Fighting a Speeding Ticket: How Was Your Speed Measured?

The key to challenging a speeding ticket knowing how the officer measured your speed. Generally, your speed and the officer's measurement method will be on the ticket itself. But you could also ask the officer or later request the officer's notes through a process called "discovery."

Here, we discuss the five most common methods of speed detection.


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 that the officer must 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

Now let's discuss the most common ways pacing can be shown to be inaccurate.

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 pace. Otherwise, the officer's speed won't match that of the car he or she is pacing. Keeping a constant distance between the officer's front bumper and the suspect's rear bumper is sometimes called "bumper pacing."

Correct bumper pacing requires both proper training and good depth perception, and it 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 being detected by the driver.

So, if you know an officer was close behind you for only a short distance, your best tactic in court might be to try to show the officer's supposed "pacing" speed was really just a "catch-up" speed. To establish this defense, you'll need to first ask the officer the distance over which he or she tailed you. If the officer admits it was, say, only one-eighth mile (between one and two city blocks), it will help to testify (if true) that you noticed in your rearview mirror that the officer was closing the gap between your car and the patrol car very quickly. The officer doing so would have the effect of giving him or her an accurately high speedometer reading.

Pacing at Dusk or Night

Pacing is much more difficult in low-light situations unless the officer is right on your tail. Basically, the accuracy of pacing depends in large part on the officer's ability to see clearly. Under dark conditions, the officer just can't see as well as during the daytime.

Road Conditions Can Affect Pacing

Pacing is easiest and most accurate on a straight road with no hills, dips, or other obstacles and that can obstruct of the officer's continuous view of the vehicle he or she is pacing. Under these ideal conditions, the officer can keep the patrol car at a constant distance behind you while pacing your speed. Hills, freeway interchanges, dips, curves, busy intersections, and heavy traffic make for a poor pacing environment. All these obstacles can be used to challenge the accuracy of the pace speed the officer reported.

Aircraft Speed Detection

In certain remote areas, law enforcement uses aircraft to enforce speeding laws. In other words, law enforcement aircraft spot a vehicle that's speeding and calls in a patrol car to make the stop. Here are several possible ways to challenge an aircraft-assisted speeding ticket.

How Aircraft Speed Detection Works

Law enforcement aircraft use two methods for determining a vehicle's speed. The first is to calculate a vehicle's speed by timing how long it takes the 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. As we'll see, this second method is typically the less accurate of the two.

How Aircraft Speed Detection Fails

There are several ways to challenge tickets based on an aircraft's measuring your speed.

Stopwatch and Reaction Time Error

If the timing is not performed properly from the aircraft, the speed of the vehicle on the ground will be wrong. Since this speed is calculated by dividing the distance by time, the shorter the distance your speed was measured over, the more significantly a timing error on the part of the sky cop will affect the estimated vehicle speed.

EXAMPLE: Officer Aircop sees Dawn Driver pass between two markings an eighth of a mile apart. At a speed of 65 miles per hour—the speed limit—Dawn's car should cross the two marks in 6.9 seconds. But if Officer Aircop starts the stopwatch a second too late or stops it a second too early and gets 5.9 seconds, he incorrectly figures Dawn's speed to be (0.125 mile/5.9 sec.) x 3,600 = 76 miles per hour.

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.

Using the Aircraft to Pace You

Pacing from aircraft is less accurate than timing a car's passage between two points for the following reasons:

  • Inconsistent distance while pacing. It's much more difficult for an aircraft pilot than for the driver of a police car to maintain the same distance behind the paced vehicle.
  • Difficulties in determining the speed of the aircraft. An aircraft officer's estimate of a ground vehicle's speed is based on the speed of the aircraft. So, it's imperative that the aircraft speed is accurately known. Wind conditions and many other factors can affect how accurately the aircraft speed can be determined. Any error in measuring the speed of the aircraft will translate into error estimating a vehicle's ground speed.

Problems for Patrol Car Officer Identifying the Vehicle

Given that license plate numbers are too small for the airborne officer to see, and many modern cars look very much alike, there's a real possibility of vehicle mixups.

Often aircraft officers relay information on several speeding cars at the same time. This, of course, increases the possibility that the ground officer might confuse different cars.


Most states allow police officers to catch speeders using a technology called "VASCAR" (Visual Average Speed Computer and Recorder). Basically, VASCAR amounts to 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.

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 devices. The human input greatly increases the possibility of error.

VASCAR works like this: The officer measures the distance between the two points by using a measuring tape or 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.

EXAMPLE: An officer picks one point at a marked crosswalk and another at a manhole cover in the street. The officer drives the distance between the two points, making sure to press the distance button on the VASCAR unit when driving over the crosswalk and again when reaching the manhole cover. The odometer connected to the VASCAR unit measures a distance of 0.12 mile or 633 feet and records this in its memory. The officer then picks a hidden spot that has a clear view of both points and waits. A motorcycle passes the crosswalk line, and the officer clicks the "time" button, then clicks it again when the vehicle crosses over the manhole cover 6.78 seconds later. The VASCAR unit calculates the speed as 633 feet in 6.78 seconds, or 93.4 feet per second (63.5 miles per hour).

How VASCAR Fails

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, while accurately observing when your vehicle and the patrol car pass over the same two points, the officer can accurately estimate your speed.

Because speed is defined as distance traveled per unit of time, timing an object's passage between two measured points seems like a foolproof method to measure speed. But because VASCAR measurement depends entirely on human input—accurately pushing the button for "time" and "distance"—it's easy for errors to creep in. 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.


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

How Radar Works

"Radar" stands 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. Why so high? Because the higher the frequency, the straighter the beam, the truer the reflection, and the more accurate the speed reading. It's important to know this because, as we discuss below, the primary defense to a radar speeding ticket is to attack its accuracy.

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.

But no matter where the unit is mounted, 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. So, slowing down once you've gone through the beam won't do any good—the officer already has the reading.

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. In moving mode, the radar receiver measures the frequency of two reflected signals: the one reflected from the target vehicle—as in the stationary mode—and another signal bounced or reflected off the road as the patrol vehicle moves forward. Using these two signals, the unit automatically calculates the driver's speed.

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.

How Radar Fails

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.

One good way to point out all the pitfalls of radar readings is to subpoena the radar unit's instruction manual. The manufacturer will usually include a page or two on inaccurate readings and how to avoid them. If you study the manual, you may find a way to attack its reliability in court using the manufacturer's own words.

The following are descriptions of common malfunctions and sources of inaccurate readings.

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 bogus 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 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 bogus 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.

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.

Since tuning forks can easily become inaccurate, it's important that they are protected from damage. (A good scratch or dent can render one inaccurate.) Keeping the forks in a sturdy box usually protects them.

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.

False Ground Speed Reading in Moving Radar

A radar unit used while a patrol car is moving must take into account:

  • the speed of an oncoming vehicle relative to the patrol car, and
  • the speed of the patrol car relative to the ground.

The errors previously discussed (for example, with weather and dust particles) can contribute to inaccurate readings of the relative speed between the target vehicle and patrol vehicle. But these kinds of errors can also contribute to inaccurate readings of the patrol car relative to the ground.

EXAMPLE: A patrol car is doing 70 miles per hour southbound and passing a truck going at 50 miles per hour. You are going 65 miles per hour northbound, in the opposite direction. Your car approaches the officer's car at a combined speed of 70 + 65, or 135 miles per hour. The officer's unit detects this 135-mile-per-hour speed and should subtract the patrol car's 70-mile -per-hour ground speed to get your true speed of 65 miles per hour. Instead, the officer's ground-speed beam fixes on the truck ahead and measures a false 50-mile-per-hour ground speed. It subtracts only 50 miles per hour from the 135 miles per hour, to get 85 miles per hour for your speed, even though you're doing only 65 miles per hour.

Pulling You Over as Part of a Group of Cars

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 upfront about this, saying that he or she saw the vehicles behind following at the same speed.

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.

About Radar Detectors

No discussion of radar would be complete without a few words on the technology of radar detectors—little black boxes that consist of a sensitive radio receiver adjusted to pick up signals in the radar frequency range. But instead of powering a loudspeaker, this type of radio circuit activates a beeper or light to warn that your speed is being monitored.

Radar detectors are illegal in Virginia and the District of Columbia but legal in all other states for most drivers. However, federal regulations, which apply in all states, prohibit commercial drivers from using them. Where radar detectors are illegal, you can usually be ticketed for having one and have it confiscated.

Even when radar detectors are perfectly legal, some people believe that officers are more likely to issue a ticket—as opposed to a warning—when they see a radar detector in your car.

LIDAR (Laser Speed Detectors)

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 LIDAR is that the light beam is narrower than a radar beam, meaning that it can be more precisely aimed. 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. So, for this measurement 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. And it's impossible to be sure that it's been accomplished because the officer can't see the beam.

EXAMPLE: Officer Krupke fixes her laser gun on Jane's car, which is traveling 60 miles per hour, about 90 feet per second. It travels about 30 feet in the one-third-of-a-second measurement the laser device uses. If the laser beam starts at the windshield and travels to the bumper, it adds about four feet to the 30-foot distance that the machine otherwise would have measured if it had stayed pointed at the windshield. It would incorrectly calculate that Jane went 34 feet or 68 miles per hour in the one-third of a second it took to measure the speed of her car. The result is that the laser unit registers Jane's speed 8 miles per hour faster than it was actually going.

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