Police officers must be able to accurately measure vehicle speeds. Methods vary, but most fall under the general types listed below.

Radar: Radar is the primary method of speed enforcement in the United States. Radar guns aim an electromagnetic signal at a target vehicle and pick up the return signal reflected off the vehicle. The Doppler effect causes the frequency of the return signal to shift by an amount dependent on the relative speeds of the source of the original signal and the target. Speed radar devices measure the frequency of the reflected signal and compare it with the frequency of the original signal to determine the speed of the target vehicle.

Laser: Laser devices, also known as LIDAR (light distance and ranging), use a time/distance calculation to measure speed. The devices aim a narrow band of light at the target vehicle and measure the time it takes to receive the reflected light. Because the speed of both the original light pulse and its reflection are traveling at the same speed (the speed of light), differences in the time it takes the transmitted light to strike the target vehicle and return can be used to calculate the speed of the vehicle. Lasers can pinpoint specific vehicles in heavy traffic. Because laser light cannot be picked up by radar detectors, drivers who have purchased radar detectors no longer can assume they can speed with impunity. Speeders ticketed by police using laser devices were four times more likely to have radar detectors than motorists apprehended by officers using conventional radar.¹ Although detector manufacturers are marketing laser detectors, the narrowness of the laser beam reduces the likelihood that a laser detector can identify the beam in time to provide drivers with enough advance warning to slow down and avoid a ticket.

Speed cameras (also known as photo radar): Radar signals can be used to trigger cameras that photograph speeding vehicles as they pass a specified point. These devices generally use a low-powered Doppler radar speed sensor to detect speeding vehicles and trigger a motor-driven camera and flash unit to photograph vehicles traveling faster than a set speed. The date, time, location, and speed are recorded along with a photo.

Video: automated traffic law enforcement

VASCAR: A vehicle average speed calculator and recorder uses a portable computer to accurately clock, calculate, and display speed based on the time a vehicle takes to travel a known length of road.

Aerial speed measurement: Officers in light aircraft measure vehicle speeds based on the time it takes to travel between two or more pavement markings spaced a known distance apart. Information is transmitted to officers on the ground who then issue speeding citations.

The number of drivers and vehicle miles traveled have risen faster than the availability of officers whose routine duties include traffic law enforcement. In some jurisdictions, available traffic enforcement resources have declined. Other police priorities such as apprehension of violent criminals and, more recently, homeland security efforts can limit resources available for traffic enforcement. It may be difficult to observe speeds at the worst places and times, and police officers may be diverted to other duties, especially during periods of heavy traffic. In congested areas, there may be no place to pull over speeding vehicles without creating hazards.

Yes, radar is highly reliable and accurate. The reliability of radar speed measurements has been repeatedly upheld by the courts. However, motorists sometimes can challenge tickets because it may be difficult in heavy traffic to pinpoint specific vehicles.

Radar detectors are radio receivers tuned to the frequency range used by police radar guns. Radar detectors are bought and sold for the sole purpose of helping speeders avoiding speeding tickets.

Institute research has shown that interstate highway drivers with radar detectors reduced their speeds by at least 5 mph or activated their brake lights when suddenly exposed to police radar. Before exposure, vehicles with detectors were traveling significantly faster than those without detectors. By 1 mile past the radar, more than three-fourths of the vehicles with radar detectors were traveling at least 5 mph faster than the speed limit.¹ Clearly, the only purpose of a radar detector is to avoid speed law enforcement.

Research shows that drivers with radar detectors consistently are overrepresented among the fastest speeders.² Use of a detector demonstrates an intention to speed that distinguishes users of these devices from drivers who speed occasionally or inadvertently. In a survey of users, more than half admitted to driving faster than they would without their "fuzz busters." Surveys of trucks in 24 states conducted before the federal government banned radar detectors in commercial vehicles involved in interstate commerce found that about half of all trucks on interstates had radar detectors in use.³

Since January 1994, the US Department of Transportation has prohibited radar detector use in commercial vehicles involved in interstate commerce. Radar detectors also are banned in all vehicles in Virginia and the District of Columbia and in large trucks in New York and Illinois.

Institute researchers measured speeds and radar detector use in large trucks in 17 states in 1990 before the federal ban and found that more than half of all trucks, including half of trucks carrying hazardous materials, were using radar detectors. Use rates ranged from 39 percent in California to 69 percent in Oklahoma.⁴ An earlier study in Virginia and Maryland showed that trucks with radar detectors were more likely than those without them to be traveling at illegal speeds.² On interstates with 65 mph speed limits, trucks using radar detectors were twice as likely as those not using detectors to travel at least 5 mph faster than the limit, and 3 times as likely to travel at least 10 mph faster.

The human and economic costs of truck crashes weigh heavily on other road users. In 2006, 4,812 people died from injuries in crashes involving large trucks, and 83 percent of them were not truck occupants. For all vehicle types, speed increases stopping distance. But, because of their heavy weight (often 20-30 times the weight of cars), loaded trucks take much longer than cars to stop. Trucks' stopping distances are even longer at higher speeds. Higher truck speeds also may exacerbate the consequences of faulty brakes; out-of-adjustment brakes are the most common reason for trucks being placed out of service by law enforcement personnel.⁵

States have a legitimate interest in banning the possession and use of radar detectors, and well-written laws stand up in court. Despite claims by manufacturers that prohibiting radar detectors violates numerous constitutional principles, no court has held that the concept of radar detector bans, either by statute or regulation, is restricted by the Constitution. In fact, a US Court of Appeals in 1995 unanimously upheld the federal government's ban on radar detector use in commercial vehicles operating in interstate commerce. In its opinion, the court wrote that the Federal Highway Administration "promulgated the rulemaking in order to reduce speeding and thereby reduce the severity of accidents when they occur." The court added that, because commercial vehicles "are much larger and heavier than other vehicles, the damage they cause…in accidents at excessive speeds is much greater."

Few other countries allow passenger vehicle drivers to use radar detectors. Among 21 nations surveyed by the Institute in 1994, the United States, Iceland, Japan, the Netherlands, New Zealand, Spain, and some Canadian provinces allowed radar detectors.⁶ In most other countries, it is illegal to sell or use radar detectors. Fines for violations can be as steep as thousands of dollars, and the prohibited devices may be confiscated. Punishment can be tough; in France, a vehicle may be seized if a radar detector is found.

Police can use radar detector-detectors to identify vehicles equipped with radar detectors. These electronic devices work by identifying the characteristic microwave radiation emitted by radar detectors.

Lasers can pinpoint specific vehicles in heavy traffic. Because laser light cannot be picked up by radar detectors, drivers who have purchased radar detectors no longer can assume they can speed with impunity. Speeders ticketed by police in Charleston, South Carolina, using laser devices were four times as likely to have radar detectors as motorists apprehended by officers using conventional radar.⁷ Although laser detectors are marketed, the narrowness of the laser beam reduces the likelihood that a laser detector can identify the beam in time to provide drivers with enough advance warning to slow down and avoid a ticket.

These are electronic devices operated by police to enforce speed limits. Speed cameras, also known as photo radar, monitor the speeds of passing vehicles and are programmed to photograph vehicles traveling a set amount above the speed limit. Unlike other methods of traffic law enforcement, speed cameras do not require offending motorists to be pulled over. There are two methods for deploying speed cameras. Mobile speed cameras are accompanied by police and moved around among various locations; fixed cameras are unaccompanied and photograph vehicles speeding at specific roadway locations. Speed cameras have been used for more than 30 years in countries including Australia, Austria, Canada, Germany, Greece, Italy, the Netherlands, Norway, South Africa, Spain, Switzerland, Taiwan and the U.K.

Speed cameras are used in about 35 US cities and are used statewide in Arizona. Cameras currently are in use in Arizona, California, Colorado, Maryland, Massachusetts, Ohio, Oregon, Tennessee, and the District of Columbia. Speed cameras in action

Institute studies show that automated speed enforcement can substantially reduce speeding on a wide range of roadway types. Six months after implementation of speed cameras on residential streets and school zones in Montgomery County, Maryland in 2007, the proportion of drivers exceeding speed limits by more than 10 mph declined by about 70 percent.⁸ Implementation of a 9-month pilot program using fixed speed cameras on a busy urban freeway in Scottsdale, Arizona, in 2006 was associated with up to a 95 percent decrease in the odds that drivers would travel more than 10 mph above the posted 65 mph speed limit.⁹ And, within 6 months of the implementation of speed cameras on streets throughout the District of Columbia in 2001, the proportion of vehicles exceeding the speed limit by more than 10 mph declined 82 percent.¹⁰ Research conducted outside the United States also shows large effects of speed cameras on traffic speeds. For example, in Victoria, Australia, speed cameras were introduced in late 1989, and police reported that within 3 months the number of offenders triggering photo radar decreased 50 percent.¹¹ The percentage of vehicles significantly exceeding the speed limit decreased from about 20 percent in 1990 to fewer than 4 percent in 1994.

No. Speed cameras usually are programmed so they will not be activated unless a vehicle is traveling significantly faster than the posted limit — often 10 mph faster. A visible police presence typically accompanies photo radar in the United States to maximize the deterrent effect. Portable units are placed at the roadside in or near a marked police car, and signs usually announce that photo radar is in use.

Public opinion regarding speed cameras is an important factor in the formulation of related traffic laws and enforcement policies. Telephone surveys conducted in three US jurisdictions with speed camera programs show a majority of drivers support the use of automated speed enforcement. A survey conducted 9 months after speed cameras were introduced in Washington, DC, showed that 51 percent of drivers favored cameras and 36 percent opposed them. Support for camera enforcement was higher among middle-age and older drivers, among drivers who had not received a speeding ticket in the mail and did not know anyone who had, and among drivers who said speeding was a problem.¹² A survey conducted 6 months after speed cameras were deployed in Montgomery County, Maryland, found that 62 percent of drivers were in favor of speed cameras on residential streets. Support was higher among females and drivers 65 year of age and older.⁸ In Scottsdale, Arizona, 63 percent of drivers surveyed prior to the start of automated enforcement said speed cameras should be used on an urban freeway where camera enforcement was planned; after speed cameras were operational, 77 percent of drivers supported their use.⁹ In telephone surveys conducted in Washington, DC, Montgomery County, Maryland, and Scottsdale, Arizona, the proportion of drivers who said speeding was a problem ranged from about two-thirds to 80 percent.

The effects of automated speed enforcement on crashes has been the subject of considerable number of prior research efforts, as summarized in two recent systematic reviews of the international literature. A 2005 review analyzed data from 14 studies and found crash reductions in the immediate vicinities of camera sites, ranging from 5 to 69 percent for all crashes, 12 to 65 percent for injury crashes, and 17 to 71 percent for fatal crashes.¹³ A 2006 review published by the Cochrane Collaboration (an international organization that conducts systematic reviews of the scientific literature on public health issues) analyzed data from 21 studies and found reductions ranging from 14 to 72 percent for all crashes, 8 to 46 percent for injury crashes, and 40 to 45 percent for crashes involving fatalities and serious injuries.¹⁴ A 2007 NHTSA-sponsored review of 13 published studies reported injury crash reductions of 20 to 25 percent for fixed speed cameras and 21 to 51 percent for mobile speed camera programs.¹⁵

VASCAR provides an average speed measurement over a greater distance than is possible with radar. VASCAR enables police officers to identify specific speeding vehicles and can be used in various modes, including from patrol cars following speeders. VASCAR can detect speeding vehicles going in the opposite direction. VASCAR also can be useful in conditions where radar is not practical and, when used correctly, is very reliable. It emits no radiation, so it cannot be picked up by radar detectors.

Like VASCAR, aircraft patrols are based on visual detection and identification of target vehicles. Police time such vehicles over a measured distance, usually marked by lines painted across the road. An officer in a low-flying aircraft radios speed information to a patrol officer who makes the stop. Aerial surveillance can provide very accurate speed measurements and allow officers to focus on the fastest vehicles, but it is costly and can be difficult to use in locations with high traffic volumes.

Yes. Roadside electronic signs that display vehicle speeds to warn drivers they are speeding also show promise for reducing speeds and crashes at high-risk locations. Institute research shows mobile roadside speedometers can reduce speeds at the sites of the speedometers as well as for short distances down the road.¹⁶ When used in conjunction with police enforcement, the effect of speedometers can last longer. Signs warning truck drivers that they are exceeding maximum safe speeds on exit ramps also show promise, as they reduce the numbers of trucks traveling greatly above maximum safe speeds.¹⁷