Assessment of the impact of speed limits and traffic calming on road capacity and traffic accidents

Abstract

This article examines the impact of speed limits and traffic calming measures on two key performance indicators of urban road networks: capacity and the level of traffic accidents. The relevance of the study is driven by the practical need for cities to simultaneously reduce the severity of crash consequences while maintaining an acceptable level of traffic service under conditions of increasing motorization. The aim of the study is to develop an approach to the quantitative assessment of the “safety–efficiency” trade-off for typical measures, including speed limit reduction, installation of pedestrian refuge islands, raised crosswalks, lane narrowing, chicanes and local speed control devices (speed humps), as well as the implementation of 30 km/h zones. The methodology is based on a combination of before–after analysis with adjustments for trends and seasonality, as well as traffic flow microsimulation to assess changes in capacity and delays. For the safety assessment, it is proposed to use indicators such as the number of accidents, the number of injuries, a severity index, as well as proxy conflict metrics (e.g., based on time-to-collision and spatial proximity) when video data are available. For capacity evaluation, average delay at intersections/road segments, queue length, average speed, and traffic flow stability are considered. Particular attention is given to the influence of traffic composition (share of heavy vehicles and public transport), the presence of on-street parking, and pedestrian activity. The result is a generalized procedure for selecting traffic calming measures depending on the functional class of the street, initial speed conditions, and accident levels. It is shown that even a small reduction in average speed can lead to a significant decrease in the risk of severe accident outcomes, while the impact on capacity is often manifested through increased flow variability and additional delays at local bottlenecks. The practical significance of the study lies in its applicability for substantiating decisions in urban road safety programs (Vision Zero), developing traffic management projects, and evaluating the effectiveness of implemented measures based on measurable indicators.