The impact of variable speed limits posted on dynamic message signs in the study area was evaluated by collecting traffic volume and speed data from loop detectors and machine vision license plate detectors before and after deployment.

FINDINGS

Variable Speed Limits

The variable speed limit system deployed as part of this project was implemented to the south of the work zone, enabling the adjustment of speed limits for northbound vehicles approaching the work zone during the morning rush. DMS were also deployed on the southbound portion of the freeway, allowing speed limits to be adjusted in response to recurring afternoon congestion at an interchange at the southern end of the study area.

The VSL system experienced considerable technical difficulties during the deployment which likely had a negative influence on the impacts of the system. Problems included displaying incorrect speed limits for up to 18 percent of the morning rush hour periods, and difficulties in responding to congestion during the afternoon peak periods (DMS signs going blank, or displaying incorrect speed limits).

Review of speed data collected during peak periods collected for one week before and after deployment of the VSL system determined that motorists do reduce speeds in response to the dynamic speed limits, by amounts generally less than 5 km/h depending on vehicle speed prior to the signs. These small changes and the bigger speed adjustments made during periods of traffic congestion meant that speeds became more homogenous only under periods of heavy traffic and congestion.