Q&A: Speed — law enforcement

April 2013

1 What devices and methods are used to enforce speed limits?

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.  Radar is highly reliable and accurate. However, it can be difficult to pinpoint specific vehicles in heavy traffic, and some motorists use radar detectors to help them speed without getting caught.

Laser: Laser devices, also known as LIDAR (light detection 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. Devices to detect lasers have been marketed, but 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.

VASCAR: VASCAR stands for visual average speed calculator and recorder. It 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.

VASCAR provides an average speed measurement over a greater distance than is possible with radar. It enables police officers to identify specific speeding vehicles and can be used from patrol cars following speeders. VASCAR can detect speeding vehicles going in the opposite direction. When used correctly, it is very reliable. It emits no radiation, so it can't be picked up by radar detectors.

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.

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.

Speed cameras: Most speed cameras measure the speed of a vehicle at a single spot. In the United States, a majority of speed cameras are fixed and use either radar or detectors embedded in the road surface to measure a vehicle’s speed. Mobile cameras are placed at the roadside in marked or unmarked police cars, containers, poles, etc., and use radar or LIDAR to measure speeds. Some communities require mobile cameras to be manned.  In either fixed or mobile systems, if a vehicle is traveling faster than a predetermined speed, a motor-driven camera goes off. The date, time, location and speed are recorded along with a photo of the vehicle.

More recent technology can measure average speeds over a certain distance. In this case, cameras located at two or more points record time-stamped images of all vehicles that pass them. Automatic license-plate recognition is used to match individual vehicles so that average speeds between the two points can be calculated. Time-stamped pictures of speeding vehicles are used as evidence of speeding. Point-to-point speed cameras have been used to enforce speed limits in Australia and the United Kingdom.

Video: automated traffic law enforcement

2 What are the advantages of speed cameras?

Traffic volumes and the number of drivers have risen faster than the availability of officers whose routine duties include traffic law enforcement. In some jurisdictions, available traffic enforcement resources have declined as apprehension of violent criminals and homeland security efforts take priority. In addition, it may be difficult to observe speeds at the worst places and times. In congested areas, there may be no place to pull over violators without creating hazards.

Automated speed enforcement can substantially reduce speeding on a wide range of roads. Institute studies in Maryland, Arizona and the District of Columbia found that the proportion of drivers exceeding speed limits by more than 10 mph declined by 70, 88 and 82 percent, respectively, after cameras were introduced. Retting, R.A., Farmer, C.F. and McCartt, A.T. 2008. Evaluation of automated speed enforcement in Montgomery County, Maryland. Traffic Injury Prevention 9(5):440-5. Retting, R.A.; Kyrychenko, S.Y.; and McCartt, A.T. 2008. Evaluation of automated speed enforcement on Loop 101 freeway in Scottsdale, Arizona. Accident Analysis and Prevention 40(4):1506-12. Retting, R.A. and Farmer, C.M. 2003. Evaluation of speed camera enforcement in the District of Columbia. Transportation Research Record 1830:34-37.

 A 2010 review published by the Cochrane Collaboration, an international public health organization, examined 35 studies from various countries. The authors concluded that speed cameras, including fixed, mobile, overt and covert devices, cut average speeds by 1-15 percent and the percentage of speeding vehicles above the speed limits or designated speed thresholds by 14-65 percent compared with sites without cameras. Wilson, C.; Willis, C.: Hendrikz, J.K.; Le Brocque, R.; and Bellamy, N. 2010. Speed cameras for the prevention of road traffic injuries and deaths. The Cochrane Library 2010, Issue 10. Oxfordshire, England: The Cochrane Collaboration.

3 Do speed cameras reduce crashes?

Yes. In 2010, the Cochrane Collaboration reviewed 28 studies that reported the effect on crashes and found reductions of 8-49 percent for all crashes, 8-50 percent for injury crashes and 11-44 percent for crashes involving fatalities and serious injuries, in the vicinity of camera sites. Wilson, C.; Willis, C.: Hendrikz, J.K.; Le Brocque, R.; and Bellamy, N. 2010. Speed cameras for the prevention of road traffic injuries and deaths. The Cochrane Library 2010, Issue 10. Oxfordshire, England: The Cochrane Collaboration.  Over wider areas, the review found reductions of 9-35 percent for all crashes, and 17-58 percent for crashes involving fatalities and serious injuries. Reviewed studies with longer duration showed that these trends were either maintained or improved with time.

4 Are speed cameras used to ticket motorists going 1 or 2 mph faster than the speed limit?

No. Speed cameras usually are programmed so they will not be activated unless a vehicle is traveling significantly faster than the posted limit — typically 10 or 11 mph faster, although in certain places such as school zones the grace levels may be lower. 

5 Are speed cameras widely used in the United States?

Currently, speed cameras are used in 131 U.S. communities in 13 states (Alabama, Arizona, Colorado, Illinois, Iowa, Louisiana, Maryland, Missouri, New Mexico, Ohio, Oregon, Tennessee, and Washington) and the District of Columbia, up from only four Arizona and Utah communities in 1995. In addition, cameras are used statewide in highway work zones in Illinois, Maryland and Washington.

U.S. cities with speed cameras

U.S. communities with speed cameras, 1995-2013

 

6 Does the public support the use of speed cameras?

Telephone surveys conducted by the Institute in jurisdictions with speed-camera programs show a majority of drivers support them.

A 2012 IIHS survey of 801 District of Columbia residents found strong support for speed cameras. Cicchino, J.B.; Wells, J.K.; and McCartt, A.T. 2013. Survey about pedestrian safety and attitudes toward automated traffic enforcement in Washington, D.C. Arlington, VA: Insurance Institute for Highway Safety. DC has used speed cameras since 2001. In the survey, 88 percent of residents said that speeding was a serious threat to their personal safety. Seventy-one percent of residents who had driven a car in DC in the past month and 90 percent of residents who had not driven supported speed cameras. Residents older than age 65 were more likely to support speed cameras than those ages 18-44 (88 percent vs. 65-76 percent), and women were more likely to support them than men (82 percent vs. 69 percent).

 A survey conducted six months after speed cameras were deployed in Montgomery County, Md., found that 62 percent of drivers were in favor of speed cameras on residential streets. Support was higher among women and drivers 65 and older. Retting, R.A., Farmer, C.F. and McCartt, A.T. 2008. Evaluation of automated speed enforcement in Montgomery County, Maryland. Traffic Injury Prevention 9(5):440-5.

 In Scottsdale, Ariz., 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. Retting, R.A.; Kyrychenko, S.Y.; and McCartt, A.T. 2008. Evaluation of automated speed enforcement on Loop 101 freeway in Scottsdale, Arizona. Accident Analysis and Prevention 40(4):1506-12.

7 What other technologies are available to help reduce speeding?

Roadside electronic signs that display vehicle speeds to warn drivers they are speeding may reduce speeds and crashes at high-risk locations. IIHS research found that mobile roadside speedometers can reduce speeds at the sites of the speedometers, as well as for short distances down the road. Casey, S.M. and Lund, A.K. 1993. The effects of mobile roadside speedometers on traffic speeds. Accident Analysis and Prevention  25(5):627-34.  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 reduce the numbers of trucks traveling greatly above maximum safe speeds. Freedman, M.; Olson, P.L.; and Zador, P.L. 1992. Speed actuated rollover advisory signs for trucks on highway exit ramps. Arlington, VA: Insurance Institute for Highway Safety. Some GPS navigation systems and cellphone applications can provide real-time speed alerts. Insurance Institute for Highway Safety. 2012. Speed-alert devices plus incentives can curb speeding. Status Report 47(9):6-7. Drivers also can use a vehicle's cruise control settings to avoid speeding.

8 What is intelligent speed adaptation (ISA)?

Intelligent speed adaptation (ISA) describes a class of systems that work with either GPS devices linked to speed limits or on-board sensors, for example, cameras, that "read" speed limit signs. ISA systems then integrate speed limit data with vehicle speed in real-time. ISA systems differ in how much control drivers have in deciding whether to speed. The least automated systems provide alerts to drivers. Biding, T. and Lind, G. 2002. Intelligent Speed Adaptation (ISA), Results of large-scale trials in Borlänge, Lidköping,Lund and Umeå during the period 1999-2002. Publication 2002:89 E. Swedish National Road Administration. A second approach is to introduce resistance to gas pedals, making it harder but still possible to speed. Varhelyi, A. and Makinen, T. 2001. The effects of in-car speed limiters: Field studies. Transportation Research Part C, 9, 191-211.   The most automated systems limit gas flow to engines so drivers can accelerate up to but not over the speed limit. Carsten, O.; Fowkes, M.; Lai, F.; Chorlton, K.; Jamson, S.; Tate, F.; and Simpkin, B. 2008. ISA-UK, Intelligent Speed Adaptation, Final Report. London, England: Department for Transport.   A fourth option is to give drivers incentives, such as auto insurance discounts, to slow down. Reagan, I. J.; Bliss, J.P.; Van Houten, R.; and Hilton, B.W. 2013. The effects of external motivation and real-time automated feedback on speeding behavior in a naturalistic setting.  Human Factors 55(1), 218-30.

During the past 25 years, researchers have completed many ISA field assessments. Trials typically involve drivers using ISA-equipped vehicles on public roads for periods lasting weeks to months. The pattern of results indicates significant reductions in speeding when driving with ISA. The largest research effort to date studied several thousand Swedish drivers for more than a year and reported a decrease in speeding violations of approximately 50 percent. Biding, T. and Lind, G. 2002. Intelligent Speed Adaptation (ISA), Results of large-scale trials in Borlänge, Lidköping,Lund and Umeå during the period 1999-2002. Publication 2002:89 E. Swedish National Road Administration. A U.S. study indicated that driving with a modest monetary incentive and an alerting ISA led to an increase in the percent of time driving at or below the speed limit from 70 percent to 83 percent of the time. Reagan, I. J.; Bliss, J.P.; Van Houten, R.; and Hilton, B.W. 2013. The effects of external motivation and real-time automated feedback on speeding behavior in a naturalistic setting.  Human Factors 55(1), 218-30.

The individual components needed to make ISA work are available now. The largest technical barrier is the absence of large areas with a GPS-linked speed-limit database. The European New Car Assessment Program (Euro NCAP), which provides vehicle safety ratings, includes ISA as one of the safety features that automakers can use to qualify vehicles for the Euro NCAP Advanced designation. Carsten, O. 2012. Is intelligent speed adaptation ready for deployment? Accident Analysis and Prevention 48:1-3.

9 What are radar detectors?

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 avoid speeding tickets.

10 What is the problem with radar detectors?

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 one 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. Teed, N.; Lund, A.K.; and Knoblauch, R. 1993. The duration of speed reductions attributable to radar detectors. Accident Analysis and Prevention 25(2):131-7.  Radar detectors cannot pick up laser light. Speeders ticketed by police in Charleston, South Carolina, using laser devices were 4 times as likely to have radar detectors as motorists stopped by officers using conventional radar. Teed, N. and Lund, A.K. 1993. The effect of laser speed-measuring devices on speed limit law enforcement in Charleston, South Carolina. Accident Analysis and Prevention 25(4):459-63.  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. Freedman, M.; Williams, A.F.; Teed, N.; and Lund, A.K. 1990. Radar detector use and speeds in Maryland and Virginia. Arlington, VA: Insurance Institute for Highway Safety.  In a survey of users, more than half admitted to driving faster than they would without the devices. Opinion Research Corporation. 1988. A survey about radar detectors and driving behavior. Princeton, NJ: Opinion Research Corporation.  

11 Are there laws banning radar detectors?

Since January 1994, the U.S. 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.

12 Why are radar detectors banned in large trucks?

A total of 3,373 people were killed in crashes involving large trucks in 2011, and most of them were not truck occupants. High speeds play a big role in truck crashes because they increase the already-long distances required to stop a large truck. Speed also exacerbates the size and weight differences of trucks and passenger vehicles, leading to more severe crashes.

Institute research focusing on large trucks, conducted before the federal ban on radar detectors, found that trucks often had radar detectors and that these trucks were more likely to be exceeding the speed limit.  Institute researchers measured speeds and radar detector use in large trucks in 17 states in 1990 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.  Trucks with radar detectors exceeded the speed limits more often than those without radar detectors. Teed, N. and Williams, A.F. 1990. Radar detector use in trucks in 17 states. Arlington, VA: Insurance Institute for Highway Safety.  An earlier study in Virginia and Maryland also showed that trucks with radar detectors were more likely than those without them to be traveling at illegal speeds. Freedman, M.; Williams, A.F.; Teed, N.; and Lund, A.K. 1990. Radar detector use and speeds in Maryland and Virginia. Arlington, VA: Insurance Institute for Highway Safety.  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.