Variable message signs (VMSs) installed on roadways are commonly used to convey specific messages to drivers about traffic conditions. During incidents with significant congestion such as major crashes, VMSs are sometimes used to divert traffic to alternative routes. Researchers analyzed drivers’ diversion responses to different VMS messages commonly used in Utah during crash incidents for a 57-mile segment of I-15 in the Salt Lake City, Utah area. Message histories from 21 VMS devices and associated crash data for this segment were analyzed for the period 2016–2020. The diversion rate was defined as the difference between the highest proportion of vehicles exiting while a VMS message was active and the proportion when no message was displayed. A statistical model was used to estimate the increase in diversion rate with respect to message content, number of frames used to display the message, time difference between crash incident and message display, distance between crash incident and VMS device, message display duration, time of day, day of week, and roadway characteristics during message display.
- Select standardized message content specifically chosen to encourage or reduce diversion according to the desired goal. Informing drivers of a crash or traffic ahead, the number of miles to crash, crash location or lane, and delay information was associated with higher diversion rates. A message combining miles to the crash and “prepare to stop” had the highest diversion rate. Messages such as “use caution,” a speed suggestion, or “prepare to stop” by itself had lower diversion rates.
- In crash prone areas, increase the number of VMS devices to decrease the average distance between a sign and crash incidents. The diversion rate was greater when the distance between the VMS and incident was shorter.
- Reduce the use of VMS messages unrelated to crashes. From the VMS historical data, less than two percent of VMS messages were classified as related to crashes. The influence of VMS messages on driver behavior may be reduced if there is an oversaturation of non-specific safety messages, or non-safety messages.