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PMSS Project Details


Project ID:FHWA-PROJ-03-0003
Project Name: Surrogate Safety Assessment Model
Status: Completed
Last Name:Chen
First Name:Shyuan-Ren (Clayton)
Office: Office of Safety Research and Development
Team:Safety Management Team
Roadmap/Focus area(s): Intersections
Project Description:This project aims to produce a software tool that computes surrogate safety measures from vehicle trajectories that were tested with several widely used traffic simulation models and validated with real crash data.
Start Date: September 9, 2003
End Date: July 17, 2010
Goals:Safety of traffic facilities is most often measured by counting the number (and severity) of crashes that occur. It is not possible to apply such a measurement technique to traffic facility designs that have not yet been built or deployed in the real world. The purpose of this project is to develop a software tool for deriving surrogate safety measures for traffic facilities from data output by traffic simulation models.
Background Information: The surrogate measures developed in this project are based on the identification, classification, and evaluation of traffic conflicts that occur in the simulation model. Surrogate Safety Assessment Model is a technique combining microsimulation and automated conflict analysis that analyzes the frequency and character of narrowly averted vehicle-to-vehicle collisions in traffic.
Test Methodology: An open-standard vehicle trajectory data format was designed, and support for this format has been added as output options by four simulation model vendors/developers: PTV (VisSim), TSS (AIMSUN), Quadstone Paramics, and Rious Engineering (TEXAS). Eleven "theoretical" validation tests were performed to compare the surrogate safety assessment results of pairs of simulated design alternatives. In addition, a field validation exercise was completed to compare the output from Surrogate Safety Assessment Model with real-world crash records.
Other Information: Enhanced Surrogate Safety Assessment Model software is available at
Fieldtest:Eighty-three intersections were modeled in VisSim and simulated. The processed conflict results were then compared with the crash records in a number of different statistical validation tests. In addition, sensitivity analysis was performed to identify differences between the Surrogate Safety Assessment Model-related outputs of each simulation model vendor’s system to the same traffic facility design.
Expected Benefits:The Surrogate Safety Assessment Model (SSAM) can assess the safety of traffic facilities without waiting for a statistically above-normal number of crashes and injuries to occur. By comparing one simulated design case with another, SSAM allows an analyst to make statistical judgments about the relative safety of the two design alternatives.
Deliverables:1. Name: Surrogate Safety Assessment Model (SSAM) and Validation: Final Report
Product Type(s): Research report
Description: This document is a final report on research and development of an alternative safety assessment approach utilizing conflict analysis as a surrogate measure of actual crash data.
Publication URL(s):
2. Name: SSAM Software User Manual
Product Type(s): Research report
Description: This document is a software user's manual for the Surrogate Safety Assessment Model (SSAM). It provides guidance on the installation and use of the software.
Publication URL(s):
3. Name: Surrogate Safety Assessment Model (SSAM) TechBrief
Product Type(s): Techbrief
Description: This document is a technical summary of the Federal Highway Administration's Surrogate Safety Assessment Model (SSAM) Final Report, FHWA-HRT-08-051. The initial version was developed in May 2008 and further updated in October 2009.
Publication URL(s):
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Keywords: Safety
Traffic Safety
Traffic Simulation
Subject Areas: Safety and Human Factors