Between 2010 and 2020, many transportation agencies began displaying safety messages and crash facts to the traveling public using roadside dynamic message signs (DMS). The content of these messages varies from annual crash and fatality statistics to more detailed messages regarding specific driving behaviors or risk factors. Despite the widespread use of DMS for safety messaging purposes, evaluations of potential impacts on driver behavior and resultant impacts on traffic safety have been limited. This research addresses this gap through a series of investigations to determine the degree to which the use of DMS for safety messages impacts various aspects of driver behavior. Driver behavior was examined in response to different DMS messages while considering critical contextual factors, such as the type of messages displayed, traffic flow conditions, and roadway geometric characteristics. The first set of field evaluations examined driver behavior as vehicles approached in-service emergency and MDOT service vehicles parked on the roadway shoulder in consideration of the state's move-over law. Compliance was measured in terms of speed reduction and lane selection and comparisons were made based upon a series of targeted messages displayed on upstream DMS. Logistic regression models were estimated to assess driver compliance with the law while considering important contextual factors, such as the type of vehicle on the shoulder and the message displayed on the DMS. The results indicate that drivers were more likely to move over or reduce their speeds when a police car was parked on the shoulder as compared to a transportation agency pickup truck. In general, the type of message displayed had minimal impact on driver behavior. The one exception showed that drivers were less likely to exceed the speed limit when targeted move over messages were shown as compared to standard travel time messages. For all message types, both speed and lane compliance were improved if the roadside vehicle was a police car. The second study examined cell phone use rates in consideration of enforcement activities that were conducted in conjunction with the display of targeted safety messages on roadside DMS. The results showed that cell phone use rates were lower during and, particularly, after the enforcement activities were conducted. Use rates were also found to vary based on age, gender, and race, allowing for the identification of target groups for public awareness and outreach campaigns. Cell phone use rates were also lower at freeway exit ramps compared to signalized and stop-controlled surface street intersections. Furthermore, cell phone-specific safety messages were associated with lower use rates than other message types.Ultimately, the findings largely reinforce federal guidance on the use of DMS for secondary purposes, which include displaying road safety messages. As a stand-alone measure, DMS provide marginal impacts on driver behavior. Such messaging strategies are likely to be more effective when used as a part of active safety campaigns with a limited duration as compared to more frequent and continuous display of generic message types. The outcome of the field investigations also shows that combining targeted messaging with the presence of enforcement results in the most substantive improvements in driver behavior.
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