A Safety Performance Function (SPF) is an equation that predicts the expected number of crashes at a location (intersection or road section), based on its traffic volume and design characteristics. The prediction of crashes may be for all crashes or for specific crash types or severities. SPFs are developed using large databases of crash counts, traffic volumes and site layout or geometric information.
For example, an SPF for total crashes on rural two-lane roads could be:
Where,
Length is the road segment length
AADT is the average daily traffic on the road segment
By entering the length and AADT of a road segment into the equation, the expected number of crashes per year at similar locations can be found.
SPFs can be used to make better decisions to improve road safety. One application is to use SPFs as part of Network Screening to identify locations that have the greatest potential for improvement. A second application is to use SPFs to determine the safety impacts of design changes at the project level (e.g., adding a turn lane to an intersection). A third application is the use of SPFs in evaluating the safety effects of countermeasure treatments << hyperlink to Monitoring and Evaluation module when ready>>implemented.
An agency can develop their own SPFs or they can adopt an existing SPF. In the latter case, the SPF should be calibrated using local data before use. SPFs are available for many facility types, for example:
• Signalized and stop-controlled intersections
• Roundabouts
• Two-lane and multi-lane roadways
• Freeways
• Urban and rural environments
Sources of existing SPFs include:
• Highway Safety Manual for many location types
• NCHRP report 572 for roundabouts
• Transportation Research Record publications
The data required to apply an SPF in a specific situation depends on what variables are included in the SPF. An SPF always includes vehicle traffic volume but may also include pedestrian or bicycle volumes and site characteristics. Examples may include: lane width, shoulder width, radius/degree of horizontal curves, presence of turn lanes (at intersections), and traffic control (at intersections).
The application of SPFs does not require special skills although some background knowledge about how they are developed, their limitations and how to interpret the results is necessary.
The calibration of an existing SPF to local data requires little to no statistical experience. However, developing new SPFs requires personnel with a background in statistical modeling. Agencies without such in-house expertise should hire a qualified consultant.
There are various sources of information available related to calibrating, estimating and applying SPFs.
1. FHWA Safety Data and Analysis YouTube Channel
This YouTube channel provides five videos related to applying the predictive method for crashes included in AASHTO’s Highway Safety Manual (HSM). These include: 1) The Predictive Method; 2) Observed, Predicted and Expected Crashes; 3) Application of CMFs; 4) Selecting a Method to Analyze Multiple CMFs; 5) Applying a Method to Analyze Multiple CMFs.
2. Safety Performance Function Development Guide: Developing Jurisdiction-Specific SPFs.
The guidebook discusses the process to develop jurisdiction specific SPFs. It is intended to be of use to practitioners at state and local agencies and to researchers.
3. Safety Performance Function Decision Guide: SPF Calibration vs SPF Development
This guidebook is intended to provide guidance on whether an agency should calibrate the SPFs from the Highway Safety Manual (HSM) or develop jurisdiction-specific SPFs. The guidebook discusses the factors that need to be considered while making the decision. It is intended to be of use to practitioners at state and local agencies and to researchers.
4. Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results.
The Guide contains guidelines for (a) quantification of the reliability of SPFs for practitioner use;
(b) interpretation of model reliability; and (c) application of SPFs accounting for, but not
limited to, assumptions, data ranges, and intended and unintended uses.
Term | Definition |
Countermeasures | Interventions applied to reduce crashes, e.g. rumble strips |
Facility | Infrastructure provided for road user movements, e.g. roads, bicycle lanes, sidewalks |
Hotspots | Locations identified as having a high number of crashes compared to other locations |
Rumble strips | Textured strips installed on the road to alert drivers through tactile vibrations if they unintentionally veer off the roadway or across the centerline |
Cable median barrier | A safety barrier installed in the median of a divided highway composed of high-tension cables supported by posts |
High friction surface treatments | Applications of specialized materials or coatings on the road surface in increase friction between vehicle tires and the pavement |
Curve warning signs | Traffic signs used to warn drivers in advance of upcoming curves in the road |
Signal backplates | Panels mounted behind traffic signal heads to enhance the visibility of traffic signals |
Countdown pedestrian signals | Pedestrian crossing signals that display a numerical countdown indicating the time remaining for the pedestrian walk signal |
Educational campaigns | Public awareness initiatives designed to inform, educate and change behaviour related to road safety |
Crash tree diagram | A visual representation or chart that illustrates the frequency of crashes by crash types and other involved factors |
Road diets | A reallocation of road space by reducing the number of through lanes and adding a two-way left-turn lane, often with the addition of bicycle lanes or other facility |
Optical speed bars | Visual speed indicators painted on the road in the form of bars that provide optical cues to encourage motorists to reduce their speed |
Speed tables | A traffic calming device similar to a speed bump but that is longer and with a flat top |
Centreline hardening | A form of traffic calming that reduces the turning radius for vehicles using physical measures on the roadway to encourage slower speeds |
Gateway treatments | Physical measures taken where a rural road meets a more urban area to increase driver awareness that posted speed limits are changing, such as landscaping, signage or road markings |
