Variable speed limits (VSL) is an active traffic management (ATM) strategy that uses dynamic speed limit signs to slow traffic before and through adverse conditions on the freeway to improve safety and keep traffic moving efficiently.
There are two common types of VSL:
- Congestion-Responsive VSL are used to manage traffic flow in response to incidents or other congestion-causing conditions such as rush hour traffic. The VSL system steps down speed limits gradually and in advance of the congestion point to minimize hard decelerations and stop-and-go conditions. Reducing speeds uniformly and gradually helps prevent rear-end collisions at the back of a queue. When a return to normal speeds is warranted, the VSL system increments the speed limits back up to the default speed limit.
- Weather-Responsive VSL are used on roads where weather factors such as rain, snow, ice, or fog create hazardous driving conditions. Upon detecting deteriorating weather conditions, the VSL system reduces speeds with the goal of minimizing collisions.