Mobility and safety are often identified in transportation agency mission statements as core values. Traffic operations management has many metrics including level of service, volume to capacity ratio, and travel time reliability measures. Safety performance is often measured in the number of crashes and the injury outcome of crashes. Operational improvements aim to keep vehicles moving freely, often resulting in higher vehicle speed. As agencies consider the Safe System approach, which has a focus on reducing vehicle speed to reduce crashes and the impact energy when crashes do occur, competing interests between safety and operations could arise. These seemingly opposed goals of safety and mobility need to be balanced.
The safety performance functions (SPF) included in the AASHTO Highway Safety Manual (HSM) indicate that as capacity increases, crashes will typically be reduced in frequency and severity on freeway and arterial segments. Vehicle speed is another metric that can be used to understand both operational improvements and changes in crashes. In addition, systemwide changes in factors related to use of the roadway network, such as traffic volume, peak hours, mode choice, and road user risk factors, can affect the safety performance of the system and of individual facilities.
The objective of this research is to produce a tool to assess how implementation of operational measures impact crash outcomes and to develop associated guidance and procedures that will improve the understanding of the complex relationship among existing operational and safety performance measures. This research has the unique opportunity to use data from the pandemic period to assess how unintended changes in operational performance led to negative impacts to safety performance. This assessment could provide insight into how operational improvements—that lead to reduced congestion, improved travel times, and changes in peak period traffic—affects crashes in a real-world environment.
Suggested research activities include: (1) compile existing research findings regarding the relationship and correlation between operational measures and safety performance measures; (2) investigate how crash frequency and severity changed due to various types of projects that improved operations—such as projects to alleviate congestion and bottlenecks to improve travel time reliability—and produce an outcome model exhibiting the findings; (3) develop a quantitative analysis procedure to understand the tradeoffs between capacity improvements (or societal changes that impact traffic volumes) and resulting change in crash frequency or severity; (4) complete an analysis of the level of service during peak hours and off-peak hours and change in crash typology and severity to produce a quantitative or graphical representation of this relationship; (5) produce an operational and safety performance measure correlation model to be used as decision-making and countermeasure prioritization tool; and (6) develop outreach materials to support transportation agencies’ use of the research results, including items such as models, worksheets, analysis tools and presentations.
Transportation agencies need tools to support decisions concerning the impacts of mobility, capacity, and crashes and to help understand the systemwide impacts of changes in external factors that affect use of the system. Corridor operational improvements have led to many research studies that show mixed results with respect to reducing crash frequency and severity. This research will provide clarity to this topic.
This research is needed to analyze the performance of the roadway system more thoroughly from the perspective of various safety and operational performance measures, and to develop a tool to assist transportation agencies in performing this type of analysis for decision-making. This research will help transportation safety professionals more fully understand the relationships between operational performance measures and traffic fatalities and to provide agencies with insights on the competing or complementary nature of the performance measures. This knowledge can be used to combine performance measures and will be important for using state departments of transportation resources efficiently to maximize fatality reductions while still prioritizing mobility for all road users.