The Distance Between Consecutive Variable Speed Advisory (VSA) Signs Installed on State Highways Should Not Be Greater Than One-Half Mile According to a Caltrans Field Study.

A Field Test on State Route 78 in San Diego, California Offers Lessons Learned on Data Collection, Location, and Physical Characteristics of VSA Signs.

Date Posted

Field Experiment of Variable Speed Advisory (VSA)

Summary Information

 Field tests of Variable Speed Advisory (VSA), an Active Traffic Management strategy, were conducted under the lead of the California Department of Transportation (Caltrans) District 11. The test site is located along a 10.8-mile section on State Route 78 Eastbound which goes through the City of Vista to the city of Escondido in San Diego County, California. Data collection included raw traffic data such as occupancy, flow, and speed from loop detectors, radar speed data from VSA signs, and fused traffic state parameters from the VSA algorithm. Seven VSA signs installed on the roadside displayed VSA and traffic speed.

The following were lessons learned from field testing.

  • Implement VSA sign with radar detection to better estimate the mainline traffic and to evaluate driver compliance using traffic speed and displayed speed.
  • Increase the size of the VSA signs for better viewability for drivers. A more effective VSA deployment should use larger and more prominent signs, or use speed enforcement.
  • Increase the size of the Changeable Message Signs (CMS) upstream to allow a better description of the reason for following the speed advisory.
  • Adjust the distance between two consecutive VSA signs to be about 0.4 to 0.5 miles, but not greater than one-half mile.
  • Ensure sufficient time for the field test period.  A three-month test would provide a better opportunity for the public driver to adapt to the VSA system, to achieve better performance improvement in traffic control.
  • Consider more than one week for the initial test period for system and algorithm tuning.
  • Consider the use of a website as a useful tool for observing traffic behavior and possible problems with the VSA system.

Portable Roadside Variable Speed Advisory (VSA) Sign

Figure 1. VSA sign used in the study

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