Iowa Specific CMF Development for Signalizing High-Speed Intersections

Abstract: Between 2016 to 2020, nearly 2,900 fatal, serious injury and minor injury crashes occurred at paved, all-way stop control and high-speed, partial stop control intersections in Iowa. This increases to nearly 7,800 fatal, serious injury and minor injury crashes when considering all paved, unsignalized intersections in the state. At the national level, 18% of all fatal crashes occurred at unsignalized intersections in 2018. Several factors can contribute to crashes at unsignalized intersections, including drivers failing to recognize the intersection or selecting inappropriate gaps. Additionally, reaction times are reduced as speeds increase, and the severity of crashes are greater. In fact, the safe system speed for “Car/car (side impact, intersections)” crashes is only 50 kilometers per hour (~31 mph) [Fildes et al. (2005)]. Installing a traffic signal can be a viable safety countermeasure at high-speed intersections. The FHWA Crash Modification Factors Clearinghouse has several crash modification factors (CMFs) pertaining to installing traffic signals. However, the CMF values can vary widely, originate from studies of various quality levels and analysis periods and employ data from single geographic regions or states.
Since Iowa has a robust intersection database, high-quality crash data, and prior experience with intersection safety performance function (SPF) development, this research focuses on creating Iowa-specific CMFs for signalizing high-speed intersections. A five-step methodology was implemented, starting with identifying treatment and reference sites and collecting relevant data. Based on this data, an objective decision-making process was used to select an appropriate CMF development method. Crash types and severities most relevant to signal installation were identified, and intersections were grouped into homogeneous sets to develop high-quality SPFs, from which CMFs were obtained through cross-sectional analysis. Findings showed signal installation at high-speed intersections reduced broadside crashes by 45%, while rear-end and same-direction sideswipe crashes increased by 70% and 80%, respectively. Additional CMF values were derived for subsets of high-speed intersection types, including divided, undivided, 3-leg, and 4-leg intersections. This research paper showcases a comparative analysis of signalization impact on different classes of high-speed intersections. The study's results were validated through hypothesis tests of proportion analysis and comparison with existing literature.