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Publication Number: FHWA-RD-03-050

Surrogate Safety Measures From Traffic Simulation Models

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10. Report Summary

This report has presented a review of the literature in surrogate safety assessment, a review of the capabilities of traffic simulation models for obtaining surrogate safety measures, functional requirements for a surrogate safety assessment software tool, and a set of surrogate measures to be collected from traffic simulation models to assess the safety of various intersection designs and timing strategies. The approach suggested here is to collect detailed data on all conflict events that occur between two vehicles at the intersection. Two requirements must be satisfied for an undesirable event to have occurred:

  • One of the vehicles must take evasive action to avoid a collision.
  • The resulting surrogate measures must be of significant value (i.e., less than a user-specified threshold, as guided by the literature).

The surrogate measures proposed are:

  • Minimum time TTC during the conflict event.
  • Minimum PET during the conflict event.
  • Maximum speed of the two vehicles (MaxS).
  • Maximum difference in the speed of the two vehicles during the conflict event (DeltaS).
  • Initial DR of the reacting vehicle.
  • Location of the starting and ending points of the conflict event (CPL, CLSP, and CLEP).

All possible situations where the traffic conflict events occur were presented. These situations can be categorized as conflict points, conflict lines, and rear-end conflict lines. Conflict points describe times when a crossing flow impedes the progression of the right-of-way vehicle at just one point in the path of the vehicle. Conflict lines describe the other situations where a vehicle merges into the traffic stream in front of the vehicle with the right-of-way and causes evasive action by the following vehicle to avoid a collision. Rear-end conflict lines are a special case of conflict lines where the two vehicles are already in the same lane (no merging activity takes place first) and the leading vehicle takes some action that causes the following vehicle to react evasively.

Computational algorithms for calculating the surrogate measures for each of the conflict event situations were presented. Example diagrams were shown that illustrate the calculations of the surrogates graphically. Conflict points are the simplest case for algorithm computations, since the TTC and PET values are defined uniquely. Conflict lines and rear-end conflict lines require multiple computations of the surrogate measures and a minimum (or maximum in the case of MaxS and DeltaS) function must be applied to select the "worst case" value for recording.

Given these computational algorithms and surrogate measure definitions, a format for an "event file" that could be produced by simulation models was presented. The event file format identifies a list of data that could be supported by the simulation model developers with a minimum of effort that could be read by the surrogate safety module and post-processed.[13] The data are essentially a time history of the speed, acceleration, and location of vehicles in the intersection that are candidates for conflict events. Each time a vehicle:

  • Changes lanes.
  • Accepts a gap to cross the right-of-way flow.
  • Accepts a gap to join the right-of-way flow.
  • Brakes and causes a following vehicle to brake.
  • Turns and causes a following vehicle to brake.

An event record is created and written to the event file until the end of the event. If more logic of the computational algorithms for surrogate measures could be integrated within the simulation model, the event file format would have to be modified.

The event file is imported into new software for performing surrogate safety analysis by comparing the performance of various intersection designs by making graphs, charts, tables, and a distributional analysis of the surrogate measures. The entire process of computing, extracting, and analyzing the surrogate measures from the traffic simulation models has been denoted as SSAM.

Finally, future validation activities for assessing the connection between surrogate measures and the safety of intersections were presented. Three hypotheses for surrogate safety measures were listed with the associated steps required to complete the acceptance or rejection of the hypothesis.

The first hypothesis verifies that the simulation models of two different intersection designs produce different distributions of surrogate measures. The second hypothesis is that the surrogate measures produced by the simulation model is correlated with the occurrence of traffic conflicts in the real world, as would be measured by a traffic conflicts study. The third hypothesis is that the surrogate measures produced by the simulation model would predict (be correlated with) the difference in traffic conflicts, as experienced in the real world, between a "before" condition of an intersection design and the "after" condition of an intersection design after improvements were made to improve the safety of the facility (improvements that could, of course, be represented in the simulation model).


[13] This event file format would also support alternative derived surrogates, such as the TET and TIT measures derived from TTC measurements (49). [back]

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