Validation of the Surrogate Safety Assessment Model for Assessment of Intersection Safety

Abstract

The surrogate safety assessment model (SSAM) is a software application that reads trajectory files generated by microscopic simulation programs and calculates surrogate measures of safety. This approach eliminates the subjectivity associated with the conventional conflict analysis technique and allows assessment of the safety of a facility under a controlled environment before accidents occur. The specific goal of this research was to validate SSAM as a tool for accident prediction at urban intersections. Two methods were used for validation. The first method compared the simulated number of conflicts from the use of SSAM and the predicted number of injury accidents from analytic models in three reference intersection layouts (four-leg priority intersection, four-leg staggered intersection, and single-lane roundabout). The second method compared SSAM results with conflicts observed on site in four real intersections: two priority ones and two roundabouts. The results indicate that, despite some limitations related to the nature of current traffic microsimulation models, SSAM analysis is an extremely promising approach to assessing the safety of new facilities or innovative layouts.In urban areas, traffic accidents are usually concentrated at intersections. Traditional approaches to estimating the potential risk of traffic accidents at intersections, which are based on historical crash data, include before-and-after analyses and accident prediction models. Both approaches have important limitations related to the complexity of safety factors and the poor quality of data, thus (a) compared with other traffic events, accidents are quite exceptional in that they result from a series of unhappy improbable actions and situations; (b) accidents are rare events, so it is troublesome to base traffic safety analyses at individual sites on accidents alone; (c) not all accidents are reported, and the level of underreporting depends on the accident's severity and types of road users involved; and (d) information on the circumstances preceding an accident is seldom available (1). In addition, the fact that accidents must occur before one can determine the risk of locations is, from an ethical point of view, a dis advantage (2). Therefore, specific evaluation techniques are required to take into account changes in traffic regulations or in the geometrical design of infrastructure so as to estimate the true effects of safety improvements. Moreover, regression models may not be transferable because they implicitly reflect road users' behavior, the vehicle fleet, and driving rules, which vary from country to country and even from site to site.The traffic conflict technique is an approach that overcomes the lack of good, reliable accident records, relying instead on observations of conflicts. A "conflict" is defined as a situation in which two or more road users approach each other in time and space such that they risk colliding if their movements remain unchanged (3). The necessary evasive action is usually braking but may also b...

Assessment of the Speed Management Impact on Road Traffic Safety on the Sections of Motorways and Expressways Using Simulation Methods

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A brief review: traffic conflict techniques and the challenges of the studies in Indonesia

“…Studies related to SSAM validation have been carried out by a number of researchers. For this reason, they compared the conflict data of SSAM simulation results with traffic accident data as well as traffic conflict data obtained from field observations [3,[11][12][13]. The result of the validity testing above is that the use of SSAM still has several limitations, but it is promising to be developed further in road safety measurement.…”

Evaluation of Roundabout Safety Performance through Surrogate Safety Measures from Microsimulation

“…The results confirmed that the use of simulation models to evaluate road safety without proper calibration should be avoided, and more work is needed to confirm that simulated conflicts represent safety measures beyond what can be expected from exposure. Vasconcelos et al [16] evaluated the potential of the SSAM approach to assess the safety performance at urban intersections and roundabouts. Model validation was accomplished by comparing the number of conflicts obtained with the SSAM both with the number of crashes predicted by analytic models and with conflicts observed on existing intersections.…”