Assessment of Variable Speed Limit Implementation Issues
Variable speed limits (VSL) are adjusted to reflect current road and
environmental conditions, and this may enhance both road safety and
driver compliance. The objectives of this project are to (1) assess
the impacts of and the implementation issues associated with deployment
of variable speed limits for a limited number of driving situations
and (2) develop operational test plans for the most promising applications.
The phase one report has been submitted to and approved by the sponsor.
We have conducted an exhaustive review of previous work on VSL and automated
speed enforcement (ASE). Among the most salient findings are:
• VSL increases safety (reduces crashes) by reducing average speeds
and speed variance.
• VSL reduces the number of speed violations.
• VSL is effective in weather related settings where it is difficult
for drivers to detect the hazard.
• VSL applied in congested settings can smooth traffic flows and
reduce the incidence of stop-and-go driving.
• Drivers view VSL systems favorably.
• ASE is an effective speed control tactic which is widely used
in Europe.
• There is resistance to using ASE in the U.S.
We have identified the most promising settings for employing VSL, in
general and for a VSL field test. Situations warranting VSL all have
some varying factor that makes it advantageous to modify the speed limit
relatively frequently. These may include severe weather, activity at
work zones, or non-recurrent congestion due to incidents. The benefits
and costs of VSLs will differ across settings presenting these types
of factor variations. Costs will be higher where real-time condition
monitoring is required.
We have examined alternative implementation approaches for the promising
settings. These areas have been studied in detail:
Speed determination – how to set the desired speed limit. This
will vary with both setting and desired level of technical sophistication,
and can eventually become fully automated;
Signs and Control Devices for VSL, particularly the technology and placement
of variable message signs; Failsafe characteristics of a VSL system;
VSL enforcement strategies, including the use of automated as well as
traditional enforcement tools, technologies available, sources of resistance,
and reactions of enforcement agencies; and
VSL system evaluation, preliminary assessment of the benefits and costs
of VLS implementations.
Evaluation results suggest that VSL systems will be cost-effective in
common traffic settings where the influencing factors change quickly
and significantly. Benefits will be greater for larger traffic volumes,
but even at moderate volumes some applications will be attractive. The
display systems represent the largest cost component.
This work continues with more detailed analysis of issues related to
implementing and enforcing variable speed limits, estimation of the
effects on safety, speed, speed variance, capacity, and speed limit
compliance for combinations of variable speed limits and enforcement
and the design and implementation and evaluation of a field test of
VSL in a work zone. The project team is in the process of negotiating
with a large transportation agency to secure a field test site.
National Cooperative Highway Research Program
Researcher(s): Joseph Schofer, Alexander Weiss, Athanasios Ziliaskopoulos, Richards Raub, Roy Lucke, Dawn Marincic, Huajing Shi and Brian Lee.
