Speed Measurement for Speeding Tickets

When you receive a speeding ticket, you may question whether the officer correctly measured your speed. Officer errors can arise both in the accuracy of their speed measurement or estimate, as well as in their use of speed measurement equipment and techniques.

How is Speed Measured

For a speeding ticket, speed may be measured through techniques ranging from a visual estimate of speed through the use of a speed measuring device (SMD), such as a radar or laser device, designed to scientifically measure speed with a high degree of precision.

Speed may sometimes be measured by one officer, sometimes even from the air, with another officer located further down the road informed of which cars to pull over and ticket.

Estimation of Speed

A police officer who issues traffic tickets will often be trained in the visual estimation of speed. At a traffic ticket hearing, following the officer's explanation of his training and experience, the officer may be permitted to testify much more specifically about a vehicle's speed based on a visual estimate.

It is not unusual for officers to make visual estimates of a vehicle's speed prior to measuring a driver's speed with a SMD, and to record both the estimated speed and measured speed on the ticket. When that occurs, even if there is a technical problem with the SMD or the officer's use of the SMD, it remains possible to be convicted of the speeding offense based on the officer's visual estimation.

Most courts will allow laypersons (non-experts) to testify about their estimation of a vehicle's speed, although not ordinarily in relation to precise speeds.

A typical person can do a decent job of figuring out that a vehicle, traveling at 50 MPH in a 30 MPH zone, is proceeding at a rate of speed well above the speed limit. That same person is unlikely to be able to state with any certainty that the actual speed of the vehicle was in the vicinity of 50 MPH, and some witnesses will estimate a speed that is considerably different than the vehicle's actual speed. Laypersons sometimes overestimate the speed of a vehicle based on sound, with louder vehicles being perceived as traveling at higher rates of speed.

Tickets based on a lay witness's estimate of speed will rarely require proof of the vehicle's precise speed, as opposed to the witness's testimony being potentially used in support of a charge such as careless driving with vehicle speed being only one of the factors alleged to constitute the offense.

Types of Speed Measurement Devices

The most common speed measurement technologies drivers will encounter are:


A radar SMD works by transmitting sound waves which bounce off of the targeted vehicle. The device uses the Doppler effect, the change in the frequency of sound waves as the targeted vehicle moves toward or away from the SMD, to calculate speed. Some radar devices are designed to be operated from a stationary position, while others can be used from a moving patrol vehicle.


Laser or lidar SMDs measure distance by illuminating a target with a laser, focusing a beam of light in the near-infrared range at the target vehicle. The light reflected back from the vehicle to the SMD is analyzed to calculate the vehicle's speed. A lidar unit is operated from a stationary position.

Pacing or Clocking

The officer follows a vehicle while matching its speed, then records the speed as measured by the patrol vehicle's speedometer.


A time-based circuit is implemented to measure the amount of time it takes for a vehicle to pass between two defined points, allowing the calculation of the vehicle's average speed (distance / time).

Challenging Speed Measurement

Some drivers attempt to challenge an officer's measurement of their speed by claiming that the officer did not have a clear view of their vehicle, or could not have measured their speed from the position in which the officer reports being stationed. As the officer's response is almost certain to be that he could and did measure the vehicle's speed as described, absent some compelling evidence this type of defense is rarely successful.

Drivers will also scrutinize the records for a SMD to see if the unit was serviced and calibrated in a manner consistent with the manufacturer's recommendation, and not less often than required by state laws or regulation. Even a patrol vehicle's speedometer should be periodically tested and, if necessary, calibrated for accuracy. SMDs should also be verified to be in working order usually both at the beginning and end of the officer's shift. An officer who uses an SMD should be trained in its proper use.

SMD Evidence in Traffic Court

When it comes to laying a foundation for the admission of scientific evidence there are broad similarities between state laws. The prosecuting authority must provide some evidence that:

  • The officer was using a reliable means of measuring speed,
  • The equipment was properly serviced, maintained and tested, and
  • The officer is competent to testify about the speed measurement technique that was used in the case.

A motorist fighting a ticket must aware of the need for a foundation for the submission of evidence - if the motorist is not paying attention during court proceedings, a court may admit evidence of speed based upon a foundation that would have been deemed inadequate had the motorist made a timely objection.

When more than one officer is involved in the issuance of a speeding ticket, as occurs when one officer measures a vehicle's speed (from the ground or air) and another officer stops the vehicle to issue the ticket, the officer who submits testimony to the court should be competent to testify to the speed of the vehicle. That will normally mean that the officer who measured the speed must appear and provide testimony.

When a driver is facing an informal hearing some states will allow the foundation to be established primarily or exclusively through the submission of documents and affidavits. At a formal hearing, or for a misdemeanor criminal prosecution, higher evidentiary standards will apply.

Preparing for Traffic Court

When preparing a defense to a speeding ticket, it is crucial to understand what rules of evidence apply in the state where your ticket is pending and in the type of court hearing that is scheduled on your ticket.

When you prepare for court on a speeding ticket, and are hoping to challenge a SMD measurement, you should take advantage of any discovery rules that exist in your state. Discovery is the process of obtaining information from the other side, in this case the prosecuting authority, in advance of a hearing or trial. Some states offer relatively generous pretrial discovery, others offer limited discovery, and some do not offer any discovery for non-criminal traffic tickets. When discovery is permitted, you must to take care that you follow your state's procedures for preparing and serving your discovery request, including serving it on the correct recipient and in a timely manner.

For states that permit discovery it is important to make a timely request, generally as early in the proceeding as you can. For states that restrict or don't allow traffic court discovery you need to consider other means of obtaining information as you prepare for trial, such as using your state's public records law (a/k/a sunshine law or freedom of information act) to obtain records from government agencies.

Copyright © 2015 Aaron Larson, All rights reserved. No portion of this article may be reproduced without the express written permission of the copyright holder. If you use a quotation, excerpt or paraphrase of this article, except as otherwise authorized in writing by the author of the article you must cite this article as a source for your work and include a link back to the original article from any online materials that incorporate or are derived from the content of this article.

This article was last reviewed or amended on May 8, 2018.