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Road Management Journal
Copyright 1997 by TranSafety, Inc.
October 1, 1997
TranSafety, Inc.
(360) 683-6276
Fax: (360) 683-6719

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Warnings Combined with Enforcement Can Reduce Speeding

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Warnings Combined with Enforcement Can Reduce Speeding

Traditional speed management techniques are inefficient and have failed to lower highway driving speeds effectively. Such methods are labor-intensive and produce only temporary changes in driving behavior. Efficiency increases when speed warnings and enforcement are automated. Tests conducted in the Netherlands demonstrated that automatic systems can reduce both driving speeds and the percentage of speeders. Test data showed that the more often the enforcement system was used, the more drivers would comply with speed limits. For speed management systems to be effective, enforcement must have a greater priority.

A study commissioned by the Netherlands Ministry of Transport and provincial road authorities resulted in a report, "Automatic Speed Management in the Netherlands," by Hway-liem Oei. This report appeared in the Transportation Research Board's Transportation Research Record 1560, Traffic and Highway Safety: Occupant Restraints, Safety Management, and Emergency and Commercial Vehicles, published in 1996. The research included road authorities and police and was evaluated by the Institute for Road Safety Research (SWOV).


The objective of a Multi-Year Road Safety Program, first introduced in the Netherlands for 1987-1991, was "to reduce the number of traffic victims by 25 percent in the year 2000 compared with 1985." By this date, the target was to decrease:

  • the average speed by 5 to 10 percent,
  • the percentage of speeders to less than 10 percent,
  • fatalities by at least 150, and
  • injuries by 2,000.

Speed limits in the Netherlands are 120 kilometers per hour (km/hr) (75 mph) for cars on motorways, 100 km/hr (62 mph) on more congested roadways, 80 km/hr (50 mph) for all vehicles on rural roads, and 50 km/hr (31 mph) for all vehicles on urban roads. Speed data compiled before this study showed 40 to 70 percent of cars and 30 to 50 percent of trucks and buses were speeding. On urban roads the percentage of speeders was 60 to 80 percent.


The researcher cited studies in the U.S., Sweden, and Finland that showed disproportionate reductions in crash and injury rates were related to lowered speed limits. He predicted a crash rate reduction of 20 to 50 percent in the Netherlands when the national speed goal is reached.


Some methods of controlling speed are:

  • warning systems,
  • enforcement, and
  • information campaigns.

Speed warning systems vary from static methods such as a fixed sign reading "Lower your speed" to dynamic methods such as a switchable sign, triggered by a speeding vehicle, that reads "You are speeding." To be effective, these signs must be backed up by enforcement. Even with no enforcement, some drivers follow the speed limit. As enforcement increases, however, the percentage of compliers rises.

Enforcement methods can be mobile, stationary, nonautomatic, or automatic. Mobile police may stop speeders on the roadway, or stationary cameras may photograph licenses so that drivers can be fined later. In the Netherlands, police often stop the speeders, a task that requires large numbers of personnel. Still the percentage of speeders is high, and drivers resume their higher speeds after leaving an enforcement area and shortly after anti-speeding campaigns end. Based on this data, the Dutch government recommended increasing the efficiency of speed control methods.

Changing driver behavior requires consistent enforcement in conjunction with information campaigns to make drivers aware of speeding laws and the consequences of speeding.

The researcher stated that lowered vehicle speed would result in:

  • More time for perception, judgment, and action; linear relation between speed and time available;
  • Shorter braking distance;
  • Reduction of accident risk;
  • Lowering impact speed;
  • Reduction of injury (and fatality) risk; and
  • Reduction of injury severity.

Speeding detection through a warning and enforcement system would result in:

  • Increase in attention level of drivers,
  • More drivers complying with the rules,
  • Speed behavior that conforms with the rules, and
  • Disproportionately large reduction in accident rate.


Automated Speed Signs

Dynamic, automated speed signs warn only speeding drivers. They have the potential to be effective; however, their effectiveness depends on the information given--with better results when there is a specific reason for the speed warning. Such reasons might include: road maintenance, a sharp bend in the road, an intersection ahead, or a dangerous stretch of road.

Automatic Enforcement

Automatic enforcement programs only require police to process photos or videos. Drivers exceeding the speed limit by 30 km/hr (19 mph) or less are fined, while those caught driving more than 30 km/hr over the speed limit face criminal charges and may have to appear in court. A drawback to this system, however, may be that many violators will not pay the fines.

To be most effective, speed management systems should combine:

  • information campaigns that educate all road users,
  • fixed speed signs that inform all road users,
  • dynamic, automated signs that warn all violators, and
  • automatic enforcement that catches all persistent violators.

Local Speed Warning System

Urban Intersection

The study evaluated an urban intersection where school children cross. Researchers used three types of signs:

  • continuous 50 km/hr (31 mph),
  • 50 km/hr during school hours, and
  • 50 km/hr that flashed when an approaching vehicle was speeding during school hours.

Before starting the experiment, researchers conducted an information campaign. For comparison purposes, they measured speed before the signs were installed and three weeks after the start of operation. The best result occurred using the flashing sign-- which lowered the average speed by 5 km/hr (3 mph). Crash potential, depending on approach speed, showed a reduction of 24 to 65 percent.

Rural Intersection

Researchers studied the intersection of "a two-lane rural priority road with a speed limit of 100 km/hr [62 mph] and a road with a low traffic function." The test included:

  • an information campaign,
  • a permanent sign displaying a lowered speed of 70 km/hr (43 mph) approaching the intersection,
  • a permanent sign telling drivers to lower their speed, and
  • a flashing warning sign.

A police car periodically watched the intersection. The mean speed decreased from approximately 80 km/hr (50 mph) to 64 km/hr (40 mph), but the percentage of speeders remained about the same.

Provincial Road Stretches

The researcher conducted experiments on four two-lane rural roads with a speed limit of 80 km/hr (50 mph) to reduce:

  • high and low speeds on two roads closed to slow vehicles and
  • high speeds on two roads open to tractors.

The experiments began with an information campaign that included a press conference followed by newspaper and television coverage. The publicity emphasized the high risk of detection. The speed-reduction program incorporated automated warning signs and added automatic enforcement after 3 months. (The speed cameras also helped in the arrest of two persons suspected of vandalizing the system. The vandalism had no impact on the study.)

The system cost $40,000 (U.S.). With average savings of $14,000 per injury and $4,500 per property-damage-only incident, "[t]he breakeven point [was] reached at a savings of three injured victims or nine material-damage-only vehicles per annum."

The research showed a "consistent reduction in the characteristics of the speed distribution" at most locations on the experimental roads. On control roads, the researcher also observed a tendency of drivers to slow when they approached the radar posts and increase their speed after they passed.

Three years after this experiment, speeds had increased slightly and the percentage of speeders had increased by 5 percent. The experiment compared crash data for the seven months studied with the same seven months during the previous three years. The study found a 35 percent decrease. A long-term study showed that this decrease continued until 1993. The benefit was about three times the cost of the experiment.

Enforcement on Road Networks

The researcher characterized speeders as structural and incidental; structural speeders are those who speed most of the time. Structural speeders can be caught speeding anywhere along the roadway; however, periodic changes of enforcement sites will create unpredictability and aid speeder detection.

Speed enforcement experiments were performed in three provinces. Working groups included police, public prosecutors, road authorities, and regional organizations for road safety. SWOV supervised the project. The groups chose 120 road sections with high speed and crash rates. During daylight hours, radar and cameras performed the enforcement from a parked, unmarked car. Beyond the enforcement site, a sign informed motorists that their speed had been checked.

This enforcement project reduced the 85th-percentile speed by 4 to 5 km/hr (2 to 3 mph) and the percentage of speeders from 42 to 31 percent. The "absolute level" of speed, however, was still high. Data from the three provinces will be combined for a crash evaluation.

To learn the optimal level of enforcement, a randomly selected group of drivers was asked to participate in a survey. Surveys showed that:

  • 75 percent accepted automatic speed enforcement,
  • approximately 50 percent said they complied with speed limits,
  • 70 percent said they would comply if speed were enforced twice per year, and
  • "almost all" said they would comply if speed were enforced every week.


To change driver behavior effectively and reduce driving speeds, the author concluded that speed enforcement would need to be a higher priority. In addition, enforcement must be automated to provide the efficiency that will make increased enforcement practical.

Copyright 1997 by TranSafety, Inc.

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