More than 76 percent of drivers on an interstate route in northern California indicated that messages displayed by an advanced curve warning system were useful.

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.


Survey results indicated that truck drivers in the corridor generally drove it frequently and were familiar with the curves, while passenger car drivers were most often infrequent travelers unfamiliar with the curves. Seventy-three (73) percent of truck drivers responding to the first survey and 70 percent responding to the second survey indicated that the signs were useful. During the two visits, 76 percent and 69 percent, respectively, indicated that they reduced their speed in response to the signs. Seventy-eight (78) percent of the passenger car drivers responding to the first survey and 85 percent in the second survey stated that the system was useful. Sixty (60) percent participating in the first survey and 69 percent in the second survey indicated that they reduced their speed in response to the signs.

The authors note a potential flaw in the survey. The number of drivers who reduced their speed in response to the signs may not completely represent the effectiveness of the signs because the survey fails to assess the impact of the sign on travelers who were already traveling below the advisory speed displayed on the signs.

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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: 2007-00415