How the Officer Measured Your Speed

Most speeding tickets are for violation of the absolute speeding limit, meaning you were driving faster than the posted speed limit. The most important thing to find out is how the officer determined your driving speed.

Often times, the means by which a police officer measured your speed can be contested in court. If a flaw can be found in the determination of speed, then the case can be dismissed and the ticket will be defeated.

The most common forms of measuring speed are pacing, radar, aircraft detection, laser and VASCAR. Each of these have weaknesses that can be attacked in court.

Speed Detection Methods


How does it work?

Pacing means that the officer drives behind you and accelerates until his speed matches yours and the gap between the cars is no longer widening. That officer then determines that your speed and his are the same. The officer will follow the target vehicle for a distance of at least two-tenths of a mile. To obtain an accurate speed-reading, the officer should be behind the target vehicle and maintain equal distance between the police cruiser and the target vehicle. Once this is accomplished, by looking at the cruiser speedometer, the officer can determine the target vehicle speed.

Can it be inaccurate?

Pace cases are more prone to human error as they rely upon the officer’s ability to multitask and accurately determine a target vehicle’s speed. Flaws with a pace case may include the officer determining a target vehicle’s speed while accelerating to catch the vehicle. The officer may make a mistake while conducting the pace while parallel to the target vehicle as opposed to behind and not conducting the pace for an adequate distance to obtain an accurate speed-reading.

Another unique problem with pace cases happen more often at night when only headlights are visible. A natural response from a driver may be to increase speed in response to the officer’s close proximity of the vehicle.

Aircraft Detected

How does it work?

Aircraft detected speeding violations work by clocking the time it takes a vehicle to travel between two fixed points on the ground. Two marks are placed on the highway. One at the beginning and one at the end of a marked and measured area. A vehicle is observed first, to be travelling at a rate of speed higher than the surrounding traffic. Once the vehicle crosses the “beginning” mark, a stopwatch like device is triggered to measure time. When the vehicle crosses the “end” point, the time is switched off. Basically, the amount of time it takes to pass between the marks is calculated and an exact speed is immediately available.

Can it be inaccurate?

There are a few disadvantages to airplane speed detection, which can work to your benefit in court. Usually, the officer uses the airplane to pace the vehicle on the ground to obtain their speed. You can point out to the court that airplane speeds are measured in air speed, which is relative to the surrounding air. If the airplane is traveling into the wind, the speed is slower than if the aircraft was producing the same amount of power with a tailwind. It may be difficult to determine whether it was actually your vehicle that was spotted from the air, since many cars look alike from such a great distance.

Another problem is that this system relies on two different officers. Both officers need to be present in court for a conviction. If both officers show up at your hearing, you can request that one of them be removed so that each may be questioned individually. If their testimony contradicts one another, this may show cause for reasonable doubt.


How does it work?

VASCAR (Visual Average Speed Computer and Recorder) is a small computer that will calculate the speed of a vehicle based on the time it takes to travel a specific distance. It is usually hooked into the speedometer of the patrol car. An officer may pass you on the highway at a significantly higher rate than you are traveling. A few miles down the road, the officer may be waiting for you. He knows the exact distance he has traveled, and the distance that you have traveled. Therefore, he can compute your speed and issue you a speeding ticket. In some states, this is considered a speed trap.

Can it be inaccurate?

Police officers must trigger switches that start and stop the timing devices. Device error, often blamed on the officer using a unit, is often brought up as a defense when the motorist challenges the charge. Some of these devices do not have Electro Magnetic Compatibility certification; therefore, it is possible that any device, which produces radio waves, such as mobile phones, pagers, radio and television transmitters may interfere with them.

In addition, the officer uses a visual estimate when starting and stopping the timing device. They are relying on their ability to visually estimate speed. This can be disproven by the following theory. Take an object and hold it straight out at arms length from you shoulder. Drop the object and ask the officer to tell you how fast the object was traveling before it hit the ground. You can make the test more difficult by using two different items, a heavy one and a light one and repeat the test. If you receive two different answers, you know he is guessing because all items fall at the same rate of 32 feet per second squared, regardless of their weight.


How does it work?

Radar is able to pick up your speed by sending out a radio wave signal and then waiting for it to be reflected back to the machine. It uses a man-made pulse of radio energy to map distance based on the length of time it takes the pulse to return from the source. Radar is based on the principle of sending long wavelength radiation from an antenna and then detecting that energy after it bounces off a remote target. When the signal comes into contact with your moving vehicle, the radar frequency will suddenly change in keeping with the speed your vehicle is traveling at the time that the radar made contact. Thus, radar can be an accurate way of measuring driving speeds.

Can it be inaccurate?

Some defendants have successfully challenged radar readings in court. One inherent weakness of radar is that it typically does not distinguish a specific vehicle. Radar transmits a radio frequency, which, when passed through, is used to determine vehicle speed. If multiple vehicles enter the stream at the same time, it becomes the officer’s challenge to determine which vehicle is registering a particular speed with the radar device. For this reason, officers usually create a visual track history of the vehicle prior to its entry into the radar stream. However, if the officer is not paying attention, this may become a weakness to the prosecution’s case.

As with any mechanical device, after time, its accuracy may become compromised. They must be frequently checked to ensure that there is no flaw with the device. A key to challenging a case involving the use of radar is in the officer’s procedure or documentation relating to the calibration of the radar. All officers must be certified to issue tickets using a radar device.


How does it work?

Another way that radar may operate is with the use of laser light. The abbreviation for the laser units is LIDAR, which stands for (light detection and ranging). LIDAR is different from conventional radar in that is uses laser light to detect vehicle speed and measures the distance from the gun to the target several times. It can calculate the speed of a passing vehicle from the change in distance. Light from a laser speed gun travels faster than the speed of sound.

Laser technology has the benefit of being vehicle specific. The officer will maintain a fixed position off the road and will aim the device at a specific vehicle with the assistance of a viewfinder on the LIDAR device. It then measures the distance of the vehicle from the point of the device through laser technology and then by using simple mathematical formula or rate x time = distance, calculates the speed of the vehicle measured. The device will typically show two readings: the vehicle’s speed and the distance measurement of the vehicle from the device.

Can it be inaccurate?

Distance measurement pertaining to the accuracy of LIDAR may become compromised at greater distances. Like radar, LIDAR requires frequent checks to make sure it is functioning properly so as to avoid wrongful convictions. While it can be an accurate device, it is not without its flaws. One of the keys to success in challenging a LIDAR case includes the careful scrutiny of the documentation provided by the officer showing the accuracy of the device.

The laser beam is far less scattered than the radar frequency, and the beam can lock onto the target immediately. The police usually target the license plate of the vehicle. The accuracy can rarely be disputed. Police claim that these systems serve as a deterrent against traffic violations. In addition, they are a good source of revenue and don’t cost much to maintain the system once it’s installed.