Abstract
This study examined the effectiveness of weather warning messages posted on a changeable message sign (CMS) as a speed reduction countermeasure for motorists approaching freeway bridge overpasses during winter weather conditions. Two weather warning messages were tested, including “Bridge Ices Before Road / Reduce Speeds” and “Slippery Road Conditions / Reduce Speeds,” in addition to the default travel time messages. The messages were evaluated through a series of field studies conducted in Michigan at three rural freeway bridge overpasses with a CMS on the approach during the winter seasons of 2023 and 2024. Speed measurements were obtained for each subject vehicle both upstream of the CMS and also at the start of the bridge. The study found that the winter weather warning messages were effective toward reducing the speed of vehicles approaching the bridge overpass during winter weather conditions. The most substantial impacts, in relation to both the speed reduction likelihood and magnitude, were observed when the “Slippery Road Conditions / Reduce Speeds” message was displayed on the CMS. With this message displayed, drivers were twice as likely to reduce their speed before reaching the bridge and the speed reductions were 0.6 to 0.7 mph greater, on average, compared with the default travel time messages. In addition, the strongest speed reduction were observed for the fastest group of drivers, which is the most vulnerable group from a crash severity risk standpoint and, thus, are considered the most highly targeted driver group for speed reductions.
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