Description

The safety of intersections, interchanges, and other traffic facilities is most often assessed by tracking and analyzing police-reported motor vehicle crashes over time.  Given the infrequent and random nature of crashes, this process is slow to reveal the need for remediation of either the roadway design or the flow-control strategy.  This process is also not applicable to assess the safety of new roadway designs that have yet to be built or new flow-control strategies that have yet to be applied in the field.

This web page provides a summary and resources related to Siemens’ research and development of the Surrogate Safety Assessment Model, or SSAM, a technique combining microsimulation and automated conflict analysis—analyzing the frequency and character of narrowly averted vehicle-to-vehicle collisions in traffic—to assess the safety of traffic facilities without waiting for a statistically above-normal number of crashes and injuries to actually occur.

This research has been funded by the Federal Highway Administration, and to date consists of two projects:

• Surrogate Safety Measures from Traffic Simulation Models
• Surrogate Safety Assessment Model (SSAM) and Validation (ongoing)

Detail

A conflict is, simply put, a scenario where two road users will likely collide without evasive action.  An example is illustrated in Figure 1, where a vehicle is angling across two lanes to the left turn bay and has abruptly cut in front of another vehicle that must decelerate to avoid a collision.

Traffic Studies

Conflict scenario from SSAM application.

Traffic conflicts have been studied since the late 1960s as a technique to assess the safety of a location, with the understanding that conflict frequency is correlated with the risk of actual collision.

Software Applications for Analysis

Conflict studies traditionally utilize personnel trained to identify and record conflicts observed at an intersection.  In this research, the SSAM software application was developed to automate conflict analysis by directly processing vehicle trajectory data.  Researchers specified an open-standard “universal” vehicle trajectory data format designed to provide the location and dimensions of each vehicle approximately every 10th of a second. It is hoped that video processing technology will, in the coming years, be capable of automatically extracting vehicle trajectory data adequate for SSAM processing.  However, the trajectory file format is currently supported as an export option by four popular traffic microsimulation packages—AIMSUN, Paramics, TEXAS, and VISSIM.