An advanced curve warning system on an interstate route in northern California caused over 68 percent of drivers to reduce their speed.

April 2000
Interstate 5, shasta county,California,United States

Summary Information

This report summarizes an evaluation of an advanced curve warning system installed at five curves along Interstate 5 in a mountainous portion of rural northern California. Traffic volumes through the curves are low, ranging from an average daily traffic of 7,650 to 9,300 vehicles. The advanced warning systems consisted of variable message signs installed before each of the curves displaying warning messages about the upcoming curves. Using data from a radar unit mounted near the signs, the system also displays the actual speed of vehicles approaching the curves. Several other alterations to the conditions at the curves may have influenced the results of the evaluation. The speed limit for passenger cars at three of the five curves was raised from 55 to 65 mph. The California Department of Transportation added median barriers at two curves and also resurfaced the roadway at two of the curves. Rain and wet pavement conditions may also have impacted the speed measurements taken at two of the curves during the final speed collection visit.

The evaluation consisted of a comparison of speed data gathered before and after installation of the warning system, and surveys taken approximately 2 months and 10 months after installation. Speed data were collected using stopwatches to determine the speed of vehicles approaching the curves as the vehicles traveled over measured distances. Speed measurements were collected 9 months prior to the system’s installation and again 2 months, 5 months, and 10 months after operation began. Surveys of vehicle drivers were taken at a rest area within the corridor. One hundred and fifty-three (153) truck drivers completed surveys regarding the system 2 months after it began operation and 162 completed the second survey 8 months later. Eighty-nine (89) passenger car drivers completed surveys 2 months after operation began, and 77 completed the second survey. The report also discusses the crash statistics for the curves during the five years preceding installation of the warning system, but the short evaluation period following installation makes comparison of accident statistics invalid.


For three of the five curves, the reduction in the speed of trucks was statistically significant for at least one of the three data collection periods after the warning signs began operation. The two curves that demonstrated a significant reduction in truck speeds for all three visits after installation had downgrades greater than five percent. The speed reductions were smaller for the later visits to the curves, possibly indicating the drivers were becoming accustomed to the signs and paying less attention to them. Passenger vehicles also demonstrated significant speed reductions at two of the five curves. One curve recorded statistically significant speed reductions during the three site visits after installation of the warning system, while two of the post-implementation visits to another curve revealed significant speed reductions.

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An Evaluation of Dynamic Curve Warning Systems in the Sacramento River Canyon: Final Report

Author: Tribbett, Lani, et al.

Published By: California DOT

Prepared by the Western Transportation Institute and Montana State University for the California DOT New Technology and Research Program

Source Date: April 2000



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Benefit ID: 2000-00161