Portable variable speed limit system in Utah successfully reduces speed limit by 15 to 25 mi/hr near work zones.

Utah Department of Transportation (UDOT) implements dynamic management of portable variable speed limit (PVSL) technology to reduce regulatory speed limits through an active work space (AWS).

Date Posted

Use of Portable and Dynamic Variable Speed Limits in Construction Zones

Summary Information

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, 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.

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

UDOT has completed two years of PVSL system deployment testing in four separate construction work zones to evaluate the effectiveness of the system. The four selected projects are described below:



Posted Speed

PVSL Low Speed

Location Type of Construction AADT and Number of Lanes

Posted Speed Limit = 80mi/hr

PVSL Low Speed = 45mi/hr

I-15 (Honeyville to Tremonton) Mill, paving, and bridge repair 22,000 AADT 4-lane divided highway

Posted Speed Limit = 80mi/hr

PVSL Low Speed = 50mi/hr

I-15 (Baker Canyon to Meadow) Mill and paving 12,000 AADT 4-lane divided highway

Posted Speed Limit = 80mi/hr

PVSL Low Speed = 50mi/hr

I-15 (Corinne to Honeyville) Mill and paving 22,000 AADT 4-lane divided highway

Posted Speed Limit = 80mi/hr

PVSL Low Speed = 50mi/hr

I-80 (milepost 20 to 30) Mill and paving 7,500 AADT 4-lane divided highway

"Before" and "After" traffic data was collected using Doppler radar technology. UDOT collected the same vehicle count and average vehicle speed data collected during the baseline data collection period during the full PVSL system deployment period.



  • Significant speed reductions, within the limited length of the AWS, were observed for the five-minute rolling average speed, ranging 10 to 15 mi/hr higher than the reduced posted speed. Overall average speeds near the AWS were 15 to 25 mi/hr lower than the original roadway posted speed limit.
  • Overall, the use of the PVSL system reduced the impact on drivers by limiting the length and duration the drivers had to reduce their speed and encounter the maximum speed limit reduction. The queue warning subsystem successfully demonstrated automatic triggering of the messaging to advise travelers of slow/stopped traffic ahead.


Project Speeds <10 mi/hr Over

Posted Speed
10 mi/hr <Speeds<15mi/hr Speeds >=15 mi/hr Over Posted Speed
  Baseline PVSL Baseline PVSL Baseline PVSL
Tremonton Overnight

(12- Hour Period)
39% 41% 16% 20% 45% 39%

(12-Hour Period)
23% 37% 10% 15% 67% 48%
Meadow 88% 58% 9% 29% 3% 13%
Honeyville N/A 60% N/A 29% N/A 10%
I-80 43% 81% 42% 14% 15% 6%
Goal Areas
Deployment Locations