Risk Factors Associated with High Potential for Serious Crashes
Report No. FHWA-OR-RD-16-05
There is a need to better understand the different risks associated with factors and features along low-volume roads. In understanding where risks are present in the system, a proactive approach may be employed to make improvements that can translate into reduced (or prevented) crashes in the future. Overall, six main tasks were completed to address this need.
First a literature review was conducted that sought to identify what factors and features have been identified in past work as presenting a risk to drivers as well as what prospective countermeasures exist to address them. Additionally, this literature review identified and summarized existing approaches to developing risk or safety indices that can be used in identifying where current features pose a potential risk to drivers.
A review of roadside, cross section and alignment features found that several have been identified through past research as posing varying risks to drivers including:
- Clear zone/lateral clearance to obstacles
- Roadside features – trees, culverts, ditches, slopes, utility poles, etc.
- Lane width
- Shoulder width
- Pavement edge drop-off
- Sight distance
- Signage and marking
- Speed limit
- Passing zone presence / frequency
- Signage adequacy
- Pavement marking condition
- Land use
- Pavement surface condition
- Driveway density
- Vertical curves
Based on this list, a review of the various countermeasures available to address the various safety issues associated with each feature was also completed. Finally, different approaches were reviewed and summarized that have been developed to establish risk or safety indices. The findings from the literature review provided a basis for understanding which road characteristics may influence crash risk on Oregon’s low-volume roads. These risk influencing features were then investigated along with other potential factors not cited in the literature during the data collection and analysis tasks.
The data collection process was extensive and included gathering road geometry data, roadside feature data, 10-year crash history, and 10-year traffic data for approximately 830 miles of Oregon’s low-volume roads at 0.05 mile resolution. These data were then used in the data analysis task to quantify their effects on crash risk.
The data analysis task completed a number of analyses including overall descriptive statistics of the sample roads and crash characteristics, crash rate analyses leading to a quantification of the effects of individual features on crash risk, and multivariate regression and correlation analyses all of which helped define the crash risk index. The features effecting crash risk and the weights associated with each feature were quantified based on results of the crash analyses.
The crash risk index development task defined an index based on the extensive data analysis efforts. The crash risk index is a function of the geometry of the road and roadside features, crash history, and traffic exposure. The crash risk index quantifies the crash risk and expresses it as a value from 0.15 to 1.0 with higher values corresponding to greater crash risk. The weights of the individual geometric features are based on data analysis and are not recommended for alteration, however, the overall weights that define the proportion of the CRI that is due to geometric features versus crash history versus traffic exposure may benefit from agency specific input.
An economic analysis of potential low-cost safety countermeasures was also completed. The analysis of the proposed safety measures for the low-volume rural road sample has shown certain treatments to be potentially economically beneficial and feasible for implementation. Based on the cost and benefit data gathered and considering the low traffic volumes for these types of roads, the most economically beneficial safety treatments for the low-volume road sample were found to be:
- Install Shoulder Rumble Strips
- Install Centerline Rumble Strips
- Install Object Markers for Objects Near the Roadway
- Install Centerline Markings
- Install Safety Edge
- Widen Centerline Markings
- Install Edge-line Markings
- Install Edge-line and Centerline Markings
- Widen Edge-line Markings
- Stabilize Shoulders
- Remove Objects Near the Roadway
- Widen Un-Paved Shoulder
- Flatten Side Slopes
- Relocate Objects Near the Roadway
- Horizontal Alignment Signs
- Horizontal Alignment Signs with Static Advisory Speed
- Flashing Beacons for Curve Warning
- Post Mounted Delineators for Curves
The results of the economic analysis are highly depended upon the costs of the treatments. While all efforts were made to choose accurate treatment costs, again agency and region specific cost information could improve the localized benefit / cost ratio results that generated these recommended low-volume treatments.
Lastly, three case studies were completed to illustrate the use of the crash risk index on real world roads in Oregon. Overall, this effort provides a quantification and guidance for improving the safety of low-volume roads.