Transportation agencies have decades of experience using crash histories to identify locations for safety improvements. Intersections and roadway segments with concentrations of fatal and serious injury crashes warrant attention, and crash records provide the foundation for understanding what happened and why.
The limitation is largely one of sample size. Severe crashes are comparatively rare, and on a large roadway network they may be scattered across hundreds of intersections and roadway segments. A location with characteristics associated with severe crashes may therefore have little or no severe-crash history of its own.
Road safety analysis addresses this challenge by examining what severe-crash locations have in common. If certain facility types, roadway characteristics or operating conditions are disproportionately represented in severe crashes, agencies can screen for those conditions elsewhere. That screening can identify locations that may never stand out on a crash map.
Identifying Patterns in Road Safety Data
When severe crashes are spread across a large network, reviewing individual crash locations may not reveal the patterns among them. A recent
safety analysis for the Southeast Iowa Regional Planning Commission looked for shared characteristics between fatal and serious-injury crash sites across 31 communities in four Iowa counties.
The analysis used crash data and
Geographic Information System (GIS) screening to develop a High Injury Network and then applied a representation ratio methodology to identify roadway and intersection characteristics that appeared disproportionately in fatal and serious injury crashes. The characteristics that appeared disproportionately in severe-crash data could then be identified elsewhere in the study area. A location with little severe-crash history of its own might warrant attention if it shares characteristics with facilities disproportionately represented in severe crashes elsewhere.
The analysis produced 10 recommended countermeasures, along with
planning-level cost estimates and project concepts that could be advanced as funding becomes available.
Accounting for Exposure in Road Safety Analysis
For pedestrian and bicycle safety, crash counts can be difficult to interpret without some understanding of exposure. A corridor where many people walk and a corridor where almost no one walks should not be evaluated the same way simply because they have recorded the same number of pedestrian crashes.
This challenge was central to pedestrian and bicyclist safety work for a large Southwestern city, where pedestrians accounted for 46% of fatal crashes in the study area. This underscored the need to better understand where risk existed beyond locations with a history of severe crashes. The analysis developed both High Injury Networks and High Risk Networks. High Injury Networks identified locations with concentrations of fatal and serious injury crashes. High Risk Networks looked beyond crash history to identify locations where roadway and community conditions suggested elevated risk, even without a history of severe crashes. To identify those locations, the analysis incorporated roadway characteristics, proximity to schools and transit, demographics and estimated pedestrian demand. Estimates of pedestrian demand considered factors such as land use, access to jobs, vehicle ownership, network connectivity and trip length.
When the resulting risk networks were compared with U.S. Department of Transportation Areas of Persistent Poverty, less than half of all study segments were located within those areas. By comparison, approximately 75% of locations identified as high risk for pedestrians and 65% of locations identified as high risk for bicyclists were located within Areas of Persistent Poverty.
The findings revealed disproportionate concentrations of pedestrian and bicycle risk that were not apparent from crash history alone and informed development of a nine-point safety checklist, policy recommendations and a countermeasure selection tool. The methodology was then tested at five locations that scored highly in the risk analysis, helping translate network-level findings into more focused evaluations of potential improvements.
From Safety Analysis to Traffic Engineering Improvements
Network screening helps identify where risk may exist, but it does not determine the appropriate solution. Before selecting a countermeasure, engineers still need to understand the specific conditions contributing to risk at each location.
That distinction was evident in an Illinois roadway study which analyzed seven years of crash records and linked severe crashes to high operating speeds, horizontal geometry, truck traffic, limited sight distance and the absence of dedicated turn lanes. Recommended improvements addressed those conditions through added turn lanes, improved sight distance and bicycle-friendly shoulders as part of a larger
transportation reconstruction project.
However, applying the same countermeasure at every location with a similar crash type overlooks the site-specific conditions that contribute to risk. Depending on site conditions, improvements can vary significantly, from incremental safety enhancements to broader roadway redesign efforts. Crash modification factors can provide evidence about expected safety performance, while site-specific
traffic engineering addresses practical considerations such as right of way, drainage, utilities and constructability. Together, those factors help determine which improvements are both effective and feasible.
Prioritizing Roadway Safety Improvements
Identifying additional risk is only part of the challenge. Agencies must still determine which improvements can deliver the greatest safety benefit within available funding and implementation constraints.
The challenge of prioritization was evident in a safety analysis conducted for a Southwestern community where more than 10,000 crashes were recorded between 2018 and 2024. Among them were 59 fatal crashes and 170 serious injury crashes. Initial screening identified corridors with concentrations of severe crashes, followed by development of High Injury Networks and High Risk Networks to examine safety needs more broadly. However, the resulting locations and potential countermeasures still had to be prioritized. Crash severity and exposure should be considered alongside expected safety benefits, countermeasure effectiveness, cost and implementation feasibility when evaluating which improvements to advance. The result is often a mix of project types, including lower-cost treatments suitable for deployment at multiple locations and larger improvements requiring additional design and capital investment.
Looking Beyond the Crash Map
Historical crash locations remain an important starting point for roadway safety work. A systemic approach expands on that information by identifying roadway characteristics, operating conditions and patterns of exposure associated with elevated risk across the broader network.
For agencies responsible for allocating limited safety funding, road safety analyses provide a starting point by identifying locations that warrant closer investigation, even without an established history of severe crashes. Lessons from crashes that have already occurred can then inform decisions at other locations where the same risk factors are present.
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