Evaluating the Safety Impacts of Reducing Posted Speed Limits on Signalized Intersections: A Calibrated Simulation Approach

Author(s): Alozi, Hussein

Slidedeck Presentation:

Alozi _ Hussein Presentation



The Traffic Conflict Technique has been well established in the safety literature as a means of proactive safety evaluation. This approach involves observing near misses (traffic conflicts) between road users and using several indicators to quantify the severity of those conflicts. The observed conflicts can be used in several safety applications as a surrogate for actual collisions.


This study aims to investigate the effectiveness of reducing the posted speed limits on the safety level of signalized intersections, using the Traffic Conflict Technique. In addition, the study examined the effectiveness of protected left turns, as a commonly used countermeasure to enhance pedestrian safety at signalized intersections with different speed limits.


To achieve the aforementioned objectives, we developed and calibrated a simulation model based on data collected from six signalized intersections. The data utilized in this study involves over 100,000 conflicts spanning a period of 21 days. The simulation is carried out on the virtual testbed of Vissim software. The model parameters were calibrated based on several mobility indicators (e.g., operating speed distribution, arrival type, and delay) and safety indicators (i.e., traffic conflicts) to ensure that the simulation model mimics the actual field conditions. We then reviewed the literature to assess the relationship between the driver’s behavior (i.e., operating speed and reaction time) and different posted speed limits (specifically, 5, 10, and 15 km/h below the existing speed limit). Four simulation models were developed to extract road user trajectories at the studied intersections under the current speed limit and the three reduced speed limits. The simulated road user trajectories were evaluated in the Surrogate Safety Assessment Model (SSAM) to extract the predicted conflicts and assess the safety benefits of the speed limit reduction. Afterwards, four other models were developed to test the impact of implementing protected left turns at the six intersections on pedestrian safety for the current speed limit and the three reduced speed limits.


The results indicate a direct correlation between the posted speed limits and both the frequency and the severity of conflicts. The safety gains of protected left turns, although minor, were found to be more significant at higher posted speed scenarios.


The resulting impact of the posted speed limit on safety levels coincides with the findings of previous studies. Nevertheless, the calibrated Vissim model provides a powerful tool for understanding the impact of posted speed limits on the safety of different road users and the performance of different safety countermeasures at signalized intersections.


Traffic conflicts prove to be an effective surrogate to investigate the impact of proposed safety countermeasures. Based on the results of this study, reducing posted speed limits can be considered a viable option to reduce the frequency and the severity of conflicts at a signalized intersection, which in turn could reduce the risk of potential collisions. These findings can be used by practitioners and planning engineers to make informed decisions on road design based on the individual characteristics of each signalized intersection.