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Publication Number:  FHWA-HRT-17-027     Date:  January 2017
Publication Number: FHWA-HRT-17-027
Date: January 2017


Open Source Surrogate Safety Assessment Model, 2017 Enhancement and Update: SSAM Version 3.0

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Publication No. FHWA-HRT-17-027  |  HRDO-20/1-17(200)E
FHWA Contact: Joe Bared; 202–493–3314  |  joe.bared@dot.gov

This document is a technical summary of the Enhanced Surrogate Safety Assessment Model (SSAM) software; originally developed by the Federal Highway Administration (FHWA), FHWA-HRT-08-051.


This TechBrief describes the development of an open source surrogate safety assessment model (SSAM) software with improved safety measures, 3D conflict graphics, and advanced computing enhancements.


The ETFOMM (Enhanced Transportation Flow Open Source Microscopic Model) Cloud Service (ECS) is a software product sponsored by the U.S. Department of Transportation in conjunction with the “Microscopic Traffic Simulation Models and Software—An Open Source Approach” project. ETFOMM’s primary components are the core microscopic traffic Simulation Engine (ESE), a graphical user input editor (ETEditor), a 3D traffic visualization tool (ETAnimator), and a database in cloud service environment. Moreover, the SSAM is being converted into open source software, enhanced, and integrated as part of ESE.

SSAM is a popular safety tool box that utilizes the microscopic traffic simulation vehicle trajectories to generate safety performance measures such as:

These safety performance measures are shown on a vehicle trajectory plot, graphing time vs. distance in figure 1.

This graph illustrates safety performance measures on a vehicle trajectory plot, time vs. distance. Time is represented on an infinite x-axis, and distance is shown on an infinite y-axis. Two curves are drawn illustrating the trajectories of two vehicles, one moving through and the other crossing an intersection. The safety measures that can be generated by SSAM are indicated along the intersection of the two trajectory curves. Vertical lines indicate important moments in time in this interaction (labeled time 1, time 2, etc.). As time progresses and the vehicles move towards the intersection, time 1 indicates the beginning of encroachment, when the driver first has the opportunity to identify a potential conflict. Time 2 indicates the vehicle beginning to break at a defined deceleration rate, determining the maximum speed and maximum relative speed differential. The distance between lines time 3 and time 4 represents the minimum time to collision (TTC), with time 4 representing the projected arrival at the conflict point. Time 5 is the actual time the conflict point is reached and the encroachment ends.

Figure 1. Graph. SSAM Safety Measure.

Open Source SSAM Features

Integrated with ETFOMM

Vehicle trajectories from ETFOMM are collected and converted into SSAM input files; the trajectories are then analyzed by the SSAM program.

Re-implemented in C++

The SSAM algorithm source code was reprogramed from Java to C++ and restructured to separate declaration and definition of classes. Referenced Java libraries are replaced with C++ libraries, and memory management modules were added. The C++ version of SSAM has reduced analysis time up to 50 percent.

Parallel Computing

The C++ source code was designed to facilitate parallel computing by using OpenMP directives. The new parallel computing capabilities decreases the computation time up to 90 percent. Table 1 shows the analysis time improvement.

Table 1. Computation Time Improvement with Updated SSAM Software
Vehicle Trajectory Records Used in Comparison Improvement (percent) *
119,458 90.63
656,710 88.91
694,527 88.45
67,472,183 87.69
*Percent improvement represents the improvement of computation time from the original SSAM 2.16 in Java to the new parallel implementation in C++.


3D Conflict Map

The conflict map from the original SSAM was converted from 2D Java graphics into a 3D display, programmed using OpenSceneGraph library based on OpenGL. Conflict points are illustrated on detailed intersections generated from the ETFOMM Intersection Model. An optional image can be imported as the background map, as shown in figure 2.

This chart shows a SSAM 3D Conflict Map. The conflict points are illustrated as different shapes with different colors, according to conflict types: red cones for crossing conflicts, yellow boxes for rear end conflicts, and blue spheres for lane change conflicts. The conflict points are placed at corresponding positions on a detailed link and intersection chart generated by ETFOMM, positioned on top of an imported background map image. Most of the (red) crossing conflicts occurred within the intersection and upstream of the intersection on a single approach. The (yellow) rear end conflicts are densest at each approach. The (blue) lane change conflicts were scattered throughout the approaches, but many occurred within the intersection.

Figure 2. SSAM 3D Conflict Map.(1)

New Features in SSAMs



  1. “Clays Mill Rd & Cecil Way, Lexington, KY” Map. Lexington, KY. January 7, 2014. Accessed January 7, 2014. http://maps.google.com. Scale: 1:500 ft. The original map is the copyright property of Google® Earth™. Colored dots have been added to create the SSAM conflict map.

  2. Federal Highway Administration. “Open Source Application Development Portal: SSAM.” Website. McLean, VA. http://www.itsforge.net/ssam. Last accessed December 2, 2016.

  3. NGSIM: Source Forge. (Updated 2014). “ETFOMM.” Website. https://sourceforge.net/projects/etfomm/. Last accessed December 2, 2016.

  4. New Global Systems for Intelligent Management Corp. (September 2016). “Enhanced Transportation Opensource Flow Microscopic Model (ETFOMM).” Website. http://www.etfomm.org. Last accessed December 2, 201

Researchers—This software was developed by New Global Systems for Intelligent Management Corporation, Contract No. SBIR DTRT5715C10005. The president is: Dr. Li Zhang (662–341–5725, ngsim@ngsim.com). The FHWA Contracting Office Representative is Joe Bared (202–493–3314, joe.bared@dot.gov); contact him with any questions.

Distribution—This TechBrief is being distributed according to a standard distribution. Direct distribution is being made to FHWA Division Offices and Resource Centers.

Availability—The publication associated with this TechBrief is “Surrogate Safety Assessment Model (SSAM) and Validation: Final Report”, Report No. FHWA-HRT-08-051. It is available in print for specific distribution, and online at www.fhwa.dot.gov/publications/research/safety/08051/08051.pdf.

Keywords— Surrogate Safety Assessment Model, vehicle conflict analysis, and traffic simulation.

Notice—This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document. The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers’ names appear in this report only because they are considered essential to the objective of the document.

Quality Assurance Statement— The Federal Highway Administration (FHWA) provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.




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