Assessing Mobility and Safety Impacts of a Variable Speed Limit Control Strategy

Abstract

With the recent advances in active transportation and demand management, variable speed limits (VSLs) have been identified as an active traffic management strategy for improving freeway mobility and safety. Several heuristic VSL strategies have been proposed and evaluated. This paper proposes a model predictive VSL control strategy and evaluates its safety and mobility impacts. The strategy uses second-order traffic flow models to predict the traffic state and to provide a speed for optimizing corridor operational performance. A sensitivity analysis of the VSL update frequency and the safety constraints for the VSL strategy was performed to determine the best scenario in terms of safety and mobility. A stretch of Whitemud Drive, an urban freeway corridor in Edmonton, Alberta, Canada, was selected as the study area. The proposed VSL strategy was implemented in the microsimulation platform with a special software module. A real-time collision prediction model was developed for the same study area by using a matched case-control logistic regression technique to estimate the collision probability for each scenario. The results indicated that the proposed VSL control strategy can improve safety by approximately 50% and mobility by approximately 30%. A VSL update frequency of 5 min and a maximum speed difference of 10 km/h between successive time steps yielded the best performances. This finding can be useful for field implementation of VSL control.

Towards a Safe and Efficient Motorway System: Literature Review on Variable Speed Limits

“…Studies by Allaby et al (2007) and Lee et al (2004) showed that safety was improved at the expense of increase in travel time, However studies by Habtemichael and Picado-Santos (2013) and Islam et al (2013) showed improvements in mobility as well as safety after implementation of VSL.…”

“…The authors reported that total travel time, total travel distance and flow were improved by 15%, 6% and 7% respectively. Islam et al (2013) investigated safety and mobility effects of another MPC based VSL algorithm and reported that total travel time, total travel distance and flow were improved by approximately 32%, 3% and 3% respectively.…”

Variable Speed Limit Control Strategy with Collision Probability Assessments Based on Traffic State Prediction

“…The predicted traffic state variables, along with the previous traffic state variables, were evaluated with the safety assessment model. A precursor-based collision prediction model, following the methodology presented in earlier work (5), was adopted. A similar case-control logistic regression technique was adopted for modeling traffic incidents.…”

“…However, during adverse traffic conditions, such as excessive demand or reduced roadway capacity (such as with construction), the posted speed limit may not be the optimal operating speed (1). Through field implementations and simulations, earlier studies reported that by enhancing the speed homogeneity effects, VSL improves traffic flow in terms of safety (1-3) and, potentially, mobility (4,5). A statistical crash probability model is usually used to assess the safety benefits of a VSL control.…”

Exploring the impact of a coordinated variable speed limit control on congestion distribution in freeway

“…Recently, proposed a cell transmission model (CTM)-based VSL control and documented a 10%e15% travel time reduction and 5%e7% flow improvement. Based on this MPC-based VSL control, Islam et al (2013) proposed several modifications in the METANET model design for relieving congestion caused by active bottlenecks to find significantly traffic safety improvement, travel time reduction and flow improvement.…”