Simulation Study Estimates a Combination of Connected Vehicle Control and Variable Speed Limit Strategies Can Reduce Rear-End Crash Risk under Fog Conditions by 48.7 Percent.

Researchers in Florida Used Driving Simulator Experiments and Microsimulation to Assess Connected Vehicle Technologies Implemented in Tandem with Variable Speed Limit Strategies.

Date Posted

The Impact of Connected Vehicle Market Penetration and Connectivity Levels on Traffic Safety in Connected Vehicles Transition Period

Summary Information

Connected Vehicle (CV) crash warning systems have the potential to improve vehicle safety by alerting drivers of imminent situations so they can take timely crash-avoidance actions. Researchers used a driving simulator to evaluate the effectiveness of a head-up display (HUD) and audio warning system on drivers’ crash-avoidance performance. The simulator scenario focused on a situation when a lead vehicle makes an emergency stop under adverse weather conditions, specifically fog conditions with reduced visibility. Furthermore, an integrated variable speed limit (VSL) and CV control strategy was developed and tested in a microsimulation environment to assess potential reductions in rear-end crash risk at freeway bottlenecks, also under fog conditions.


In the driving simulation, throttle release time, brake transition time, perception response time, brake reaction time, minimum modified time-to-collision, and maximum brake pedal pressure were analyzed for 54 participants who drove scenarios that included fog conditions. Each participant performed the experiment under three warning conditions:

  • No Warning
  • HUD Only Warning
  • HUD and Audio Warning

For the integrated VSL and CV strategy, the proposed VSL strategy was tested for a freeway section with a bottleneck through microsimulation, and the driving simulator was employed to build the CV environment. Time-to-collision at braking (​​​​​TTC𝑏𝑟𝑎𝑘𝑒) and total travel time (TTT) were captured from the simulation to assess the proposed combined control strategy. In addition, TTC𝑏𝑟𝑎𝑘𝑒 percentage was defined as the proportion of TTC𝑏𝑟𝑎𝑘𝑒 values below a 2 second threshold, representing the measure of rear-end crash risk.


  • The warning system reduced drivers’ brake response time and increased the minimum modified time-to-collision as compared to the no warning condition. However, there was no significant difference found between the HUD Only warning and the combined HUD and Audio warning. No significant gender effect could be identified in this study.
  • Compared with the scenario without any speed controls, the VSL control strategy reduced the rear-end crash risk, as measured by TTC𝑏𝑟𝑎𝑘𝑒 percentage, under fog conditions by 29.5 percent and 6.3 percent for low- and high-volume conditions respectively, when the VSL compliance rate is 100 percent. However, because VSL control reduces speeds to enhance safety, this strategy increased TTT by 26.9 percent and 3.5 percent for low- and high-volume conditions, respectively.
  • Implementing VSL in a CV environment could further enhance safety. When CV technology was implemented in combination with the VSL strategy, the rear-end crash risk was reduced by 48.7 percent and 6.6 percent for low- and high-volume conditions, respectively, when the VSL compliance rate is 100 percent. However, at lower VSL compliance rates, control performance was found to be variable in the high-volume scenario.
  • The inclusion of CVs could diminish the increase in travel time caused by VSL control, especially for high-volume conditions. In contrast to a 3.5 percent increase in TTT when only using the VSL strategy, the VSL and CV combined strategy could decrease TTT by 55.2 percent under high-volume conditions.
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