Fight Your Speeding Ticket: Determining Your Speed

How Did the Police Detect Your Speed?

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

  • Were you charged under an “absolute,” “presumed,” or “basic” speed law? (Don’t worry, we explain this jargon in this article.)
  • How did the cop determine your speed—through pacing, aircraft, radar, laser, VASCAR, or other means? In this article we explain these methods of speed detection

There may be only one way to speed, but there are many ways to detect your speed. The five most common methods are explained in more detail, below. 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 mph 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 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.

  • Road configuration may help prove inadequate pacing. Hills, curves, 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 at a curve, not allowing enough distance to properly pace the vehicle. Similarly, if you were ticketed within 500 feet of starting up from a stop sign or light, the officer will not be able to prove having paced your car for a reasonable distance. 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 you are being paced. The officer’s speedometer reading, after all, means nothing if the officer is driving faster than you are in an attempt to catch up with you. That’s why an officer is trained to “bumper pace” your car by keeping a constant distance between the patrol car’s front bumper and your rear bumper. Doing this correctly requires both 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 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. You will want to 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. This would have the effect of giving the officer a high speedometer reading. When your turn comes to testify, emphasize (if true) how you initially saw the patrol car some distance back in your rearview mirror, then saw it bear down on you quickly. Be sure to also testify (if true) that you periodically glanced at your speedometer, which indicated a steady speed, and that you didn’t slow down when you saw the patrol car. During your final argument, you should emphasize the point that your testimony and the officer’s both show that the officer was actually closing in on you when the officer claimed to be measuring your speed, not truly pacing you at a constant speed. Then, if the above formula will result in your speed being below the limit, explain that there is a simple mathematical formula to show your true speed. Show how it is derived (see above), and how, when the numbers are plugged in, it shows your speed was below the speed limit.
  • Pacing at Dusk or Night. Pacing is much more difficult in the failing light of dusk or in complete darkness, unless the officer is right on your tail. In 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 will have trouble keeping the same pair of taillights in view. In this article, we include a few cross-examination questions to bring this out during the trial.
  • Road Conditions Can Affect Pacing. Pacing is easiest and most accurate on a straight road, with no hills, dips, or other obstacles and where the officer can see your vehicle continuously as the officer follows you. This allows the officer to 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 of these obstacles can be used to challenge the pacing of your vehicle for accuracy.

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. As we’ll see, this second method is less accurate and therefore easier to attack.

Under either system, if a car is found to be speeding, a waiting ground patrol car is radioed. If that ground patrol car does not independently verify your speed, your chances of successfully fighting your ticket go up. For starters, that’s because both the aircraft and ground officer will have to be present in court. The aircraft officer must testify as to how he or she measured your speed, and the ground officer must say that you were, in fact, the driver. If the pilot appears in court but the ground officer does not, the prosecution cannot prove its case in the majority of states that treat traffic cases as minor criminal violations. In part, this is because you are not required to testify, because the Fifth Amendment to the Constitution gives you the right to remain silent. However, in states that treat traffic violations as “civil offenses,” you may not have this right to remain silent. Fortunately for you, there are several good ways to challenge tickets based on an aircraft’s measuring your speed.

  • Ask for a 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. Even if both officers show up, you still may have a decent opportunity of winning a case where an airplane is involved. To maximize your chances, ask the judge to exclude one officer from the courtroom while the other is testifying. Don’t worry, you are not being impolite but only exercising your right to prevent the two cops from taking cues from each other’s remarks.
  • Stopwatch Error/Reaction Time. If the timing is not performed properly from the aircraft, the speed of your vehicle will be wrong. 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 on the part of the sky cop will result in a too-high speed reading. 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. The longer the distance between the ground markings, the more accurate the officer’s reading is likely to be. A one-second error in starting the stopwatch will result in only about a 1-mph error where the distance between markers is a mile. (See this article for cross-examination questions that highlight this error.)
  • Difficulty in Keeping Your Car in View. If two markers are a mile apart, it takes a car doing 75 mph some 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. Hopefully, you will learn that the officer must keep a log for every vehicle he or she paces, recording the vehicle’s basic description, the time between the two points, and the calculated speed. In short, the officer is usually also keeping track of other cars. If you establish this during cross-examination, you can argue in your final argument that the officer might have started to pace your car but mistakenly focused on another car that looked like yours after looking up from taking notes. (See this articleon cross-examination.)
    • Using the Aircraft to Pace You. The second method by which an officer in an aircraft can determine your speed involves two steps: (1) timing the aircraft’s passage over two separate highway markings a known distance apart to get the aircraft’s speed and then (2) using the aircraft to “pace” your vehicle. For example, if the aircraft passes over two markings a mile apart in 60 seconds, the aircraft’s speed is 1 mile/60 seconds, or 0.0167 mile per second. Since there are 3,600 seconds in an hour, this 0.0167 mile per second is multiplied by 3,600 to get miles per hour, or 60 mph. If the car below stays ahead of the aircraft, it’s going 60 mph; if it’s pulling away, it’s going faster. The officer in the plane then radios this information to the officer on the ground. This method is less accurate than timing a car’s passage between two points for the following reasons: (1) 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. (2) Inaccuracy in ascertaining reference points from the air. For the officer in the air to determine his speed, he or she has to time the passage of the plane over two markers several thousand feet below. This is done by starting a stopwatch as the plane passes the first marker on the roadway and by stopping the watch as the second marker is crossed. The speed is then determined by dividing the distance between the markers by the elapsed time. This sounds reliable enough, but it often isn’t. For starters, it is difficult for a pilot to know exactly when the plane passed a spot on the ground. An inconsistency in the aircop’s body position within the aircraft, by even a few feet, as he or she times the passage, can add several miles per hour to your estimated speed. (3) Wind conditions can also affect the speed of the aircraft. If a headwind comes up after the aircraft has timed its passage over two markers its airspeed would be decreased. That would make it appear to the aircop as if you were going faster than you actually were.
    • Problems Identifying the Vehicle. After testifying about how the speed was computed, the aircraft officer will next testify about radioing the information to the ground officer who stopped you. Here you’ll again want to raise the possibility that the ground cop stopped the wrong car. Given that license plate numbers are too small for the airborne officer to see, and many modern cars look very much alike, this is a real possibility.
    • Ask the pilot how many cars he or she was tracking. 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. If the ground officer is excluded from the courtroom, that officer will take the copy of the ticket along, since he or she issued it. This means the aircraft officer won’t be able to use the ticket to “refresh his or her memory” while testifying. In this article we discuss cross-examination techniques, including suggested questions for this situation.

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. It really does pay to examine this log. Here are some things to look for:

•  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.

If the patrol officer on the ground testifies to have independently checked your speed by means of pacing, try to establish that the officer overly relied on the radio report and didn’t really pace you for an extended period. See above for how to challenge the accuracy of pacing.



Most states allow police officers to catch speeders using technology called VASCAR (Visual Average Speed Computer and Recorder). Despite the fancy name, 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.

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. As we will see, this also greatly increases the possibility of mistakes.

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

A VASCAR unit is normally connected to an officer’s odometer to allow the measurement of a distance between two preselected points while driving past them. This also allows an officer to use the unit while moving. VASCAR units are engineered to take into account the police unit’s speed and the suspected vehicle’s speed by pressing the “time” switch twice as your car passes the two preselected points, and by pressing the “distance” button twice as the patrol car traverses those same two points.

The officer can use a VASCAR unit in five ways:

  • While stationary. The officer manually measures a certain distance with a tape or other measuring device, dials that measurement into the VASCAR unit, then clicks the “time” switch when the car passes the first and second distance marks.
  • While stationary, after having driven a set distance in his or her vehicle and using the odometer to enter that distance into the VASCAR unit. Again, the cop clicks the “time” switch when the car passes the first and second distance marks.
  • While following you and allowing the VASCAR unit to take into account that the patrol car is also moving.
  • While ahead of you, by pressing the “distance” switch twice as the officer passes between the two points, then the “time” switch twice as the officer watches you—through the rearview or side-view mirror—pass over the same two points.
  • While driving in the opposite direction, by clicking the “time” switch as you pass a point well ahead of the patrol car and by simultaneously pressing the “time” and the “distance” buttons as your cars go past each other—setting the second point. Then the officer presses the “distance” switch as he or she reaches the first point where he or she started to time you. (The officer then makes a quick U-turn to pull you over.)

VASCAR is obviously a much more flexible tool than pacing, since the officer does not 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 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.


Different Types of VASCAR Errors

Short Distances. At short distances—generally less than 500 feet—reaction-time error is most likely to produce an incorrect VASCAR result. If the officer is late to the trigger when you cross the first measuring point, but accurate as you cross the second point, you will be clocked as going faster than you actually were. For this reason, a federally commissioned study of VASCAR recommends that to obtain accurate VASCAR readings, officers measure speeds over elapsed times of at least four seconds for stationary police units and five seconds for moving units.

The name of this study is “Analysis of VASCAR” and it is available for download from the U.S. Department of Transportation’s On Line Publications website, at To find the study, go to the site’s main search page and plug in the publication’s number (DOT HS 807 748) or the keyword “VASCAR.”

Long Distances. When VASCAR is used at distances greater than 1,500 feet, reaction-time errors are less of an issue. (Half a second one way or the other won’t make much difference.) Here, significant errors usually result because the officer simply doesn’t see when you pass the marker point farthest from the patrol car because it is too far away.


How VASCAR Fails

Because speed is defined as distance traveled per unit of time, timing an object’s passage between two measured points seems foolproof. But because VASCAR measurement depends entirely on human input—accurately pushing the button for “time” and “distance”—it is 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.

In its Legal Defense Kit for defending traffic tickets, the National Motorists Association of Waunaukee, Wisconsin (www includes a scientific study entitled “An Error Analysis of VASCAR-Plus,” by Kenneth A. Moore of JAG Engineering, Manassas, Virginia. Through numerous calculations, charts, and graphs, Moore demonstrates that VASCAR is most prone to error where the distance between the two clocking points is 1,500 feet or more. (He also agrees that it is prone to error below 500 feet.)

The possibility of VASCAR error is so well known that Pennsylvania lawmakers have taken action. Pennsylvania law (Title 75, Section 3368) forbids a VASCAR speeding conviction—where the speed limit is less than 55 mph—if the VASCAR speed readout isn’t more than 10 mph over the limit. That’s another way of saying, “We don’t trust the accuracy of a VASCAR unit that says ‘44 mph’ when the speed limit is 35.”

If you’re charged with speeding and the officer used VASCAR, you should try to bring up these possibilities for inaccuracy at trial. The best way to do this is to cross-examine the officer, knowing what questions to ask (see this article).

  • Officer’s Observation of Distant Point. 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. For example, while VASCAR can be used at night, the officer must be able to see when vehicle headlights pass objects that may be illuminated poorly or not at all. Obviously, this is far more difficult than watching a car pass two nearby points at noon in good weather.
  • 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. Here’s why: The shorter the distance between the two points, the lower the elapsed time a speeding car will take to pass through those two points. 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 mph to pass—a reaction-time error of a few tenths of a second will affect the accuracy by only 1% to 2%. In promotional materials, VASCAR manufacturers claim reaction time isn’t a factor, because they assume that the officer will anticipate, rather than react to, your car passing each point. They also argue that any delayed reaction will be the same for each click of the VASCAR unit, thereby canceling out the error. This is faulty reasoning. There’s no guarantee that the officer will delay the same interval when pushing the button as you pass the first and then the second points. In fact, the officer may do a much better job at the second point because the officer’s eyes have now been fixed on your car for quite some time, making the officer better prepared to press the button. The result can easily be that the officer has erroneously shortened the time and, thereby, increased your recorded speed. Reaction-time error is likely to be worst in the situation where the officer’s vehicle is approaching yours from the opposite direction. For example, if you’re doing 65 mph northbound, and an officer is doing the same speed southbound, your closing speed is 130 mph, or 191 feet per second. If you’re 500 feet away, the officer has little more than two seconds to look ahead, watch your vehicle pass one point, hit the “time switch,” then hit the “time” switch again simultaneously with the “distance” switch as your cars pass each other. The officer then has a few more seconds to hit the “distance” switch a second time, hopefully just as the officer passes the same point you passed when he or she hit the “time” switch the first time. Operating VASCAR in the opposite direction is so difficult to do well that some police agencies discourage officers from using it this way. Your main goal is to attack the officer’s reaction time through cross-examination (see this article), focusing your questions on the difficulty in timing a car’s passage past a distant point. When it is your turn to testify, tell the judge in detail (if true) that your speed was at or under the limit—or safely above it in a “presumed” speed limit state. Finally, be prepared to argue during your closing argument (see Chapters 12 and 13) how your testimony as well as the officer’s responses to your cross-examination questions raise a reasonable doubt over whether you were violating the speeding law.
  • 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. That is because the VASCAR gets its distance information via the patrol vehicle’s speedometer/odometer, to which it is connected. As the patrol vehicle moves forward, the cable linking the VASCAR unit to the speedometer/odometer turns, calculating how far the vehicle has moved from Point A to Point B. It is supposed to be recalibrated at least once a year. Tire wear and pressure can affect the accuracy of a speedometer. These factors will also affect odometer accuracy, because the odometer and speedometer both run off the same cable. For example, low tire pressure and tire wear on the police vehicle can result in a tire with a slightly smaller circumference than a new and properly inflated tire. The smaller wheel must make more revolutions to cover the same distance as a new tire. This results in erroneously high speedometer readings and in an exaggerated odometer distance reading. 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.This type of error, however, is usually fairly small. For example, a 24-inch diameter tire that has lost one-quarter inch of tread will be 23.75 inches in diameter, a mere 2% less, so that the recorded distance and speed will be only 2% high. Still, this type of error, when added to other types of errors—like the ones listed above—may well result in an erroneous VASCAR reading. So, during cross-examination, ask when the VASCAR unit was last tested. If it was not tested recently, or the officer does not know when it was tested last, you should attack the accuracy of the test in your closing argument.


Because so many speeding tickets involve the use of radar measurement systems, let’s briefly examine how radar works. Of course, the point of doing this is so you’ll be well positioned to cast doubt on the accuracy of your radar ticket. It can sometimes be an uphill battle trying to convince a judge that a sophisticated electronic radar device is fallible. But it is definitely possible to do this. After you’ve read what follows, you’ll know more about radar than most judges and some police officers, and may be able to use your knowledge to beat your ticket.

Don’t confuse radar with laser. You need to determine how you were caught. You can ask the ticketing officer what method was used, and testify to that in court. Or you can demand to see the officer’s notes, which will indicate what method was used to clock your speed. While radar and laser detection systems work in a similar way, the ways to fight them in court have significant differences. Be sure you know which one was used against you.

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. 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.

To better understand how radar works, remember what it was like to blow peas out of a straw as a kid. If you blew the peas at the trunk of a stationary car, they would (at least theoretically) take the same amount of time to bounce back and hit you in the forehead. If the car had been moving away from you, the peas would each take a longer time to hit and bounce back. The radar beam sends out billions of electronic pulses (like peas) per second and sends back reflected waves whose pulses are slightly farther apart. The greater the difference between the transmitted and reflected waves, the greater the relative speed or difference of speed between the target vehicle and the police car.

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.

Unfortunately for errant motorists, modern radar units are fairly easy to operate. Officers using them do not have to be certified or licensed. But it’s also true that to operate radar units with a high rate of accuracy under all sorts of road and weather conditions takes practice and skill. The best way to learn is with the help of an experienced instructor. It follows that it will usually look bad in court if an arresting officer admits to never having any formal instruction in the use of radar equipment. Realizing this, most officers will say (either when making their presentation or in answer to your cross-examination questions) that they have taken a course in how to use radar. It’s important for you to know that this course can range anywhere from a short pep talk by a company sales representative to a few hours or even a day of instruction at a police academy. Either way, most officers don’t receive comprehensive instruction on the important fine points of using radar.

This gives you the opportunity to use cross-examination questions to try to pin the officer down (see this article) on just how few hours were actually spent on good instruction. Assuming you succeed in doing this, you’ll then want to make the point, during your closing argument, that the officer could well have misused the unit. For example, the officer may not have realized that at a distance of a few hundred feet, a radar beam is wide enough to cover four lanes of traffic, and thus might have clocked a nearby vehicle instead of yours. And as we discuss in the rest of this chapter, there are a number of other ways officers commonly produce false radar readings.

How Radar Fails

Contrary to police department propaganda, new technology has not completely ironed out problems known to cause radar malfunctions. Most screwups 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 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. Thus, 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 mistaken reading of another vehicle’s speed is especially likely to occur if the other vehicle is larger than yours. In fact, the vehicle contributing to the officer’s high radar reading needn’t even be in another lane; if a larger vehicle, such as a truck, is rapidly coming up behind you in your lane, the officer may see your car while the radar is reading the truck’s speed. Inability of the equipment 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 are often read by radar devices. And sometimes these spurious readings can be attributed to your vehicle. You may have read newspaper stories about radar trials in which a hand-held radar gun was pointed at a windblown tree resulting in the tree being “clocked” at 70 mph! 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 bogus 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. Typically, you would attack the radar use by referring to the manual during cross-examination and getting the officer to admit that the manual says errors can occur due to adverse weather conditions. Then in your final argument, you might say something like this: “Your Honor, the officer testified that the radar unit’s accuracy can be affected by windblown rain and storm clouds, and also admitted that at the time, there were clouds and rain.”
  • Calibration Problems. Every scientific instrument used for measuring needs to be regularly calibrated to check its accuracy. 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. While this may seem a far cry from a moving car, the use of a tuning fork is scientifically sound; tuning forks, when struck against a hard object, vibrate at a certain frequency, which we hear as an audible tone. 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 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. The Connecticut case of State v. Tomanelli, 216 A.2d 625 (1965), indicates that the use of a certified tuning fork is the only scientifically acceptable method of calibrating a radar unit. The fact that an internal “calibrate” test isn’t a substitute for a tuning fork explains why it’s so important in any traffic trial involving the use of radar to cross-examine the officer and see whether he or she really did use a tuning fork before you were ticketed. Typically, they are required to use the tuning fork at the beginning and end of their shifts. If the officer says “yes,” move on to another question. But if the officer says “no,” then it’s time to ask more specific questions. (See this article for suggestions on cross-examination questions on this point.) Of course, if you discover that a tuning fork wasn’t used, you’ll want to emphasize this as part of your final argument.
  • False Ground Speed Reading in MovingRadar. A radar unit used while a patrol car is moving must take into account: (1)            the speed of an oncoming vehicle relative to the patrol car, and (2) the speed of the patrol car relative to the ground.
  • Above, we discussed common ways that a moving radar unit can incorrectly attribute high speed to your vehicle. Here we deal with the notion that radar units can also misjudge the patrol car’s speed. This can most easily occur if the radar unit mistakes a signal reflected back from a nearby car or truck for the signal reflected back from the ground.
  • 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. (“There was no change in bumper-to-bumper distances”.) Or the officer may even claim to have also used the radar unit to measure the speed of second and/or third cars. (“When they passed through the beam, there was no change in the reading.”) 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.) If the driver of the second car can truthfully testify as to how the lead car was going faster and increasing the distance, it should be a big help to establish reasonable doubt in court. And the use of radar to measure the cars 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. Here are some possible defenses: (1) If you were the driver of the lead car, you may be able to claim that the officer inadvertently locked onto a higher reading of the second or third vehicles that were gaining on you. If the second or third vehicles were larger than yours, the chances of a false reading on your car go up, because the larger vehicle will reflect a stronger signal. In this situation it may help the driver of the lead car if he or she can truthfully testify to seeing (in the rear or side mirror) the second vehicle quickly gaining from behind and suggest that the radar reading was really for that vehicle. (2) If you were the driver of one of the vehicles behind the lead car, the vehicles in front of you may have been traveling faster (as lead vehicles often do). If that vehicle was larger than yours, or closer to the officer’s vehicle, this would result in that vehicle’s reflected radar signal being stronger. You could argue here that the radar unit read the speed of the car ahead of you, not your slower speed.

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. Many of the commercially available detectors have a sensitivity control that can be adjusted to give the best compromise between trying to detect even faint, far-away police radar signals and attempting to screen out off-frequency signals that come from sources other than police radar.

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 big-rig drivers from using them. Where radar detectors are illegal, you can usually be ticketed for having one and have it confiscated. Often this occurs when officers use what, for lack of a better term, are called radar-detector detectors. These are, in essence, radio receivers that pick up the low power signal emitted by most radar detectors.

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.

At trial, ask the officer if the radar unit was on automatic. The chances of registering the speed of the wrong car go way up when an officer, who is stationary, points a unit at a highway and puts it on the automatic setting. This is true because the officer isn’t pointing at a specific vehicle, and the beam angle width means the unit could be picking up one of several cars going the same, or even the opposite, direction. In this case, ask the officer whether there was other traffic in either direction. If the answer is “yes,” ask the officer which direction. If there was traffic in the direction opposite you, follow up and ask him or her whether the unit responds to traffic in both directions. (See this article for sample cross-examination questions of this type.) Either way, if there was other traffic, be sure to raise the possibility in your closing argument that the radar unit clocked the wrong vehicle.


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. Still, there is room for error. Here’s why:

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. Moreover, it’s impossible to be sure that it’s been accomplished, because the officer can’t see the beam. As a result, the laser detector’s measurement is highly subject to error.

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. If the two cars are traveling at different speeds, the laser detector will read incorrectly. 

 Speeding Tickets and the ‘Hearsay Rule’

In challenging your ticket, you will want to be aware of a key legal rule called “hearsay,” which could help your case. The hearsay rule prohibits any testimony that quotes information from somebody other than the witness. This is sometimes called the “he said” rule because it forbids a witness from testifying to what somebody else said he saw. There is a huge catch to this hearsay prohibition—just like Perry Mason, you must affirmatively object or the judge will allow the testimony. Here are the most common situations in which a prosecutor is most likely to use hearsay evidence to prove a speed violation:

  • An officer testifies about what another driver said about your behavior.
  • The officer who wrote your ticket testifies about what another officer told him or her. This is particularly likely to happen where an aircraft monitored your speed and relayed the information to a patrol car that stopped you.
  • Where two officers were in a patrol car, and one of them observed your driving. The officer who did not see your driving may not testify to what the other officer told the first officer about your driving.


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