Estimating the speed-crash relationship has long been of interest in roadway safety analysis. Because of the many contributing factors – such as roadway geometry and context, weather conditions, human factors, and vehicle type distribution – that influence both speeds and crashes, the relationship cannot be adequately established without considering the corresponding roadway geometric features, the dynamics of the vehicle and tire, and accounting for their effects on speeds and crashes.
While a significant amount of research has been conducted to identify relationships between roadway design elements and crashes, research that has considered the contribution of operating speeds or posted speed is limited. A general subjective understanding of the contribution of operating speed of a highway or freeway, through the dynamics of the vehicle, on the severity of a crash (higher speeds are associated with more severe crashes) is known. What is desired is a quantitative understanding of how speed (both operating and posted) in conjunction with roadway geometry relates to the likelihood of a crash and crash severity.
This research would build on existing research to explore the relationships among design elements, speed, vehicle dynamics, and crashes on high-speed rural highways and freeways. These relationships will help inform future design guidance, posted speed practices, and potential safety countermeasures, which are related to desired outcomes for multiple AASHTO Committees, such as Design, Traffic Engineering, and Safety.
The greater availability of speed data along with innovative statistical analysis techniques provides the opportunity to better consider operating speed in understanding the safety relationships among geometric design, traffic volume, posted speed limit, and crashes. Recent research efforts that should be considered in scoping the plans for collecting and analyzing data for freeways and high-speed rural highways include:
• NCHRP Project 17-79, “Safety Effects of Raising Speed Limits to 75 mph and Higher”
• NCHRP Project 17-92, “Developing Safety Performance Functions for Rural Two-Lane Highways that Incorporate Speed Measures”
• FHWA Project, Development of Speed Crash Modification Factors (CMF) using SHRP-2 Roadway Inventory Database
• FHWA Project, Research Utilizing SHRP2 Data to Improve Highway Safety: Development of Speed–Safety Relationships
• USDOT, Rural Speed Safety Pilot Project
• TxDOT Project, Develop a Real-Time Decision Support Tool for Rural Roadway Safety Improvements
• FHWA’s Speed Concepts: Informational Guide (FHWA-SA-10-001)
The objectives for this research include (1) identifying the relationship(s) among roadway geometric characteristics, speed (posted and operating), and safety for high-speed rural highways and freeways; (2) determining the relative contribution of speed (posted or operating) with various roadway geometric characteristics on safety for high-speed rural highways and freeways; and (3) determining if existing crash modification factors for geometric elements can be modified with a speed component (operational, posted), or if a speed related CMFs can be developed.
To explore the relationships among roadway design, operating speed, posted speed limit, and crashes, tasks could include: (1) conducting a literature review; (2) identifying potential data sources, especially with respect to operating speed data and evaluating the quality of the data sources along with whether they can support the desired evaluation; (3) gathering data; (4) performing statistical modeling and data analysis to identify the holistic relationships among design elements, speed, and crashes on high-speed rural highways and freeways; (5) assessing the effects of study factors (posted and operating speed, and roadway geometry and characteristics) to safety and perform sensitivity analysis with respect to inclusion of covariates, model forms, etc.; (6) developing improved/enhanced CMFs or crash modification functions for geometric elements with consideration of operating speed.