Queue warnings

Queue warnings inform drivers about stopped or slow traffic ahead to provide vehicles more time to slow down safely. Queue warning systems use real-time traffic detection to identify queues and roadside variable message signs (VMS) to display the warnings.

Key characteristics




WSDOT regions

Other names

  • End-of-queue warnings

Strategy description

Queue warnings are a traveler information strategy that uses real-time traffic detection to traffic detectors strategy and warning signs to variable message signs strategy and/or flashing lights to inform drivers of stopped or slow traffic ahead. By providing early information, a queue warning system helps drivers anticipate an upcoming slowdown. This can reduce emergency braking or erratic behavior, and avoid queue-related collisions.

Based upon the sophistication of the system and the specific needs of the roadway, queue warning systems can operate automatically using local real-time traffic detectors as a trigger, or manually controlled by an operator at a traffic management center (TMC).

Queue warnings are frequently used on roads with physical characteristics such as curves, steep grades, or poor visibility (e.g., tunnel entrances), where drivers might have limited reaction time to see and respond to a queue ahead.

The real-time traffic detection and signage used for queue warnings can also be used for variable speed limits, incident management, dynamic lane assignment, and other active traffic management (ATM) and traveler information strategies, and are often implemented together as part of a larger corridor management strategy.

Work zones also benefit from queue warning with portable changeable message signs units placed ahead of expected queue points.

When to use this strategy

Queue warnings make sense for freeways or roads that are frequently congested and where queues occur frequently in predictable locations (e.g., near a major interchange). They also can be used in locations where vertical grades, curves, or poor illumination impact sight distance and may not give drivers enough time to observe and respond safely to a queue ahead.

Queue warnings can be used as a supporting strategy in conjunction with other active traffic management (ATM) and traveler information strategies, like variable speed limits, dynamic lane assignments, and variable message signs.

Portable changeable message signs can be used to provide queue warnings during construction, maintenance, or other temporary roadway activities that may create congestion.

Strategy benefits:

  • Reduces primary and secondary collisions by alerting drivers to congested conditions ahead
  • Reduces the severity of collisions by providing drivers with warning about congested conditions ahead so they can slow down in time
  • Delays the onset and shortens the duration of recurring freeway congestion by reducing collisions and improving smooth and efficient traffic flow
  • Reduces emissions and fuel consumption by decreasing stop-and-go traffic

What you need in order to implement

Policy needs:

  • Agency standards for queue warning systems, including approved products, desired messaging on the signs, and integration with other traveler information systems

Planning needs:

  • Operational analysis to identify appropriate locations for queue warnings, place signs, and establish proper messaging

Coordination needs:

  • Coordination with the applicable road owners, centralized traffic management center, and construction and maintenance groups

Equipment needs:

  • Variable message signs (VMS) to display queue warning messages
  • Overhead gantries or other mounting structures (for permanent VMS installations, not applicable to portable VMS)
  • Traffic detectors to gather traffic conditions data
  • Queue detection software to interpret traffic data, identify queues, and communicate queue warnings to field devices
  • Cameras to verify VMS messages and presence of a queue

Maintenance needs:

  • Regular maintenance of VMS, traffic detectors, and communications systems
  • Regular testing and validation of the queue detection system to confirm that queues are detected reliably and accurately and that warning messages are provided in a timely manner

Agency resources needs:

  • Traffic engineers to place signs, establish proper messaging, monitor its effectiveness,  and make modifications to the system
  • Traffic management center operators to operate and monitor the queue warning system

Learn more about this strategy

Federal Highway Administration (FHWA), Work Zone Intelligent Transportation Systems Implementation Guide (2014).

Texas A&M Transportation Institute, Queue Warning Technical Information Briefing (PDF).

About key characteristics

Location notes:

Queue warnings are useful for freeway locations with restricted site distances, locations where queues form regularly predictably, or where construction or maintenance work may result in slowdowns.

Cost notes:

The major cost driver for a permanent installation is the variable message sign and sign structure. The cost varies depending on the number of signs installed along the corridor and the extent to which existing infrastructure can be utilized. Costs for queue warning systems using PCMS are relatively low.

Technology notes:

Technology needs vary depending on the sophistication of the queue warning system, its level of automation, and the types of conditions it is designed to detect. All queue warning systems require traffic detection, software to identify formed queues, and communications to a roadway sign to display the warning.

Collaboration notes:

Collaboration needs are relatively low as the queue warning system typically operates on a single facility within a single jurisdiction.