Researchers
About the research
Between 2016 and 2020, nearly 2,900 fatal, serious injury, and minor injury crashes occurred in Iowa at paved, all-way stop control and partial stop control intersections with at least one approach having a speed limit exceeding 45 mph, i.e., high-speed intersections. 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, not complying with the traffic control, or selecting inappropriate gaps. Additionally, reaction times are reduced as speeds increase, and the severity of crashes is greater. In fact, the safe system speed for “car/car (side impact, intersections)” crashes is only approximately 30 mph. Traffic signal installation is a countermeasure that may be considered at high-speed intersections; however, national research presents mixed findings on its effectiveness.
Since Iowa has a robust intersection database, high-quality crash data, and prior experience developing intersection safety performance functions (SPFs), this research focused on creating Iowa-specific crash modification factors (CMFs) for signalizing high-speed intersections. A five-step methodology was implemented.
This research revealed that signal installations at non-ramp high-speed locations increase all but broadside crashes on most facility types. The findings showed that signal installation at high-speed intersections reduced broadside crashes by less than 64%, while rear-end crashes increased by more than 70%, depending on intersection geometric characteristics. Additional CMF values were derived for subsets of high-speed intersection types, including divided, undivided, three-leg, and four-leg intersections. The report showcases a comparative analysis of the impact of signalization on different classes of high-speed intersections. The study’s results were validated through hypothesis tests of proportions analysis and comparisons with existing literature.