihood of mistakenly calculating the speed of a nearby vehicle increases if the other vehicle is larger than the target vehicle because the radar device measures the speed of the vehicle with the most dominant reflective surface. Misidentifying the target vehicle is also more likely to occur in heavy traffic and as the distance between the radar and target vehicle increases.

Because the laser light beams used in LIDAR devices are narrower and spread less over a given distance than radar beams, the possibility of mistakenly measuring the speed of a nearby vehicle is reduced—though not entirely eliminated.

The officer's visibility and reaction time. An accurate VASCAR measurement relies on the visibility and reaction time of the officer operating the device. The officer must start and stop the device when the target vehicle passes the two points. If the officer's reaction time is inconsistent when the target vehicle passes each point, the speed calculation can be incorrect. Inaccurate speed calculations can also occur if the officer has a bad vantage point. For example, an officer who is too far away from or has an obscured view of the target vehicle and the two points might have problems using VASCAR accurately.

Incorrect aim or movement of a LIDAR device. To operate properly, LIDAR requires the infrared light to reflect back to the device. Thus, the laser must be pointed at the flat and reflective surfaces of the target vehicle, such as the license plate or headlights. If the laser is pointed at a non-reflective surface of the target vehicle, the device can produce an inaccurate reading.

Also, LIDAR devices must remain stationary to measure speed accurately. If the officer moves the device, even slightly during operation, the light beam can reflect off different parts of the target vehicle, resulting in an incorrect reading.

Interference with the Device

Another common challenge to speed-measuring devices involves arguing that the device produced a false reading because of some kind of interference.

Weather conditions. Clouds, fog, precipitation, wind, temperature, humidity, and other weather conditions can affect the accuracy of radar and LIDAR devices. Weather conditions can also interfere with a speed calculation based on VASCAR if conditions limit the officer's visibility of the target vehicle.

Reflective surfaces. Reflective surfaces, such as road signs and large trucks near the device or target vehicle can interfere with radar and LIDAR speed measurements.

Radio frequency interference. Power lines and transformers, radio transmitters, and neon lights can affect the operation of radar and LIDAR devices.

Heater or air conditioner fan. A patrol vehicle's heater or air conditioner fan can interfere with a radar speed calculation by either reading the speed of the fan or picking up the electrical signals from the fan motor.

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