Effects of Variable Speed Limits on Motorway Traffic Flow

Variable speed limits (VSLs) displayed on roadside variable message signs (VMSs) have emerged as a widespread traffic control measure on motorways in many countries, leading to substantial traffic safety benefits; however, there is no clear evidence of improved traffic flow efficiency in operational VSL systems. The available information on VSL impact on aggregate traffic flow behavior is summarized, and the issue is investigated in more detail with real traffic data from a European motorway. It is found that VSLs decrease the slope of the flow–occupancy diagram at undercritical conditions, shift the critical occupancy to higher values, and enable higher flows at the same occupancy values in over-critical conditions. Implications of the derived findings for more efficient VSL control strategies are discussed.