Variable Message Sign strategies for congestion warning on motorways – a driving simulator study

Congestion and stopped traffic on motorways constitute a potential safety hazard for drivers. In particular, unexpected hard congestion tails are the causes of serious rear-end crashes as they compromise the driver’s ability to gradually lower speed and avoid erratic maneuvers. Advance congestion warning has been proposed as a countermeasure to prepare drivers adapting to the changing traffic conditions in real time. Typically, Variable Message Signs (VMS) can be installed on a gantry (i.e., overhead sign bridge) or cantilever (i.e., road-side pole) to display a congestion warning. In this study, we used a driving simulator and eye tracker to evaluate the effect of several congestion warning message sign strategies on driving behavior on the motorway. In total, we developed seven scenarios to investigate the effect of varying message designs and warning distances on driving speed, deceleration and accident involvement. The results showed that a gantry-mounted congestion warning displaying a pictogram in combination with the word ‘congestion’ at 1km before the congestion tail had the best impact on speed, resulting in smooth deceleration and safe stopping distances. Overall, the effectiveness of congestion warning via VMS is dependent upon visibility conditions (i.e., drivers need to be able to clearly see and read the warning message) and message-incident contingency (i.e., warning messages located too far away from the congested area have no clear link with the critical event warned for anymore, and therefore, lose their effect on driving behavior).