A variable- (also changeable-, electronic-, or dynamic-) message sign, often abbreviated VMS, CMS, or DMS, and in the UK known as a matrix sign, is an electronic traffic sign often used on roadways to give motorists and pedestrians information about special events. Such signs warn of traffic congestion, accidents, incidents, roadwork zones, or speed limits on a specific highway segment. In urban areas, VMS are used within parking guidance and information systems to guide drivers to available car parking spaces. They may also ask vehicles to take alternative routes, limit travel speed, warn of duration and location of the incidents or just inform of the traffic conditions.

A complete message on a panel generally includes a problem statement indicating incident, roadwork, stalled vehicle etc; a location statement indicating where the incident is located; an effect statement indicating lane closure, delay, etc and an action statement giving suggestion what to do traffic conditions ahead.

In some places, VMSes are set up with permanent, semi-static displays indicating predicted travel times to important traffic destinations such as major cities or interchanges along the route of a motorway.

Typical messages provide the following information:

• Crashes, including vehicle spin-out or rollover
• Stalls affecting normal flow in a lane or on shoulders
• Non-recurring congestion, often a residual effect of cleared crash
• Closures of an entire road
• Downstream exit ramp closures
• Debris on roadway
• Vehicle fires
• Short-term maintenance or construction lasting less than three days
• Pavement failure alerts
• AMBER Alerts and weather warnings
• Travel times
• Variable speed limits
• Event information
• Product launch or promotion

The information comes from a variety of traffic monitoring and surveillance systems. It is expected that by providing real-time information on special events on the oncoming road, VMS can improve vehicles’ route selection, reduce travel time, mitigate the severity and duration of incidents and improve the performance of the transportation network.