Beware that work zone construction equipment operating at low speed can block and interfere with portable traffic detection systems used to control variable speed limit systems in active work zones.
Utah Department of Transportation (UDOT) implements dynamic management of portable variable speed limit (PVSL) technology to reduce regulatory speed limits in active work spaces.
Made Public Date


United States

Use of Portable and Dynamic Variable Speed Limits in Construction Zones


The Utah Department of Transportation (UDOT) was selected as a recipient of FHWA’s Accelerated Innovation Deployment (AID) Demonstration grant to fund the use of a portable variable speed limit (PVSL) system on four roadway construction projects along I-15 and I-80, with a focus to:

  • Encourage significant driver speed reductions within "active work spaces" where construction personnel are on site
  • Adjust speed limits automatically in accordance with speed harmonization principles
  • Minimize the length of the AWS for which the significant speed reductions are applied to encourage driver compliance with the speed limit
  • Increase regulatory speed limits when workers are not present
  • Provide a portable system that is easy for construction personnel to use.

Lessons Learned

This PVSL system used a dynamic variable speed limit (VSL) algorithm to raise and lower regulatory speed limits. The PVSL system also provided a queue warning algorithm that operated independent of the VSL algorithm to control roadside messages posted on portable dynamic message sign trailers.

The following lessons were gathered from the deployments:

If no data are received from portable vehicle detection equipment, post the last known speed limit. Communications between portable roadside dynamic message signs and traffic detection trailers can be interrupted when workers begin to relocate detectors to accommodate work zone progression down the roadway.

Move portable vehicle detection equipment first before moving other equipment. If the proper sequence of operation is followed, PVSL devices will have the last recommended speed limit posted until all equipment required for proper operation is relocated.

Construction equipment may interfere with traffic detection. Construction equipment operating at a low speed can interfere with automated work zone traffic control systems and generate a large number messages. A five-minute rolling average will likely work better than a one-minute rolling average when estimating traffic speeds in active work zones. Sensitivity testing of the algorithm logic used to control variable speed limits was recommended.

Goal Areas
System Engineering Elements

Focus Areas Taxonomy: