Fighting a Speeding Ticket: How Was Your Speed Measured?

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

• whether you are charged under an "absolute," "presumed," or "basic" speed law, and
• how the officer measured your speed—through pacing, radar, laser, VASCAR, or other means.

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.

How Do Police Measure Your Speed?

As noted above, police use different methods for measuring or estimating driver speed. These methods include:

• radar
• laser (LIDAR)
• pacing
• VASCAR, and
• aircraft speed detection.

We'll explain all of these, but law enforcement probably uses radar or LIDAR more often than the other three.

How Do Police Prove You Were Speeding in Court?

In traffic court, police officers prove speeding tickets by testifying. Generally, the testimony will focus on how the officer measured the driver's speed and what the driver's speed was relative to the speed limit.

Some more modern speed measurement devices generate a printout. These printouts typically show the driver's speed and details like time and date. If there is a printout, the officer might present it as evidence in court. However, in most cases, the officer sees a digital reading on the radar (or other device) and just writes it down on the ticket.

Radar Speeding Tickets

Lots of speeding tickets involve radar measurements. Radar is generally a reliable and straightforward method for measuring vehicle speed. However, despite their general reliability, radar devices aren't infallible.

How Does Radar Work to Measure Driver Speed?

"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. The machine takes this information and computes the car's speed.

Two Types of Radar Used by Police

Typically, the radar units 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."

Below, we explain how these two radar systems work.

Car-Mounted Units

Most radar units used in patrol vehicles are shaped like a side-mounted spotlight. They're 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 patrol vehicle is also in motion.

Hand-Held Radar Units

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

Most radar errors result from the radar's operation in real-world conditions, which are often 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 usually includes a page or two on inaccurate readings and how to avoid them.

More Than One Car Is Speeding—"Lack of Resolution"

Radar beams are similar to flashlight beams—the farther the beam travels, the more it spreads. 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.

Radar spread can result in bad speed readings because a spread-out beam can hit two vehicles at the same time. 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.

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 (and therefore has a greater surface area) than yours. And, automatic radar units (or those set to automatic mode) tend to produce this type of error more frequently than units the officer manually turns on and off such as with a trigger system.

Wind, Rain, and Particles in the Air Can Make Radar Inaccurate

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.

Theoretically, 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. However, most police radar units operate at frequencies that are unaffected by atmospheric charges.

Radar Calibration Problems

Every scientific instrument used for measuring needs to be regularly calibrated to ensure 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's time-consuming to calibrate a radar with a tuning fork. 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.

LIDAR (Laser Speed Detectors) Speeding Tickets

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.

How Does LIDAR Speed Measurement Work?

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.

One advantage for police officers of the laser gun is that the light beam is narrower than a radar beam, meaning 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.

Are There Any Defenses for LIDAR Speeding Tickets?

For LIDAR measurements 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 more tricky 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.

LIDAR 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.

"Pacing" Speeding Tickets

Law enforcement sometimes uses a technique called "pacing" to measure a driver's speeding. As we'll explain below, pacing is more accurately described as an estimate of a driver's speed.

How Do Police Use Pacing to Measure a Driver's Speed?

Pacing is pretty simple. The officer tries to match your speed and then looks to their own speedometer to estimate how fast you're going. For this technique to be accurate, the officer needs to maintain a constant distance between the police vehicle and your car for an appreciable distance. Some states have laws that require the officer to pace for a certain minimum distance like one-eighth or one-fourth of a mile.)

What Are Common Defenses to Speeding Tickets Where Police Use Pacing?

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

The Officer Is Too Far Back to Match Your Speed

For pacing to provide a reasonable estimate of speed, the officer needs to be certain he or she is going the same speed as you are. The further away the officer is, the harder it becomes to ensure the speeds of the two vehicles are the same. In order to avoid detection, officers don't like to get too close to the target vehicle. However, when officers leave too big of a buffer, the driver has a better chance of convincing a judge that the speed estimate is unreliable.

Pacing in Low-Light Situations can be Unreliable

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 that can obstruct 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 environment for pacing. All these obstacles can be used to challenge the accuracy of the pace speed the officer reported.

VASCAR Speeding Tickets

"VASCAR" (Visual Average Speed Computer and Recorder) is a technology used by law enforcement to catch speeders. However, its use isn't as common as it was in the past.

How Does VASCAR Speed Measuring Work?

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.

When Are VASCAR Speed Measurements Inaccurate?

Because speed is defined as the 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 errors 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 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.

Aircraft Detection Speeding Tickets

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 call in a patrol car to make the stop. But as you might have guessed, aircraft speeding tickets are rare.

How Does Aircraft Speed Detection Work?

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 Does Aircraft Speed Detection Fail?

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

Stopwatch and Reaction Time Errors

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.

Difficulty in Keeping the Target 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.

Aircraft Pacing

Pacing from aircraft is less accurate than timing a car's passage between two points for several reasons. First, 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. Second, 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 in 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.

What About Speed Control Cameras?

So far, we've been talking about how police officers directly measure driver speed. However, some jurisdictions also catch speeders using automated camera technology.

How Do Speed Traffic Enforcement Cameras Work?

Speed cameras typically use radar equipment that's linked to the camera. When the radar detects a vehicle that is exceeding the speed limit, the camera is triggered to take a picture of the vehicle. Speed cameras record the speed of the vehicle, along with the details of the violation (date, time, and location) and the vehicle's plate number.

Are Speed Cameras Accurate?

The accuracy of speed cameras depends on the accuracy of the technology they use. So, if a speed camera uses radar, the possible sources of error are, for the most part, the same as those discussed above related to officer-operated radar.

Speeding Ticket FAQs

This article covers the basics of how police measure driver speed and some causes of measurement error. Below, we've tried to answer some frequently asked questions that haven't been directly addressed above.

• How accurate are police speeding measurements?
• What should I do if I believe the speed measurement on my ticket is inaccurate?
• Can I ask a police officer to show me my speed measurement?
• Can one cop ticket me for speeding based on another cop's radaring?
• Are radar detectors illegal?

What should I do if I believe the speed measurement on my ticket is inaccurate?

If you believe the speed measurement on your ticket is incorrect, you should first find out how the officer measured your speed. With this information, you can start thinking about what factors could have resulted in an inaccurate speed measurement. As we discussed above, each type of measuring device is susceptible to error.

Talking to a traffic attorney with experience fighting speeding tickets might also be a good idea. Traffic attorneys usually have a good idea of what defenses might work in a given situation.

Can I ask a police officer to show me my speed measurement?

You can always ask a police officer to show you your speed measurement. But unless the officer's measurement device prints its readings, you're unlikely to get an affirmative response. Generally, police look at the digital reading on their device and write it down on the ticket. Also, it's important to be mindful that many officers might be annoyed by this type of request. So, the juice might not be worth the squeeze on this one.

Can one cop ticket me for speeding based on another cop's radaring?

Generally, it's legal for one officer to ticket a driver based on another officer's radar reading. As you can imagine, officers working in pairs—with one in position radaring cars and another down the line a way making the stops—can enforce the speed limit laws more effectively than if they were working alone.

However, if the officer who measured the speed doesn't come to court, you might have a good objection based on the hearsay rule. Generally, witnesses can testify only as to their personal knowledge. In most instances, personal knowledge equates to an officer actually observing a violation. When officers (or any other witnesses) try to testify about events they didn't see with their own eyes, it's normally considered "hearsay." Hearsay is generally inadmissible in court, assuming someone makes an objection.

Are radar detectors illegal?

In some states, radar detectors are illegal. And federal regulations generally prohibit commercial drivers from using radar detectors.