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UBC Theses and Dissertations

Field validation for surrogate safety assessment methodology (SSAM) using a multi-purpose micro-simulation El-Basyouny, Karim


Several approaches exist for estimating safety ranging from using accident rates to accident prediction models which relate the expected accident frequency at a road location to its traffic and geometric characteristics. In recent years, the usefulness and reliability of accident records has led several researchers to consider surrogate safety measures. Among the most common surrogate measures is the Traffic Conflict Technique (TCT). However, there some shortcomings related to the cost of collecting conflicts data and the reliability of human observers. Several efforts were taken to incorporate safety, in terms of surrogate measures such as TCT, into multipurpose micro-simulations (MPMS). As a result, the Surrogate Safety Assessment Methodology (SSAM) was developed. The main objective of this thesis is to perform a field validation of the SSAM approach using VlSSlM 4.1-12 program. This thesis provides the results of the field validation plan for the Surrogate Safety Assessment Methodology (SSAM) with an aim to compare the predictive safety performance capabilities of the SSAM approach with actual accident experience at Canadian signalized intersections. The validation plan consisted of five tests aiming to quantify the relation between the recorded accidents and simulated conflicts. The first validation test, safety ranking analysis, compares the ranking of intersections from SSAM according to predicted average conflicts rates (ACR) and the ranking of the same intersections using actual accident frequency. The second test repeats the same comparative ranking procedures as test 1, but for sub-sets of accident/conflict types. The third validation test, conflict/accident paired comparison, compares the conflict frequency predicted by SSAM to the actual accident frequency at each intersection. A regression equation that relates actual accidents to the predicted conflicts was developed. The test determines the strength of the relationship between predicted conflicts and actual accidents. The fourth validation test, conflict/accident prediction model comparative analysis, determines whether the conflict prediction model can predict risk in a manner similar to the accident prediction model for intersections with the same characteristics. The comparison included several model applications such as the identification and ranking of accident prone locations. The fifth test repeats the same comparative ranking procedures as test 4, but for sub-sets of accident/conflict types. The results of the five validation tests indicated that the safety measures computed from the simulated conflicts were poorly related to those of actual accidents. In terms of model applications, the results indicate a poor agreement between the identification and ranking of accident prone locations obtained from the conflict/accident prediction models. Furthermore, it was concluded that traffic volumes alone can explain more variation in the occurrence of accidents than simulated conflicts obtained from SSAM. The poor relation between simulated conflicts and actual accidents could be associated with SSAM’s sensitivity to the manner by which an intersection was modeled in VISSIM. A number of validation issues were investigated to demonstrate the sensitivity of SSAM’s output by varying some of the design parameters. The effects of redefining the priority rules, changing the minimum allowable gap size and the effects of changing the lateral clearance parameters were investigated. The results of these investigations showed that the number of conflicts produced by SSAM varied considerably as a result of changing these parameters. As well, it was found that the lane changing logic in VlSSlM had a significant impact on the number of simulated conflicts. In many cases, an abrupt lane changing behavior was noticed. To this time, there is no clear justification for some of the abrupt lane-changing behaviors experienced in some intersections. Although certain measures were taken into account to reduce the effect of that unusual maneuver, this behavior continued to occur and affect the number of conflicts produced by SSAM.

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