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

4

6

1

2 3

5

7

8

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

e

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