Downhill speed warnings on mountain highways warn truck drivers when to put on the brakes.

In May 1998, a downhill speed warning system was deployed on a mountainous highway outside of Denver. The system was designed to influence driver behavior and reduce vehicle speeds as trucks emerged from the I-70 Eisenhower Tunnel and approached a 10-mile section of downhill grade (5-7%).

The system used weigh-in-motion sensors, inductive loop detectors, and computer hardware and software to monitor vehicle speed, weight, and axle configuration for trucks traveling through the tunnel. A computer algorithm was used to compute a safe descent speed for each type of vehicle weighing more than 40,000 pounds. Then a recommended speed/warning message was posted on a dynamic message sign at the end of the tunnel.

To evaluate system impacts, video cameras were used to measure changes in truck speeds downstream of the posted messages, and customer satisfaction surveys were given to a small number of truck drivers to evaluate their experience with the system.

Overall, the results indicated the system was effective at warning truck drivers of dangerous speeds. Speed data collected with and without the system active indicated the average truck speed for similar sized trucks decreased by about 5.2 miles per hour with the system active. Results from a survey of a limited number of truckers indicated that most drivers thought the system was helpful.

Related reports (see link below) show the system has reduced truck-related crashes on steep sections of this roadway even though truck volume has increased by about 5% per year.

See the related ITS Benefits Database entry for more information.

See also, Section 3.1 of the report Safety Applications of ITS in Rural Areas, which discusses the Colorado system.

Past "Benefit of the Month" summaries are also available.