How do Police Measure Speed?

There are 5 ways police will claim to measure the speed of a vehicle.

1. LIDAR, or Laser

LIDAR stands for “light detection and ranging”. It is often referred to as laser, as the LIDAR unit uses a laser beam to measure speed. Much like radar, LIDAR sends a beam of light out that bounces off an object and returns to the gun. The time it takes for the laser beam to go out and return measures the speed of an object as well as the distance of the object from the laser gun.

Handheld LIDAR gun showing speed and distance, probably in km/h and meters.

Unlike radar, LIDAR cannot be operated while moving. LIDAR is always operated from a stationary position. The LIDAR unit is a handheld unit that the officer must aim at a vehicle. The beam of a LIDAR unit does not spread wide like a radar beam. The tighter beam and intentional aim make LIDAR more difficult to challenge.

LIDAR Issues

If the traffic is dense and the distance is far, it is still possible for an officer to laser the wrong vehicle.

Another issue is truth-fullness of the police officer. This could apply to any of the speed measurement methods. After all, with any of these methods, we are relying on the officer’s word that what he observed actually happened.

With LIDAR, the officer will often include a photograph of his LIDAR unit readout showing the alleged speed and distance. Well, this speed and distance could have been measured anytime. For example, it could have been measured on one car, and then the officer just doesn’t clear it. Then, the officer sees a car he wants to pull over. He pulls over the car and says he measured the car at xxx mph. The officer presents the LIDAR readout (from earlier in the day) as proof of this measurement.

Unfortunately, we have seen situations where this appears to have occurred. For example, an officer presents his photograph of a LIDAR readout claiming our client was speeding. Our client adamantly denied speeding. We looked at the physical location where the officer claims he was located. Then we used the distance noted on the LIDAR to see where the officer claimed our client was. Turns out that the space between the officer and where our client allegedly was is filled with trees!! It was simply not possible for the officer to have used his LIDAR in the way he claimed.

2. Radar

Radar is probably the most common speed measurement method.

How does radar work?

Radar is an acronym for “Radio Detection and Ranging.” In a nutshell, a radar device sends radio waves out which bounce off objects and return to the radar device. The elapsed travel time of the radio waves provides the speed of the object.

Almost all of the radar units we encounter are car-mounted units, not hand held. A car-mounted unit has antennae that face forward and rearward. The radar unit can measure traffic in front of and behind the patrol unit. The unit can measure traffic coming towards or moving away from the unit. The radar unit can also measure speed when the patrol car is in motion and when it is stationary. The variety of conditions in which radar can operate makes it very versatile.

Issues with radar

1. Beam spread, or more than one vehicle

The radar beam spreads out from the radar device like a light beam. In other words, it is not a line, but more like a piece of pie. The further away the radar beam gets, the wider it gets. This means at a distance, the radar beam may be wide enough to hit two vehicles. This raises the question of whether or not the police officer measured the correct vehicle.

A radar unit can operate in an “automatic” mode. In this mode, it is constantly sending out radar beams and measuring anything that travels through the path of the radar. This is problematic in that the police officer may not be paying attention to a particular vehicle. If there is more than one vehicle on the road, there is a good chance that the officer doesn’t know which vehicle his radar unit measured.

Another problem with radar in that it has a tendency to pick up larger objects over smaller ones. So, for example, if the defendant was far from the officer, was on a motorcycle, and right next to a semi, it’s possible the officer was reading the speed of the semi.

2. A group of cars

Sometimes a driver might be traveling in a line of cars. A police officer measures the speed of the lead car. Then the police officer either pulls over the entire line of cars, or cherry picks a driver out of the line to pull over. The bottom line is the police officer only measured the speed of the lead car. He didn’t measure the speed of any of the following cars. The lead car could have been pulling away from the following cars.

Alternatively, the police officer could have actually gotten a radar reading from one of the following cars, not the lead car. It is possible that the following cars were gaining on the lead car. If the officer pulls over the lead car, he may have over-estimated the lead car’s speed.

3. Calibration

We always check the maintenance and calibration history of a radar unit. All police officers operating a radar unit will claim to check the calibration of their units with tuning forks, both at the beginning and end of their shifts.

4. Cosign Effect

During training on radar and lidar police are trained in the Cosine Effect. The speed measured by the radar device is directly related to the cosine of the angle between the radar or lidar and the defendant’s line of travel. In English, the Cosine Effect means the more aligned the officer is with the angle the defendant is driving on, the more accurate the reading. As that angle gets wider, the accuracy of the reading goes down. So, if an officer was directly perpendicular to a defendant’s vehicle, there would be about 100% error. The downside from the defense perspective is that the increase in angle usually results in a lower speed.


As attorneys, this is our favorite speed measurement method. When measuring speed by pace, the officer is claiming that he followed the defendant’s vehicle at a fixed distance while matching the driver’s speed. The officer then uses his own speedometer to determine how fast the subject vehicle is traveling.

In theory, this makes sense. In practice, we frequently find problems with this.

Problems with measure speed by pace

Was the officer exactly matching the subject vehicles speed? Or was the officer maybe slowly gaining on the subject vehicle, thereby over-estimating the subject vehicle’s speed?

Is the officer being truthful about the alleged speed? This question comes up regardless of the method of speed measurement. However, the nice part about a pace is we can often get information to show the officer is not being truthful.

For example, all Arizona Department of Public Safety vehicles (not motorcycles) collect GPS data (or AVL data). You can read about AVL and GPS data in more detail here. If an officer claims to have paced the defendant on the I-17 at mile post 278 at a speed of 105 mph, then we would expect the GPS data to show the officer in that location, at the date and time alleged in the ticket, traveling at the alleged speed.

If the GPS data does not align with the officer’s allegations, then we have a strong argument that the officer is mistaken or not being truthful.


VASCAR stands for Visual Average Speed Computer and Recorder. I could write an entire post about how to operate a VASCAR device, but here’s how VASCAR works in a nutshell.

A VASCAR unit is a small computer that is used to measure time over distance. It can be used while the officer is in motion, or while he is stationary. The officer either measures the time it takes a vehicle to travel a known distance. This could be an area where the officer has measured out a distance between two landmarks ahead of time. Alternatively, the officer can use the VASCAR unit, which is connected to his odometer, to measure a distance while also measuring the time.

There are 3 primary issues with VASCAR

1. Reaction Time

First, the time measurement is impacted by the officer’s reaction time in starting and stopping the timer. This is especially true over short distances, where the time is also very short. If the officer is slow to start the timer, he will over-estimate the subject vehicle’s speed.

2. Observational Error

Second, the accuracy of VASCAR depends on the officer’s ability to accurately observe when a vehicle passes a certain landmark. The officer cannot actually measure the distance travelled by the subject vehicle. He is either measuring the distance using his own odometer as he follows the vehicle, or he has measured the distance in his vehicle ahead of time. This means the officer is trying to visually determine when a vehicle passes two landmarks. There will inevitably be error here because it is very difficult to observe from a distance when precisely a vehicle is passing a particular point.

3. Odometer Error

Third, any error present in the odometer of the officer’s vehicle will necessarily impact the speed measurement. The officer is using his odometer to measure the distance over which the speed of the subject vehicle is calculated. So if the officer’s odometer reads a longer distance than was actually traveled, the alleged speed will be higher than the actual speed.

5. Visual Estimation of Speed

Pretty much every police officer we have ever interviewed or cross-examined claims that they can look at a car and tell how fast it is going. We know this is simply not possible. However, it is regularly recognized by the courts. Perhaps more importantly, it’s most commonly used together with one of the methods listed above in an effort to support whatever speed the officer is alleging.

Problems with estimating speed using human eyes.

Humans are not machines. A human’s estimation of speed is essentially a guess. Some guesses may be closer to correct than others, but it is still a guess. Moreover, a human’s ability to visually judge speed gets a lot worse when the object is not traveling from side to side across the person’s field of view. It is very difficult to judge speed when the object is traveling directly towards or away from you.

Nevertheless, nearly every police officer will include in their report or testimony that they visually estimated the defendant’s speed to be xxx miles per hour. I believe police make this visual estimation claim in an effort to help overcome any potential flaws in whatever speed measurement method they used.

A word about radar and laser detectors

Using a radar or laser detector does not mean you will avoid a ticket. Here are some issues.

A radar detector can be effective when a police officer is using a radar unit in automatic mode and there are other vehicles ahead of you. A radar detector is effective when it can detect radar waves that were not directed towards your vehicle. In other words, if the officer is using radar on cars ahead of you, your radar detector will alert you with sufficient time to slow down.

radar detector on dash of car

If the officer is not using his radar in automatic mode, meaning he is looking for cars he thinks are speeding and then manually operating the radar, the first time your radar detector alerts may be when the officer radars you.

A radar detector counts on a radar beam scattering. The radar detector detects the scattered beam. A laser does not scatter like a radar beam. So if a police officer lasers a car ahead of you, your laser detector will not detect that laser.

It is even possible that a laser aimed at the bumper of your vehicle will not disperse enough to alert your laser detecter on your dash.

It appears the best defense against laser is a system that absorbs the laser. This results in the police officer getting no speed reading because the laser beam is absorbed rather than reflected back to the officer’s LIDAR unit.