Dynamic message signs (DMS) are used in conjunction with other media to communicate traffic conditions, general information, and recommended diversion strategies to motorists. Previous studies using loop detector data to estimate diversion rates attributable to advisory messages on DMS have found that diversion is minimal when vague messages are displayed or a distant alternate route is the only option. For motorists traveling on I-95 through Richmond in the Virginia Department of Transportation’s Central Region, however, when DMS alert motorists of incidents, I-295 serves as a comparable alternate route, adding no significant travel time to through trips. This scenario provides the opportunity to evaluate the effectiveness of DMS in traffic diversion without the major concerns of added trip time and the quality of the route.
This study investigated the impacts of existing message strategies to determine messages that maximize diversion for specific circumstances and to develop new messages for future deployment. An analysis was done for various message types and split into two diversion scenarios: (1) an incident on the primary freeway, I-95, encourages diversion of I-95 traffic to an alternate route, I-295; and (2) an incident on an intersecting freeway, I-295, encourages exiting
I-295 traffic to remain on I-95 as an alternate route. The results showed trends where the use of particular words in messages is more effective than the use of others in achieving diversion when percentage of diverted traffic was used as the performance measure.
The effects on traffic flow by drivers’ reactions to non-traffic messages were also investigated. Transportation agencies are frequently asked to post public service announcements on DMS when they are not being used for traffic-related purposes. It has been suggested that these messages are a distraction to drivers and result in queuing, creating mobility and safety hazards. An analysis that used speed as the performance measure showed minimal impacts on traffic flow from the display of non-traffic messages during weekday non-peak hours.
The study recommends that (1) travel time estimates for both the primary and alternate routes or the length or time of the delay be provided on DMS; (2) specific wording, as noted in the text, be used to induce diversion or simply to provide information; (3) messages be displayed in “title case” instead of “ALL CAPS” (i.e., all letters in a word are capitalized) for low-frequency messages; and (4) left-justified or “staircase” messages be used. Further, non-traffic messages should be one-phase messages and should be displayed only during non-peak periods to minimize the potential for queuing.
If the recommendations of this research are implemented, the enhanced effectiveness of diversion strategies will result in reductions of delay, fuel consumption, and emissions, as well as the potential for secondary accidents created by major incidents and other traffic flow disruptions. In 2007, the cost of delay for motorists in Richmond, Virginia, resulting from incidents was estimated at $119 million. A modest 1 percent reduction in this cost attributable to better diversion strategies that use DMS more effectively would result in an annual cost savings to VDOT of more than $1 million.