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SURROGATE SAFETY ASSESSMENT MODEL (SSAM) Prepared by: Joe Bared, FHWA
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Transcript of SURROGATE SAFETY ASSESSMENT MODEL (SSAM) Prepared by: Joe Bared, FHWA
SURROGATE SAFETY SURROGATE SAFETY ASSESSMENT MODELASSESSMENT MODEL
(SSAM)(SSAM)
Prepared by:Prepared by: Joe Bared, FHWAJoe Bared, FHWA
Event Descriptions at IntersectionsEvent Descriptions at Intersections
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2 3
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Conflict Points
Rear-end Conflict Lines
Conflict Lines
Intersection box
Calculate corresponding SSAM Calculate corresponding SSAM measuresmeasures
• 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.
AA
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 MeasuresSurrogate 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 SurrogatesConflict Point Diagram with Surrogates
conflictpoint
Through vehicle
Dis
tanc
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t1
Time
encroachmentbegin
t2
t3
Vehicle beginsbraking
encroachmentend
projected arrival atconflict pt
actual arrival atconflict pt
t4
t5
TTC
PET
Initial DecelerationRate
MaxS andDeltaS
crossing vehicle
Comparison of Two Alternative designsComparison of Two Alternative designs
Field Validation of 83 Signalized Field Validation of 83 Signalized IntersectionsIntersections
Relating crashes to conflicts
Safety Ranking of Total IncidentsSafety 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 TypeSafety 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 1Conflicts: Case 1
Conventional573 total
CFI609 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 sTTC <=1.0 sTTC <=1.5 s
Conflicts: Case 1Conflicts: Case 1T-Test: Significance of difference of CFI statistics from the conventional intersection statistics with 95% confidence interval, no crashes, conflicts of VMAX >= 10ft/s
SSAM Measures Mean Variance Sample Size Mean Variance Sample Size t value t critical SigfinicantTTC 1.163 0.133 609 1.103 0.159 573 2.693 1.66 YESPET 2.465 2.655 609 2.397 2.505 573 0.73 1.66 NOMaxS 12.541 4.447 609 12.028 3.127 573 4.54 1.66 YESDeltaS 4.712 7.87 609 4.308 6.552 573 2.588 1.66 YESDR -2.638 4.048 609 -2.766 4.683 573 1.047 1.66 NOMaxD -3.943 4.118 609 -4.197 4.541 573 2.097 1.66 YESMaxDeltaV 2.539 2.584 609 2.363 2.34 573 1.936 1.66 YESFrequencies:Crossing 0.6 0.489 10 0.1 0.1 10 2.06 1.734 YESRear-end 51.5 15.611 10 40.7 18.456 10 5.851 1.734 YESLane changing 8.8 6.178 10 16.5 4.5 10 -7.452 1.734 YESTotal 60.9 15.878 10 57.3 31.122 10 1.661 1.734 NO
CFI Layout Conventional T-Test