SURROGATE SAFETY ASSESSMENT MODEL (SSAM) Prepared by: Joe Bared, FHWA

Event Descriptions at Intersections Conflict Points Rear-end Conflict Lines Conflict Lines Intersection box

Calculate corresponding SSAM measures Calculate the angle of the headings of the two vehicles and use this data to determine the conflict type: –if angle α >= 45 degree, then it is a crossing conflict; –If α <= 2 degree, then it is a rear-end conflict; –else, it is a lane change conflict. A B α Adding configurable angle thresholds in SSAM version 2 Calculate the angle of the headings of the two vehicles and use this data to determine the conflict type: –if angle α > 85 degree, crossing conflict; –If α < 30 degree rear-end conflict; –else, it is a lane change conflict.

Surrogate Measures Minimum Time To Collision (TTC) Minimum Post-Encroachment Time (PET) Initial Deceleration Rate (DR) Maximum speed (MaxS) Maximum relative speed difference (DeltaS) Location of the conflict event (CLSP, CLEP) Maximum “post collision” DeltaV (MaxDeltaV)

Conflict Point Diagram with Surrogates

Comparison of Two Alternative designs

Field Validation of 83 Signalized Intersections Relating crashes to conflicts

Safety Ranking of Total Incidents The Spearman rank-correlation coefficient between hourly conflicts frequency and annual crash frequency is a significant 0.46

Safety Ranking Incident Type The Spearman rank-correlation coefficient for rear-end conflicts/collisions is a significant 0.47 The Spearman rank-correlation coefficient for sideswipe conflicts/collisions is a significant 0.47

Conflicts: Case 1 Conventional 573 total CFI 609 total High-speed conflicts for ten simulation runs - no crashes, conflicts at Vmax >= 10 ft/s Same as the previous slide, but with time-to-collision (TTC) composition TTC <=0.5 s TTC <=1.0 s TTC <=1.5 s

Conflicts: Case 1 T-Test: Significance of difference of CFI statistics from the conventional intersection statistics with 95% confidence interval, no crashes, conflicts of V MAX >= 10ft/s