How Do Police Determine Your Speed?

Speeding tickets are, by far, the most common moving violation. If you want to fight your ticket, you should find out how your speed was determined.

Speeding tickets are, by far, the most common moving violation. If you want to fight a speeding ticket, there are two things you must know first:

This article focuses on how police measure drivers' speed. The most common methods are explained in more detail below. However, not all methods are allowed in all places—it depends on the laws of your state.

Speed Radar

Lots of speeding tickets involve the use of radar measurement systems because it's generally a reliable and straightforward method for measuring vehicle speed. However, despite their general reliability, radar devices aren't infallible.

How Do Police Radars Work?

The word "radar" is an acronym for "Radio Detection and Ranging." In simple terms, radar uses radio waves reflected off a moving object to determine its speed. With police radar, that moving object is your car. Radar units generate the waves with a transmitter. When they bounce back off your car, they are picked up and amplified by a receiver so they can be analyzed. The analysis is then reflected in a speed-readout device. Radar systems use radio waves similar to those involved in AM and FM radio transmissions, but with a higher frequency—up to 24 billion waves per second as compared to one million per second for AM radio.

Two Types of Radar Used by Police

Typically, the radars police use are one of two types: car-mounted units that can be operated while the officer's vehicle is stationary or moving and hand-held radar "guns."

Car-Mounted Units

Most radar units used in patrol vehicles are shaped something like a side-mounted spotlight. They are usually mounted on the rear left window of the police car facing toward the rear. The officer reads your speed on a small console mounted on or under the dash. The unit has a digital readout that displays the highest speed read during the second or two your vehicle passes through the beam.

Most modern police radar units can also operate in a "moving mode," allowing the officer to determine a vehicle's speed even though the officer's own patrol vehicle is moving.

Hand-Held Radar Units

Hand-held radar guns are most often used by motorcycle officers. Radar guns use a trigger system. So, the officer just pulls the trigger when he or she wants to measure a vehicle's speed.

When Radar Becomes Inaccurate

Most radar errors result from the radar's operation in real-world conditions, which are often far less than ideal. And, of course, human error can also cause radar devices to fail.

More Than One Car Is Speeding

Radar beams are similar to flashlight beams—the farther the beam travels, the more it spreads out. And this simple fact often results in bad speed readings because a spread-out beam can hit two vehicles in adjacent lanes (or that are otherwise near each other).

In other words, if you're in one lane and a faster vehicle is in another, the other vehicle will produce a higher reading on the officer's radar unit, which the officer may mistakenly attribute to you.

Most radar units have a beam angle, or spread, of 12 to 16 degrees. So, the beam width will be about two lanes wide (approximately 40 feet) at a distance of only 160 feet from the radar gun.

The inability of the equipment to distinguish between two separate objects is called "a lack of resolution."

A few factors can make this kind of error more likely. Resolution problems are more likely to occur if the other vehicle is larger than yours simply because the other vehicle has a greater surface area. And, automatic radar units (or those set to automatic mode) tend to produce this type of error more frequently than units that the officer manually turns on and off such as with a trigger system.

Wind, Rain, and Particles in the Air

Although metal reflects radar beams better than most surfaces, pretty much any material will reflect radar waves to some extent. In fact, on windy days, windblown dust or even tree leaves can be picked up by radar devices. The same is true of rain, snow, and the like. Sometimes these spurious readings can be attributed to your vehicle.

Pre-thunderstorm atmospheric electrical charges can also interfere with a radar unit. This interference occurs when electrically charged storm clouds reflect a bad signal back to the radar unit even though they are high in the sky. If such a storm cloud is traveling at a sufficient speed, a false radar reading can result.

Radar Calibration Problems

Every scientific instrument used for measuring needs to be regularly calibrated to ensure its accuracy. Radar equipment is no exception. It must be checked for accuracy against an object traveling at a known (not radar-determined) speed.

Calibration of a radar unit typically involves using a tuning fork as the moving object. Tuning forks are supplied by the manufacturer of the radar equipment and certified to correspond to the speed marked on the fork. According to most operation manuals, a radar unit should be calibrated with the tuning forks before and after every shift. Ideally, several tuning forks vibrating at different speeds should be used to check the radar unit's accuracy.

It is time-consuming to use a tuning fork as a calibration device. So a second—but far less accurate—method has been developed to check the accuracy of radar units. This second method is a "calibrate" or "test" switch built into the radar unit itself. The unit reads a signal generated by an internal frequency-generating device called a "crystal." The resulting number is supposed to correlate with a certain predetermined speed. Unfortunately, these internal calibrating systems don't work as well as they're supposed to.


Most states allow police officers to catch speeders using a technology called VASCAR ("Visual Average Speed Computer and Recorder"). VASCAR is basically a stopwatch coupled electronically with a calculator. The calculator divides the distance the target vehicle travels (as recorded by the stopwatch) by the time it took to travel that distance.

How VASCAR Works

VASCAR is not like a radar or laser gun, which gives a readout of a vehicle's speed by simply pointing and pulling the trigger. A VASCAR unit requires far more human input than radar or laser guns—which greatly increases the possibility of mistakes.

VASCAR works like this: The officer measures the distance between the two points—typically, by using the patrol car's odometer, which is connected to the VASCAR unit. When the officer sees the target vehicle pass one of two points, the officer pushes a button to start the electronic stopwatch, then pushes it again to stop it when the vehicle passes the second point.

How VASCAR Fails

Using VASCAR correctly isn't easy. For example, it is no easy thing to accurately push the "time" and "distance" buttons while observing the target pass between two points, at least one of which is almost sure to be far away from the officer. And, of course, doing this accurately is even harder when the patrol car is moving.

The most common three mistakes that can cause error in a VASCAR measurement are:

  • the inability of the officer to accurately see when a distant car passes a distant point
  • the officer's reaction time (how long it takes him or her to push the button when a car passes a marker), and
  • the accuracy of the odometer on the officer's car.

Generally, these errors become more pronounced and lead and lead to greater inaccuracies in the final speed estimate when the distance between the two passing points is small. For example, there's less likely to be significant inaccuracy using VASCAR if the measured distance is something like 1,500 feet than for a much shorter distance like 500 feet.

Because VASCAR accuracy can depend so heavily on the officer's reaction time, it's crucial to know the distance over which the officer clocked you. You may be able to obtain this information from the officer prior to the court date by requesting it through a process called "discovery."

Laser or "LIDAR" Speed Measurements

Laser detectors are the most recent addition to the traffic officer's arsenal of speed-measuring devices.

How LIDAR Works

Built to look and act like a hand-held radar gun, a laser detector uses a low-powered beam of laser light that bounces off the targeted vehicle and returns to a receiver in the unit. The unit then electronically calculates the speed of the targeted vehicle. Laser detectors are supposedly more accurate than radar units.

Advantages of LIDAR

One advantage for police officers of the laser gun is that the light beam is narrower than a radar beam, meaning that it can be more precisely aimed. This is true even though laser detectors use three separate beams because the combined width of the three beams is still much narrower than a single radar beam at the same distance. This technology reduces but does not eliminate the chance that the speed of a nearby car will be measured instead of the speed of the car at which the operator aims the gun.

How LIDAR Fails

Laser detectors measure distance (between the gun and the target car) using the speed of light and the time it takes the light, reflected off the target vehicle, to return to the laser gun. The detector makes about 40 of these distance measurements over a third of a second, then divides the light's round-trip distance by the time, to get the speed. This means to be accurate the officer must hold the combined beams on the same part of the car during the test. While this is easier to do with radar because of its wide beam, it is tricky to do this with a narrow laser beam. Also, it's impossible to be sure that the officer has been able to accomplish this feat because the officer can't see the beam.

It's also possible (especially in heavy traffic) for one beam to hit the target car and another beam to hit a nearby car. The chances of this happening increase with traffic density and the distance between the laser unit and the measured vehicle.


Many speeding tickets result from the police officer following or "pacing" a suspected speeder and using his or her own speedometer to estimate the suspect's speed.

How Pacing Works

With this technique, the officer must maintain a constant distance between the police vehicle and the suspect's car long enough to make a reasonably accurate estimate of its speed.

Some states have rules requiring the officer to verify speed by pacing over a certain distance. (For example, at least one-eighth or one-fourth of a mile.) In practice—even in states that don't require pacing over a minimum distance—most traffic officers will usually try to follow you for a reasonable distance to increase the effectiveness of their testimony, should you contest the ticket.

How Pacing Fails

The accuracy of pacing depends on the officer's ability to maintain a constant distance from the target vehicle. So any facts that might prevent the officer from doing so can lead to an inaccurate speed estimate. For example, where an officer is a long way back from the target vehicle, it's more difficult to maintain a constant distance. Hill, curves, and traffic can also make it hard for the officer to keep a constant following distance and lead to an inaccurate speed estimate.

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