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NCHRP 17-25
Crash Reduction Factors for Traffic
Engineering and ITS Improvements

 
UNC HSRC
VHB
Ryerson University
(Bhagwant and Craig)

Project Goals

  • Develop new Accident Modification Factors where voids currently exist
  • Increase the level of predictive certainty for existing Accident Modification Factors
  • Coordination with other NCHRP and FHWA projects

Completed Tasks

  • Phase I
    – Literature Review
    – State Surveys and Interviews
  • Research Results Digest 299
    – Accident Modification Factors Knowledge Matrix
    – List of 20 “credible” Accident Modification Factors
    – High or Med-High level of predictive certainty (LOPC)

Phase II Efforts

  • Empirical Bayes Before-After Evaluations
    – Rural signal installations
    – 4-lane to 3-lane conversions (Road diets)
    – Skid resistance treatment
    – Left-turn signal phasing
    – 8" to 12" signals
    – Nighttime flash operation to normal
    – Double red signal heads (“Dollys”)
    – Median width on divided roadways
  • Analysis-Driven Expert Panels
    – Urban/Suburban
    – Rural Multilane
  • Cross-sectional models
  • GOAL – Develop Accident Modification Factors

Rural Signal Installations

Summary of data collected
StateTreated SitesUnsignalized ReferenceSignalized ReferenceYears of Data
California282,330631993-2002
Minnesota171,258211991-2002
 
 
Safety Performance Functions Developed
3-Leg Stop-Controlled (CA and MN independently)4-Leg Stop-Controlled (CA and MN independently)4-Leg Signalized (CA and MN combined)
AllAllAll
Right-AngleRight-AngleRight-Angle
Left-TurnLeft-Turn 
Rear-EndRear-EndRear-End
 
 
Crash Frequency Crash Modification Factors (standard error)
StateTotalRight AngleLeft TurnRear End
CA0.778 (0.061)0.221 (0.036)0.433 (0.065)2.474 (0.373)
MN0.488 (0.027)0.228 (0.019)0.374 (0.063)1.300 (0.141)
ALL0.559 (0.025)0.227 (0.017)0.401 (0.047)1.579 (0.142)
 
 
Economic Cost Crash Modifications Factors
CharacteristicTheta Cost
All0.265
California0.315
Minnesota0.247
3 leg0.286
4 leg0.264
2 lanes on major0.265
4 lanes on major0.265
AADT < 20,0000.314
AADT > 20,0000.253
Expected RA/Expected RE <=4.50.324
Expected RA/Expected RE >4.50.215
 
 

Skid Resistant Treatment

SUMMARY OF DATA COLLECTED
Site TypeNo. of Sites
Treated Intersection256
Reference Intersection3,993
Treated Segment36.3 miles
Reference Segment1,242.4 miles
Data collected in New York State for 1994 to 2003.
 
 
SAFETY PERFORMANCE FUNCTIONS DEVELOPED
IntersectionsSegments
TotalTotal
Wet-RoadWet-Road
Rear-endRear-end
DryDry
Rear-end wet-roadRear-end wet-road
Right-angleRear-end dry-road
Right-angle wet-roadSingle vehicle
 Single vehicle wet-road
 
 
Safety Effects At Intersections
GroupingTotal (s.e.)Wet-road (s.e.)Rear-end (s.e.)Dry (s.e.)Rear-end Wet (s.e.)Right-angle (s.e.)Right-angle Wet (s.e.)
All0.799
(0.028)		0.426
(0.030)		0.582
(0.034)		1.149
(0.051)		0.322
(0.041)		1.045
(0.078)		0.799
(0.123)
3 leg signalized0.667
(0.050)		0.372
(0.053)		0.554
(0.065)		0.959
(0.093)		0.261
(0.066)		0.787
(0.125)		0.470
(0.161)
3 leg stop-controlled0.819
(0.048)		0.355
(0.046)		0.586
(0.057)		1.302
(0.095)		0.335
(0.075)		0.828
(0.218)		0.828
(0.218)
3 leg yield-controlled0.590
(0.114)		0.217
(0.103)		0.304
(0.086)		1.392
(0.321)		0.221
(0.161)		n/an/a
4 leg signalized0.797
(0.052)		0.546
(0.070)		0.585
(0.068)		0.992
(0.081)		0.361
(0.084)		0.898
(0.117)		1.105
(0.294)
4 leg stop-controlled1.271
(0.143)		0.597
(0.137)		0.943
(0.188)		1.754
(0.242)		0.482
(0.215)		1.687
(0.323)		0.829
(0.351)
4 leg yield-controlled0.589
(0.216)		0.361
(0.371)		0.504
(0.248)		0.651
(0.273)		n/an/an/a
   
     s.e means standard errorn/a means not applicable
 
 
Safety Effects For Segments
GroupingTotal (s.e.)Wet-road (s.e.)Rear-end (s.e.)Dry (s.e.)Rear-end Wet-road (s.e.)Rear-end Dry-road (s.e.)Single-vehicle (s.e.)Single-vehicle Wet-road (s.e.)
All0.764
(0.023)		0.434
(0.024)		0.828
(0.043)		1.003
(0.043)		0.575
(0.055)		0.977
(0.068)		0.698
(0.040)		0.399
(0.039)
Rural 2 lanes0.964
(0.073)		0.852
(0.126)		1.047
-0.149		1.167
(0.114)		0.971
(0.256)		1.235
(0.219)		1.078
(0.141)		1.125
(0.287)
Rural >2 lanes0.684
(0.032)		0.346
(0.028)		0.776
(-0.068)		0.875
(0.061)		0.474
(0.079)		0.838
(0.098)		0.588
(0.046)		0.292
(0.038)
Urban 2 lanes0.599
(0.082)		0.260
(0.066)		0.612
(-0.142)		0.992
(0.195)		0.344
(0.145)		0.695
(0.216)		0.921
(0.232)		0.523
(0.247)
Urban >2 lanes0.862
(0.038)		0.538
(0.045)		0.866
(-0.059)		1.132
(0.065)		0.640
(0.084)		1.120
(0.099)		0.800
(0.083)		0.615
(0.115)

s.e means standard error

Road Diets in Urban Areas

Previously discussed

Winston-Salem Empirical-Bayes Evaluations

  • Treatments
    – Left-turn signal phasing
    • Permissive to protective/permissive LT phasing
    • Protective/permissive to protected LT phasing
    – 8” to 12” red signal heads
    – Nighttime flash to normal
    – Added double red signal heads (“dollys”)
  • Identified treatment sites and reference group
  • Obtained crash data, geometric data, and traffic volumes for treatment and reference sites
  • Permissive to protective/permissive Left Turn
    – 11 sites
  • Protective/permissive to protected Left Turn
    – 4 sites
  • 8” to 12” signal heads
    – 26 sites
  • Nighttime flash to normal
    – 12 sites
  • Double red signal heads
    – 8 sites
  • Safety Performance Function (SPF) for total crashes
    – Developed using reference group
  • Safety Performance Function (SPF) for target crashes
    – Could not be developed with available data
    – Used proportion of target crashes to adjust SPF for total crashes
  • Empirical Bayes analysis almost complete
    – Calibrate Safety Performance Functions separately for before and after periods
    – Estimate percent change in crashes

Median Width AMF

  • Current studies are not consistent
  • Accident Modification Factor for median width and validation of the approach
  • Preliminary assessment of states – WA, CA, MN, IL

Analysis-Driven Expert Panels

  • Urban-Suburban Arterials (NCHRP 17-26)
    • 25+ treatments addressed for segments and intersections
      – Geometrics (lane/shoulder widths, median type, etc.)
      – Operations (signalization, speed limits, etc.)
      – Other (lighting, roundabouts, etc.)
    • Consensus on 9 Accident Modification Factors
    • Follow-up work on additional Accident Modification Factors (e.g., roadside hazards, speed)
  • Rural Multilane Roadways (NCHRP 17-29)
    • 15+ treatments addressed for segments and intersections
      – Geometrics (median width/type, shoulder width/type, turning lanes, alignment, etc.)
      – Operations (signalization, speed limits, access, etc.)
      – Other (lighting, roadside hazards, etc.)
    • Consensus on 3 Accident Modification Factors
    • Follow-up work identified for additional Accident Modification Factors (access, shoulder type) within both projects

< Presentation 15 | Presentation 17 >

 

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