This study was designed to evaluate the effectiveness of radar-equipped changeable message signs, now referred to as dynamic message signs (DMS), at reducing vehicle speeds at work zones. DMS units were deployed at seven work zones on two interstate highways in Virginia where construction activities were expected to last one week or less. Three work zones were studied in 1992 (I-81 South near Lexington, I-64 East near Covington, and I-64 East near Short Pump). Data collection was discontinued during the winter months. Data collection at the remaining four sites (I-81 North near Bristol, I-81 North and I-81 South near Abingdon, and I-64 East near Shadwell) was completed between May 1993 and November 1993. Each work zone lowered speed limits on the freeways by 10 mi/hr, with the exception of one location (I-81 North near Abingdon) which lowered speeds by 20 mi/hr and was excluded from the analysis.
To evaluate the impact of the DMS messages on vehicle speeds, vehicle speed and traffic volume data were collected at the beginning, middle, and end of each work zone area using pneumatic road tube counters. The radar equipped DMS units were designed to display specific warning messages if vehicle speeds exceeded preset threshold levels.
Data collected from road tubes and video tapes were used to evaluate impacts with and without treatment. Statistical tests were conducted to determine whether significant reductions in speed accompanied the use of DMS.
Overall the study found that the radar equipped changeable message signs are more effective than standard static signs (per the Manual on Uniform Traffic Control Devices) in altering driver behavior in work zones. The application of radar-equipped DMS effectively reduced the number of vehicles speeding in the work zone. The number of vehicles speeding was reduced by a statistically significant amount in each of three groups of monitored vehicles: those speeding by any amount, by 5 mi/hr or more, and by 10 mph or more. In most cases, the use of the DMS resulted in the reduction of vehicles speeding by 50 percent or more.
The messages displayed on the DMS were rated according to their level of effectiveness as follows:
- YOU ARE SPEEDING SLOW DOWN.
- HIGH SPEED SLOW DOWN.
- REDUCE SPEED IN WORK ZONE.
- EXCESSIVE SPEED SLOW DOWN.
See also: Garber, N.J., and S. Srinivasan. Effectiveness of Changeable Message Signs in Controlling Vehicle Speeds in Work Zones: Phase II, Virginia Transportation Research Council. Charlottesville, VA: December 1998.
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