Road Injury Prevention Litigation Journal
Road Injury Prevention & Litigation Journal
Copyright © 1998 by TranSafety, Inc.
May 1, 1998
TranSafety, Inc.
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Study Reports on the Effectiveness of Photo-Radar and Speed Display Boards

Since its modest inception in 1902, "speed control technology" has experienced tremendous growth and increasing sophistication. However, speeding remains a major safety concern on the nation's roadways. Nearly one in five fatal crashes is believed to be the result of driving too fast, and less than half of survey respondents reported obeying posted speed limits all the time.

To combat the speeding problem, law enforcement has enlisted the help of "two of the most technologically sophisticated forms of . . . automated speed control"--photo-radar and speed display boards. Photo-radar, in use for more than 30 years, employs radar to detect speeding, takes photographs of speeding vehicles, and issues their drivers citations by mail. Speed display boards are a newer device (developed in the late 1980s). They are widely used, having grown from about 90 in use in 1991 to more than 500 today. They also employ radar to detect speeding but rely little on speed limit enforcement. Instead drivers learn their speed when it is flashed next to the posted speed limit sign.

Widespread use of these speed control devices has not, however, resulted in definitive conclusions about their effectiveness. Research conclusions on the devices are sometimes in conflict, and the research is often flawed. A number of studies suffer from "weak methodologies," including "serious research design problems" such as not controlling for external factors and not using comparison sites. These weaknesses also appear in various studies reportedly showing that speed control devices can reduce traffic crashes. To some degree, flaws have plagued "even many well-designed studies."


In an effort to provide a more definitive answer to the question of effectiveness, a 1993 study in Riverside, California examined "the effect of photo-radar and speed display boards on traffic speed . . . on comparable streets. . . ." The study sought to determine which device is more effective (including more cost-effective) and "whether supplementing speed display boards with police enforcement makes them more effective." Steven A. Bloch reported the study's results in "A Comparative Study of the Speed Reduction Effects of Photo-Radar and Speed Display Boards," a paper presented at the Transportation Research Board's 77th Annual Meeting (January 1998). The study's "primary conclusion" was: "[W]hile both photo-radar and speed display boards can be effective in reducing vehicle speeds, display boards offer better overall results."

This study examined three approaches to speed control: "photo-radar, (unenforced) speed display board, and a speed display board with intermittent enforcement." Three sites were chosen with the stipulation that they be "as comparable as possible" in posted and actual speeds (25 miles per hour), number of lanes, traffic markings and volume, and road alignment, type, length, width, and development. The photo-radar van was equipped with "a police medallion and strobe light," and "the two speed display boards . . . were SPECTER Trailers." Data were collected at two sites on each street during four weeks in September and October. The first site was next to the experimental site, and the second was about 0.2 miles downstream--to allow determining if the slowing effect of the devices lasted for any distance. Overall, researchers recorded 9,790 speed measurements.


Findings showed that "photo-radar and speed display boards are about equally effective during deployment," reducing mean speeds by 5.1 and 5.8 miles per hour (mph), respectively, where baseline speeds averaged 34-35 mph in 25-mph zones. Mean speed reductions for both devices were less downstream, at 4.1 and 2.9 mph, respectively. Results also showed that "speed display boards become significantly more effective when supplemented with police enforcement. This effect is evident to a statistically significant degree, however, only at the downstream site." The enforced display board was "only barely less effective" downstream (5.9 mph) than at the experimental site (6.1 mph). However, the unenforced display board was significantly less effective downstream (2.9 mph versus 5.8 mph at the experimental site). The photo-radar was only "modest[ly]" less effective downstream (from 5.1 mph at the experimental site to 4.1 mph downstream).

All speed control devices produced more noteworthy results on speeds 10 mph or more over the 25-mph speed limit. At the experimental site, the photo-radar reduced these "excessive speeds" by 30.2 percent; the speed display board reduced them by 34.9 percent, and the enforced display board by 31.8 percent. However, these significant speed reduction capabilities were "typically not long-lived" after the devices were removed, with the following notable exceptions. At the experimental and downstream sites, 45.7 and 46 percent of drivers were speeding, respectively, before the enforced display board was deployed, but those percentages fell to 16.1 and 20.1 percent, respectively, after the board was removed. Researchers noted one "long-term, statistically significant effect" with the unenforced display board. A 1.7 mph decrease in speed continued at the experimental site after the display board was gone; and while 52.5 percent of drivers were speeding at the experimental site before the board was in place, the percentage dropped to 40.9 percent after the board was removed.

The study also analyzed the cost-effectiveness in three areas of the three speed controls. "Cost per deployment" represented an overall estimate for a speed control program, while "cost per mph of speed reduction" determined whether a device had been cost-effective in achieving speed reductions. "Cost per driver exposed" assessed "the cost of exposing an individual driver to a speed management device." Table 1 illustrates the overall cost estimates for the three areas. As indicated, the unenforced speed display board was the most cost-effective device on both an hourly and daily basis, and photo-radar was the least cost-effective of the three speed control devices.

Cost-Effectiveness Estimates for Speed Display Boards, Photo-radar and Speed Display boards with Enforcement

Type of Speed Control
Photo-radar (Police Costs Only) Photo-radar (Police and Equipment) Unenforced Speed Display Board Enforced Speed Display Board
Cost per deployment $155.00 $220.36 $10.29 $91.79
Cost per mph of speed reduction        
  • Each hour of deployment
$8.42 $11.98 $0.20 $1.27
  • Full 12-hour day
$119.23 $169.51 $2.39 $16.39
Cost per driver exposed $0.39 $0.55 $0.01 $0.08


The study faced "potential limitations" in four areas; however, their effect on the outcome was considered minor. First, the speed control devices were only deployed for one week. Second, many drivers may have mistakenly thought the display board was a photo-radar device. In addition, although the point of photo-radar is to give offending drivers a citation, no citations were issued in this case. Finally, "the offsetting income from photo-radar and enforced display board fines should have been included in the cost-effectiveness analysis. This income had the potential to sharply reduce the higher costs of photo-radar and enforced display boards."


Results of the study revealed "that both speed display boards and photo-radar effectively reduce vehicle speeds while deployed" and are "particularly effective in reducing the number of vehicles traveling ten or more miles over the speed limit." However, "only the display boards demonstrated carry-over effects," particularly in the long term. Already the most cost-effective of the speed control devices, the speed reduction capabilities of display boards can be greatly enhanced with "intermittent police enforcement."

While this study's potential limitations most likely had a minimal effect on the results, the author recommended five areas of research to "extend this study's findings." Additional research should examine whether the display board's "carry-over effects" hold true in other locations and "should establish optimal police enforcement levels for display boards that would maximize speed reductions at the deployment site and downstream." Further research should examine if speed control devices are also capable of reducing the number of traffic crashes. Finally, more research should focus on photo-radar devices, including their cost-effectiveness in other states and whether they "would yield greater results if supplemented with additional driver feedback, such as using a speed display board at deployment sites."

Copyright © 1998 by TranSafety, Inc.

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