What is a safety evaluation?
October 14, 2006
The transportation industry is taking positive strides toward creating and maintaining roadways that are safe for all users. Safety evaluations make up one of these strides. Engineers use safety evaluations to study crash conditions at a particular site (intersection or road segment) in detail to identify treatments that could reduce the incidence of crashes. Considered in the context of an overall roadway safety management system, safety evaluations are conducted after the entire network is reviewed to identify locations with the potential to be improved with a safety treatment. The approach to safety evaluations vary, although most follow the process outlined here.
Step 1. Crash Data Review
During the crash data review, engineers consider statistics relating the characteristics of crash conditions (i.e. crashes by type, severity, and environmental conditions). Beyond those statistics, engineers review the location of crashes using crash diagrams, condition diagrams, and/or Geographic Information System (GIS) mapping tools. Engineers study the crash data for a particular location to identify potential trends, which can assist with the field review or rule out particular trends in the crashes (e.g. no run-off the road crashes). These charts show some useful ways to summarize crash data.
Step 2. Assessment of Site Characteristics
This assessment provides engineers and jurisdictions with information about overall site conditions. Recent and planned infrastructure improvement projects, traffic operations, roadway geometry, intersection geometry and traffic control, modes of travel through the location, and records of relevant public comments are all examples of site conditions.
These are some of the elements engineers typically consider when assessing site characteristics:
- Current traffic volumes and traffic operations
- As-built construction plans and relevant design guidelines
- Relevant photo or video logs
- Recent studies in the area
- Land-use and site-access information
- Potential weather extremes
- Plans for future land use and transportation system improvements
- Current traffic operations
- Pedestrian, bicycle, and transit facilities and plans for the area
Step 3. Assessment of Field Conditions
During the field investigation, engineers first gather site information by traveling through the site to understand motorized and non-motorized circulation to and through the site. When possible, a multi-disciplinary team (e.g. engineers, police officers, and pedestrian, bicycle, or transit advocates) should participate in the field review. In addition, the team should conduct a review during daylight and darkness conditions to gather a more complete understanding of site travel. The field assessment provides engineers and jurisdictions with information essential to support or negate what they learned in the site characteristics investigation. The study of field conditions is key to identify potential improvements at the site.
Through the conduct of the first three steps of the safety evaluation, it is likely that engineers will identify potential improvements to address the observed trends and issues.
Step 4. Develop Improvement Concepts
As the potential safety improvement concepts are considered it is important that engineers strive to achieve a balance between mobility and safety by:
- Eliminating as many vehicle, pedestrian, and bicycle conflicts as possible without significantly reducing mobility for the non-auto modes of transportation.
- Designing roadways and intersections to minimize the severity of crashes if they do occur.
- Considering human factors such as strain, workload, lane tracking, speed choice and driver expectations.
Some feasible traffic engineering strategies may include adding turn lanes or medians, signal control improvements, nontraditional intersection designs (such as roundabouts or traffic circles), pavement condition and lighting improvements, sight distance improvements, and roadway striping and signs upgrades.
Step 5. Economic Analysis
Once improvement options are identified, engineers can analyze the costs and benefits to help determine the most cost-effective solution for the site. Cost/benefit analyses weigh the potential decrease in crash costs against the cost of implementing the improvement under consideration. Costs to implement improvements will include construction, right of way, and other infrastructure costs. The lower the costs and the greater the benefits, the stronger the option will appear to be. Engineers can use the October 2005 FHWA document, Crash Cost Estimates by Maximum Police-Reported Injury Severity within Selected Crash Geometries, to estimate crash costs by severity if these costs have not been estimated for the jurisdiction overseeing the safety evaluation. It is also important to consider costs and benefits that can’t necessarily be quantified such as environmental, mobility, and accessibility impacts.
While all these steps are necessary to complete a site safety investigation, the two most critical steps are: a detailed evaluation of crash data to identify or rule out trends, and daytime and nighttime field visits to understand travel experiences for all modes through the site. Finally, similar to many other aspects of engineering, the process and methods discussed should be considered a framework for the critical engineering thinking required to develop safety solutions to benefit the transportation system.