This study analyzed the effects of dynamic message signs (DMS) and in-vehicle traffic advisory systems (IVUs) on drivers traveling on Snoqualmie Pass (I-90) in Washington State. During the winter of 1997 and 1998 variable message signs were used to provide speed limit, weather, and roadway information. Based on historical accident records and loop detector vehicle speed data, several models were developed to estimate accident frequencies, accident severities, mean driving speeds, and mean driving speed deviations at sites "with" and "without" access to travel advisory information. DMS signs at sites with loop detectors were turned on or off depending on the need for traveler information, however, it was problematic to determine if drivers slowed down because of DMS information or just because travel conditions were degraded. Therefore, the differences between mean vehicle speeds and speed deviations recorded at each site were modified using a logit model and probabilistic model to account for typical reductions in speed as a result of poor travel conditions. The derived data was then analyzed along with driver behavior experiments in order to estimate the impact of traveler information on accident frequency and severity. Typical driver behavior was modeled using geometric and weather-related variables to account for varied roadway configurations and weather conditions.
Human laboratory experiments and traveler surveys were used to check the results of the modeling effort. Driver reaction to roadway variables and travel advisory information was measured using: a compact car driving simulator equipped with auditory and visual effects, 51 representative test subjects who actually drove on Snoqualmie Pass, and 444 mail-back surveys from area travelers.
In general, the modeling effort and simulator experiments estimated the impact of travel advisory information on mean driving speeds, mean driving speed deviation, and the number of accidents and accident severity in the study area.
Travel advisory information on DMS signs can decrease mean driving speeds and reduce accident severity; however, if traveler information is provided to some, but not all drivers (or DMS sites), mean driving speed deviations can increase and temper the effect of lower mean speeds.
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