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Effectiveness of Changeable Message Signs in Controlling Vehicle Speeds in Work Zones: Phase II
Srinivasan, Srivatsan.
Nicholas J. Garber
Year: 1998
VTRC No.: 98-R10
Abstract: Highway work zones have been plagued with increasing numbers of accidents in recent years. Drivers' lack of compliance with speed restrictions within work zones has been cited as one of the major contributing factors to this trend. The conventional practice for regulating work zone speeds has been static signing procedures (using regulatory or advisory speed signs). It has been found that drivers do not slow down in response to these static control measures. Changeable message signs (CMS) equipped with a radar unit can be used to display specific warning messages to speeding drivers. The radar unit detects the speed of each vehicle entering the work zone and can be programmed to activate the CMS if the speed of the vehicle exceeds a preset threshold value. This offers a more dynamic speed control environment and therefore may prove to be more effective in influencing drivers to reduce their speeds. This report is the second phase of a longitudinal research study. The first phase of the project, conducted by Garber and Patel, examined the short term effectiveness of CMS in reducing vehicle speeds in work zones. That research established that the CMS (with the radar unit) is more effective in reducing speeds in work zones than the standard Manual on Uniform Traffic Control Devices (MUTCD) signs. This study, while attempting to replicate the results obtained in phase I of the project, concentrated on evaluating the effect of duration of exposure of the CMS with radar on its effectiveness in reducing speeds and influencing speed profiles in work zones. The impact of length of the work zone and vehicle type on speed reductions was also studied. Three work zone sites in southwest Virginia, two on Interstate 81 and one on a primary highway on Route 19, were selected for the study. Speed and volume data for the population were collected using automatic traffic counters at the beginning, middle and end of each work zone. In addition, the speeds of individual drivers who triggered the CMS by exceeding the threshold speed were also recorded (using a video camera) at two other locations within the work zone to study the behavior of high speed drivers in particular and to compute their average speed reduction in response to the warning message. The results of the study indicate that the duration of exposure of the CMS does not have a significant impact on speed characteristics and driver behavior. Therefore, the CMS continues to be effective in controlling speeds in work zones for projects of long duration. It was also determined that the drivers exceeding the speed limit, in both interstate work zones have on average reduced their speeds by around 12.86 km/h (8 mph) at the middle of the work zone. At the third site (Route 19) the speed reductions at the middle of the work zone were about 16.08 km/h (10 mph). It was also found that there were no distinctive differences among the different types of vehicles with regard to speed reduction. The study also established that in longer work zones, drivers who reduced their speeds in response to the speed control effort frequently have a tendency to speed back up as they approach the end of the work zone. This indicates that very long work zones might warrant the installation of a second CMS to maintain speed reductions through the work zone.