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variable speed limit (VSL)

Contributor(s): Ivy Wigmore

A variable speed limit is a flexible restriction on the rate at which motorists can drive on a given stretch of road. The speed limit changes according to the current environmental and road conditions and is displayed on an electronic traffic sign. Signs typically indicate a maximum speed and may also list the minimum.

Variable speed limits are implemented to increase safety and provide clear guidance for motorists. Conventional speed limit signs, in contrast, usually list a single speed, generally the maximum rate. The maximum speed limit is generally considered to apply when conditions are optimal: There is no precipitation or fog and the roads are free of snow, water and ice, for example. Drivers are expected to adjust their speed to adapt to any adverse conditions, but there are no rates specified for particular situations.

Te lack of specificity of conventional signage can contribute to accidents, speeding tickets and criminal charges. Many motorists assume that posted speed limits apply at all times and drive at that speed in all conditions, but the maximum speed can be quite unsafe at other times. Charges for "excessive speed" can be the result, even though the motorist may have been driving under the posted speed limit. In the event of an accident, excessive speed could be considered dangerous driving and result in criminal charges.

Flexible speed limits solve that problem by adapting so that the posted speed limit is appropriate for the current conditions. Currently, the speed limit indicated on a variable sign is usually determined by a centralized authority, as deemed appropriate for conditions. As the Internet of Things (IoT) develops, however, networked sensors in the environment are likely to automate that process; connected and self-driving cars could also be designed to comply automatically.

This was last updated in April 2017

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