Fight Your Speeding Ticket: Determining 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 five most common methods are explained in more detail below.

However, not all methods are allowed in all places. California, for example, forbids the use of timing devices over fixed distances, outlaws VASCAR, and forbids radar on some roads. In Pennsylvania, only the state police—not local law enforcement—can use radar, and VASCAR can be used only if the measured speed exceeds the posted speed limit by 10 miles per hour or more.


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

Here are some things to consider for fighting a speeding ticket based on pacing:

  • Road configuration may help prove inadequate pacing. Hills, curves, traffic, interchanges, traffic lights, and stop signs can all help you prove that an officer did not pace you long enough. For example, an officer following your vehicle a few hundred feet behind will often lose sight of it through a curve. Similarly, if you were ticketed within 500 feet of starting up from a stop sign or light, it could case into doubt whether the officer can prove having paced your car for a reasonable distance.
  • The farther back the officer, the less accurate the pace. For an accurate "pace," the officer must keep an equal distance between the patrol car and your car for the entire time of the pacing. The officer's speedometer reading, after all, means nothing if the officer is driving faster than you are in an attempt to catch up. To avoid this problem, officers are trained to "bumper pace" your car by keeping a constant distance between the patrol car's front bumper and your rear bumper. Bumper pacing becomes more difficult the farther behind the officer is from your car. (The most accurate pace occurs where the officer is right behind you.) But patrol officers like to remain some distance behind a suspect to avoid alerting a driver who periodically glances at the rearview and side-view mirrors. So if you know an officer was close behind you for only a short distance, your best tactic in court is to try to show that the officer's supposed "pacing" speed was really just a "catch-up" speed.
  • Pacing at dusk or nighttime. Pacing is much more difficult in the failing light of dusk or in complete darkness, unless the officer is right on your tail. In the darkness, the officer's visual cues are reduced to a pair of taillights. Also, if an officer paces a speeder's taillights from far back in traffic, he or she might have trouble keeping the same pair of taillights in view.

Which pacing it defense might be best just depends on the situation. But the basic idea behind all these defenses is to point out to the judge that, under the circumstances existing at the time, the officer's use of pacing could not have produced a reasonably accurate estimation of the driver's speed.

Aircraft Speed Detection

There are two ways an aircraft officer determines your speed. The first is to calculate your speed by timing how long it takes for your vehicle to pass between two highway markings at a premeasured distance apart. The second involves a kind of "pacing" of the target vehicle, but from the aircraft. The pilot uses a stopwatch to time its own passage over highway markings that are a known distance apart. Then the aircraft is used to pace your vehicle's speed.

Under both methods, if a car is found to be speeding, a waiting ground patrol car is radioed to issue the ticket. If that ground patrol officer independently verifies your speed with a reliable methos such as radar, it generally diminishes the chances of beating the ticket. When the ground unit measures your speed, the government no longer has to rely on the less accurate aircraft speed measurement to prove the violation in court.

Here are some things to consider for fighting an aircraft speeding ticket:

  • Ask for dismissal if either officer fails to appear. If both officers are not in court, ask the judge to dismiss the case. If the prosecution tries to introduce an absent officer's police report or other written record into court in place of live testimony, simply object on the basis that it is hearsay. Without an officer present, the written report is inadmissible hearsay testimony.
  • Stopwatch and reaction-time error. If the officer's timing is not performed properly from the aircraft, the speed measurement of your vehicle won't be accurate. Since this speed is calculated by dividing distance by time, the shorter the distance your speed was measured over, the more likely it is that a timing error will result in a too-high speed reading. For example, if the officer hesitated even slightly before pushing the timer as you passed the first ground marker, the measured time would be shorter than the true time your vehicle took to traverse the distance to the second marker.
  • Difficulty in keeping your car in view. If two markers are a mile apart, it takes a car doing 75 miles per hour about 48 seconds to travel between the two markers. It's hard to stare continuously at anything for that long, especially from a plane. If many other cars are on the road, it would be easy for the sky officer to lose sight of your car while looking at the flight instruments. You should raise this possibility on cross-examination by asking the airplane officer about procedures during the flight. Your goal is to get the officer to admit to not continuously watching your car during the pacing.

More About Aircraft Tickets

When the aircraft officer identifies a car going too fast (either by pacing it or measuring its speed between two marks), the officer normally records the time, speed, vehicle color, and type, along with brief notes on the car, in what's usually called an "observation log." You have the right to request a copy of this log before trial. Here are some things to look for in the log:

• references to multiple vehicles, thus raising identity problems

• hard-to-believe identical speeds for multiple vehicles

• short distances between markers, creating a greater chance for reaction-time errors, and

• long distances between markers, raising possible vehicle-identity problems.

You may also see that the timing occurred over less than a minute, or even that your car was described as being a different make or color than it really is. Obviously, tidbits like these are extremely useful to prepare questions that cast doubt on the reliability of the pacer's observations.


Most states allow police officers to catch speeders using 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. For example, a car passing between two points 200 feet apart, over two seconds, is traveling an average speed of 200/2 or 100 feet per second, which converts to 68 miles per hour.

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.

VASCAR is obviously a much more flexible tool than pacing, since the officer doesn't have to be going the same speed as you are or follow you over any particular distance. As long as the officer manipulates the "time" and "distance" switches correctly and consistently, the officer can accurately track your speed.

But fortunately (from your point of view) 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.

Here are some things to consider for fighting a VASCAR speeding ticket:

  • Officer's observation of distant points. When an officer times the passage of a car between two points, the officer must accurately record when the car passes each. This becomes more difficult the farther the officer is from either point. This is especially true at dusk, at night, and during bad weather, particularly fog or rain.
  • Officer's reaction time. Reaction time is the time between observing something and responding to it. Especially where the distance between the two points is only a few hundred feet, an officer's reaction time will greatly affect the speed calculated by the VASCAR unit. For example, if the distance is only 100 feet, the car will pass the second point in only a second or two, meaning a reaction-time error of only a few tenths of a second will affect the accuracy by 20% or 30%. On the other hand, if the distance between the two points is 1,000 feet—which takes 15 seconds for a car going 40 miles per hour to pass—a reaction-time error of a few tenths of a second will affect the accuracy by only 1% to 2%.
  • Odometer error. The VASCAR unit's accuracy depends on the accuracy of the police vehicle's odometer, except where the distance between the two points is independently measured with a tape and dialed into the VASCAR unit. As the patrol vehicle moves forward, the cable linking the VASCAR unit to the odometer turns, calculating how far the vehicle has moved from Point A to Point B. This mechanism is supposed to be recalibrated at least once a year. Since speed is distance divided by time, an erroneously high odometer distance fed into the VASCAR unit will result in an erroneously high speed reading.

Because VASCAR accuracy can depends so heavily on the officer's reaction time, it's crucial to know the 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."


Because lots of speeding tickets involve the use of radar measurement systems (not to be confused with LIDAR systems, discussed below), it's worth briefly taking a look at how radar works.

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.

Although radar signals can be bounced off stationary or moving objects, they cannot be bent over hills or around curves. To clock your speed with radar, this means you must be in an officer's line of sight. However, don't expect to see the radar unit. Officers can hide it behind roadside shrubbery or stick it out unobtrusively from behind a parked car.

Here are some of the more common malfunctions and sources of inaccurate readings with radar device:

  • More than one target. Radar beams are similar to flashlight beams —the farther the beam travels, the more it spreads out. And this simple fact often results in false speed readings, because it's common for a spread-out beam to hit two vehicles in adjacent lanes. Most radar units have beam angle, or spread, of 12 to 16 degrees, or about one-twenty-fifth of a full circle. This means the beam will have a width of one foot for every four feet of distance from the radar antenna. Or put another way, the beam width will be two lanes wide (about 40 feet), only 160 feet distant from the radar gun. So, if you're in one lane and a faster vehicle is in another lane or otherwise close to you, the other vehicle will produce a higher reading on the officer's radar unit, which the officer may mistakenly attribute to you. The mistaken reading of another vehicle's speed is especially likely to occur if the other vehicle is larger than yours. The inability of radar to distinguish between two separate objects is called lack of "resolution."
  • Wind, rain, and storms . 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 infere with radar devices readings. And sometimes, an officer will attribute these spurious readings to your vehicle. Windblown rain can also reflect enough energy to give false signals, particularly if the wind is strong enough to blow the rain close to horizontal. The more rain or wind, the more likely an erroneous radar reading will result. Pre-thunderstorm atmospheric electrical charges can also interfere with a radar unit. That's because electrically charged storm clouds can reflect a false signal back to the radar unit even though they are high in the sky. If such a storm cloud is being blown by the wind at sufficient speed, a false radar reading may result.
  • Calibration problems. Every scientific instrument used for measuring needs to be regularly calibrated to function properly. Radar equipment is no exception. It must be checked for accuracy against an object traveling at a known (not radar-determined) speed. If the speed on the radar equipment matches the known speed, the unit is properly calibrated. In practice, the best way to do this is to use a tuning fork as the moving object. However, it's 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 consist of flicking on the "calibrate" or "test" switch built into the radar unit itself and seeing if it calibrates properly. 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, there is a big problem with this sort of calibration testing. There are two types of circuits in the unit, frequency circuits and counting circuits. Flicking the calibration switch tests only the counting circuits. In short, if the frequency circuit is not calibrated, the radar unit may well be inaccurate. Based on this shortcoming, some courts won't accept the internal calibration features of radar units as reliable verifications of the unit's accuracty.
  • Pulling you over as part of a groupofcars. In situations where several cars proceed over the speed limit, some especially zealous officers will take a radar reading on the "lead" vehicle and then pull it over, along with one or two followers. In court, the officer will try to use the reading for the first vehicle as the speed for everyone else. The officer may even be up front about this, saying that he or she saw the vehicles behind following at the same speed. Or the officer may even claim to have also used the radar unit to measure the speed of second and third cars. Either way, this is shaky evidence. To be really accurate, the officer would have had to simultaneously note the lead car's reading while also keeping a close eye on the other cars. (This is something that is especially hard to do if the officer's car was also in motion.) And the use of radar to measure the the speeds of multiple cars in a group is also problematic, since by doing so the officer admits several cars were close together and that he or she was trying to measure all their speeds almost simultaneously.

Many of these defenses are applicable only in certain situations. But, anytime an officer uses a radar to clock a driver's speed, there's the potential for a defense based on inproper calibration.

Laser or "LIDAR" Speed Measurements

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

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.

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 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.

Talk to a Lawyer

Need a lawyer? Start here.

How it Works

  1. Briefly tell us about your case
  2. Provide your contact information
  3. Choose attorneys to contact you
Get Professional Help

Talk to a Traffic Ticket attorney.

How It Works

  1. Briefly tell us about your case
  2. Provide your contact information
  3. Choose attorneys to contact you