Variable message signs

Variable message signs (VMS) are electronic roadside signs used to post traveler information messages to inform drivers of incidents, travel times, detours, special events, and other useful road conditions or travel information.

Key characteristics




WSDOT regions

Other names

  • Dynamic message signs
  • Changeable message signs
  • Portable changeable message signs

Strategy description

Variable message signs (VMS) are permanently-installed or portable electronic roadside signs operated remotely by traffic management centers (TMC) to post traveler information messages to inform drivers of incidents, travel times, detours, special events, and other useful road conditions or travel information. VMS can be used to post messages required by various demand management and traveler information strategies, including queue warnings, variable speed limitsroute choice, and dynamic lane assignment.

VMS Placement

VMS are typically connected to a central location, such as a TMC, where the signs are operated remotely and can be quickly changed to provide current information. VMS are especially useful to display in advance of locations where travelers must take action or make route choice decisions. Because VMS messages can be seen only when drivers pass by, strategic placement of the signs, especially permanently-installed signs, is critical. Portable VMS can be used to display traveler information messages for special events or other temporary conditions, such as road construction.

Message content 

VMS messages must also be concise and clear enough for the drivers to interpret at high speeds. The Manual on Uniform Traffic Control Devices (MUTCD), published by FHWA, provides detailed guidance on appropriate text size, resolution, and message content for the VMS sign based on the expected speed of drivers.

Public service announcements

When not being used to display traveler information, VMS may be used to display public service announcements, including messages about distracted driving, impaired driving, and seat belts. However, because overuse of VMS can lead to drivers ignoring the posted messages, many agencies strictly limit the types of messages that can be displayed, reserving VMS only for critical traveler information or safety messages.

When to use this strategy

VMS makes sense for locations where travelers routinely need information (e.g., the entrance to mountain passes) or before locations where travelers must make a routing decision. They can also support active traffic management strategies where having a means to provide en-route traveler information is a core element of the strategy. Portable VMS are useful during roadway construction and special events to provide information about temporary conditions to travelers.

Strategy benefits:

  • Improves traveler decision making, which can improve safety, reduce congestion and travel delay, and decrease traveler frustration
  • Reduces collisions by alerting drivers to slowed or stopped traffic ahead
  • When combined with other ATM strategies, such as variable speed limits, VMS improves driver compliance with posted speeds because drivers are more likely to obey the speed limits when they are aware of the reason for the reduced speed.
  • As compared with other technologies used to provide traveler information—such as in-vehicle navigation devices and mobile apps—VMS it is widely accessible, easily comprehended, and less likely to cause driver distraction.

What you need in order to implement

Policy needs:

  • Development of message prioritization rules to ensure that critical messages are displayed when needed and what types of messages are prohibited
  • Clear and direct language in messages that allows for quick and easy comprehension

Planning needs:

  • Analysis to determine the number, placement, and type of VMS signs necessary to provide traveler information effectively (e.g., a VMS placed well upstream of a major interchange allows travelers time to reroute after seeing the sign)

Coordination needs:

  • Coordination with external agencies, including highway patrol, on how and when to display non-traveler information messages

Equipment needs:

  • VMS displays of the appropriate size and resolution to provide the required traveler information messages
  • Overhead gantries or other mounting structures for permanently-installed VMS (not applicable for portable VMS)
  • Centralized VMS software to determine message content and communicate to multiple field devices simultaneously for events that impact wider areas
  • Cameras adjacent to VMS to verify messages posted correctly (optional)

Maintenance needs:

  • Regular maintenance of VMS to ensure the display components are functional and that all posted messages are displayed fully

Agency resources needs:

  • Traffic engineers to place signs and establish proper messaging
  • TMC operators to operate and monitor the VMS

About key characteristics

Location notes:

VMS works best for high volume corridors and primary routes where travelers would benefit from getting information about incidents, travel times, detours, or special events.

Cost notes:

VMS costs vary depending on the number of signs installed along the corridor and the extent to which existing infrastructure and mounting structures can be utilized. Costs for portable VMS are relatively low.

Technology notes:

Technology needs for VMS vary depending on the number and sophistication of active traffic management and traveler information strategies that are using the VMS for message display. Most VMS systems are centrally controlled at a traffic management center and require software to generate message content and communications to the roadway signs to display the messages.

Collaboration notes:

Operating a VMS system typically involves some collaboration with external agencies in order to display a broader range of traveler information from various sources.