Development and Implementation of Safety Performance Functions

Development and Implementation of Safety Performance Functions


Research Team:

Dr. Aditya Medury 
Dr. Praveen Vayalamkuzhi 
Sang Hyouk Oum 
Dr. Offer Grembek
Camille Salem
Lin Yang


Funding Organization:

California Department of Transportation (Caltrans). Contract #65A0633


Publications and Resources:

  • Salem, C., Vayalamkuzhi, P., Grembek, O., Medury, A., and Ensch, J., (2019). Process Mapping of Safety Applications in Transportation Organizations, Presented in the 98th Transportation Research Board Annual Meeting, January 13-17, 2019, Washington D.C., USA


Study Description

The Federal Highway Administration (FHWA) developed the Highway Safety Manual (HSM) to provide quantitative information for decision making in road safety management of state and local transportation agencies. In recent years, significant progress has been made with respect to crash prediction models for identifying such locations. In addition to providing valuable information related to factors that can potentially contribute to an increase in the likelihood of traffic collisions, the HSM explains how Safety Performance Functions (SPFs), which are the mathematical relationship describing the collision frequency, and explanatory variables are used to estimate the expected number of collisions per year for a given location. There are two types of SPFs, referred to as Type 1 and Type 2, each with its own data requirements. Type 1 SPFs use only traffic volumes to predict collisions, while Type 2 SPFs use additional site information such as road geometry and intersection design elements as explanatory variables. For example, alignment data is crucial for Type 2 SPFs, and can provide for significant improvement in predictive effectiveness in comparison to the Type 1 baselines.  These SPF’s will serve as a baseline for network screening techniques which play a major role in the transportation safety management process.

Figure 1 of IDOT Potential for Safety Improvement Graphical Definition

Figure 1. Potential for Safety Improvement Graphical Definition (Source: FHWA Safety)


Figure 2 of Road Safety Management Process

Figure 2. Road Safety Management Process by FHWA (Source: FHWA Safety Tools)

California Department of Transportation (Caltrans) is in the process of transforming from its current network screening method of Table C to the network screening based on Safety Performance Functions. The added value of the current Caltrans effort cannot be realized until the outcomes are deployed as part of a transportation safety management tool. The goal of the transportation safety management tool is to assist Caltrans in gradually deploying newly developed SPFs into the network screening processes. The idea behind this is to develop a tool that would be flexible enough to make use of any intermediate progress related to network screening capabilities. 

This project will focus on designing and developing an MS Excel-based tool that can be used for network screening by calibrating the existing SPFs for different facility and injury types. The development of the network screening framework will consider the inclusion of network screening methods, such as sliding window method, continuous risk profile, etc. The implemented network screening approaches would allow Caltrans to input the parameters which would define the high collision concentration locations, and the output of the network screening would result in a list of locations which meet the selection criteria. In addition to this, the project also provides guidelines for developing additional SPFs, re-estimating existing SPFs, and a road-map for incorporating such SPF’s into the tool.