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.
Key characteristics
Setting/Location
Cost
Technology
Collaboration
WSDOT regions
Other names
- Weather-responsive VSL
- Congestion-responsive VSL
- Speed harmonization
- Dynamic speed limits
Strategy description
Variable speed limits (VSL) is a safety and congestion management strategy that works by dynamically changing posted speed limits to reflect current road conditions. VSL systems use sensors to detect freeway congestion or weather conditions and algorithms to determine how, when, and where to implement temporary changes in speed limits.
There are two common types of VSL:
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Congestion-responsive systems 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.
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Weather-responsive systems 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.
The VSL system uses congestion or weather data to inform speed limit changes, often in increments of 5 or 10 mph. These speed changes are displayed on overhead signs mounted lane-by-lane. Both speed reductions and adjustments back to the normally posted speed are made in gradual increments until conditions return to normal. However, VSL systems would not recommend a speed limit greater than the default posted limit for that area or facility.
The VSL sign infrastructure is compatible with dynamic lane assignment strategies (e.g., using lane-by-lane overhead signs to indicate lane closures ahead) and they are often used together.
VSL also pairs with variable message signs (VMS) to provide explanations for speed reductions or other relevant traveler information. Drivers are more likely to comply with dynamic speed limits when they are aware of the reason for the reduced speed limit.
Posted speeds can be either regulatory (enforceable) or advisory (non-enforceable), depending on the operating agency’s regulatory authority or the objectives of the system. Where allowed by law, regulatory speed limits are recommended for longer stretches of road and where there is high confidence in the accuracy of the real-time data and appropriateness of the speed recommendation. Advisory speeds are recommended for isolated locations and in cases where real-time data may be less accurate or enforcement would not be possible.
VSL also pairs with variable message signs (VMS) to provide explanations for speed reductions or other relevant traveler information. Drivers are more likely to comply with dynamic speed limits when they are aware of the reason for the reduced speed limit.
Posted speeds can be either regulatory (enforceable) or advisory (non-enforceable), depending on the operating agency’s regulatory authority or the objectives of the system. Where allowed by law, regulatory speed limits are recommended for longer stretches of road and where there is high confidence in the accuracy of the real-time data and appropriateness of the speed recommendation. Advisory speeds are recommended for isolated locations and in cases where real-time data may be less accurate or enforcement would not be possible.
When to use this strategy
VSL makes sense for:
- Freeways or other major corridors that have frequent congestion
- Specific locations with known issues like high collision “hotspots”, construction or maintenance activities, bottlenecks (e.g., lane narrowing, sharp curves), or adverse weather
Strategy benefits:
- Improves safety because speed limits better reflect road conditions
- Reduces crashes associated with stop-and-go freeway traffic
- Delays the onset of congestion by allowing traffic to flow more smoothly and efficiently
- Reduces emissions caused by stop-and-go traffic
What you need in order to implement
Planning needs:
- Operational analysis to develop an appropriate speed recommendation algorithm that considers traffic volume, operating speeds, sight distance, and roadway surface condition
- Legislative and legal analysis to determine whether regulatory or advisory speeds are feasible
Coordination needs:
- Law enforcement coordination to ensure driver compliance and address potential violations
- Media and public outreach to provide education to drivers if VSL is a new concept
Equipment needs:
- VSL signs
- Overhead gantries or other mounting structures
- Centralized VSL software to calculate speed limits and communicate to field devices
- Corridor-wide vehicle detection to accurately collect real-time speed and volume data
- Environmental sensors and other weather detection to accurately collect road weather conditions
- Variable message signs to display explanations for speed reductions and other traveler information
- Closed circuit television (CCTV) cameras for verification and monitoring
Agency resources needs:
- Resources to monitor the functioning of the VSL system and adjust how it works to maximize performance
Learn more about this strategy
FHWA Guidelines for the Use of Variable Speed Limit Systems in Wet Weather, 2012.
About key characteristics
Location notes:
VSL systems are used exclusively on freeways. Speed limit signs should be mounted over each lane of traffic to ensure visibility to all vehicles.
Cost notes:
VSL costs vary considerably depending on the number and sophistication of signs and the extent to which existing infrastructure can be used. New overhead gantry structures are typically the most significant cost component of a VSL installation, so opportunities to adapt existing infrastructure (sign structures, bridges, overcrossings, etc.) can greatly reduce costs.
Technology notes:
Technology needs vary depending on the sophistication of the VSL system, its level of automation, and the types of conditions it is designed to detect, but overall VSL is a mid-range cost solution.
Collaboration notes:
Collaboration needs are relatively low as the VSL system typically operates on a single facility within a single jurisdiction.