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NCHRP Project 17-35: Evaluation of Safety Strategies at Signalized Intersections

 

 

Evaluations of Low Cost Safety Improvements Pooled Fund Study

 

PPT version for Printing

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
State Treated Sites Unsignalized Reference Signalized Reference Years of Data
California 28 2,330 63 1993–2002
Minnesota 17 1,258 21 1991–2002
 
 
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)
All All All
Right–Angle Right–Angle Right–Angle
Left–Turn Left–Turn  
Rear–End Rear–End Rear–End
 
 
Crash Frequency Crash Modification Factors (standard error)
State Total Right Angle Left Turn Rear End
CA 0.778 (0.061) 0.221 (0.036) 0.433 (0.065) 2.474 (0.373)
MN 0.488 (0.027) 0.228 (0.019) 0.374 (0.063) 1.300 (0.141)
ALL 0.559 (0.025) 0.227 (0.017) 0.401 (0.047) 1.579 (0.142)
 
 
Economic Cost Crash Modifications Factors
Characteristic Theta Cost
All 0.265
California 0.315
Minnesota 0.247
3 leg 0.286
4 leg 0.264
2 lanes on major 0.265
4 lanes on major 0.265
AADT < 20,000 0.314
AADT > 20,000 0.253
Expected RA/Expected RE <=4.5 0.324
Expected RA/Expected RE >4.5 0.215
 
 

Skid Resistant Treatment

SUMMARY OF DATA COLLECTED
Site Type No. of Sites
Treated Intersection 256
Reference Intersection 3,993
Treated Segment 36.3 miles
Reference Segment 1,242.4 miles
Data collected in New York State for 1994 to 2003.
 
 
SAFETY PERFORMANCE FUNCTIONS DEVELOPED
Intersections Segments
Total Total
Wet–Road Wet–Road
Rear–end Rear–end
Dry Dry
Rear–end wet–road Rear–end wet–road
Right–angle Rear–end dry–road
Right–angle wet–road Single vehicle
  Single vehicle wet–road
 
 
Safety Effects At Intersections
Grouping Total (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.)
All 0.799
(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 signalized 0.667
(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–controlled 0.819
(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–controlled 0.590
(0.114)		 0.217
(0.103)		 0.304
(0.086)		 1.392
(0.321)		 0.221
(0.161)		 n/a n/a
4 leg signalized 0.797
(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–controlled 1.271
(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–controlled 0.589
(0.216)		 0.361
(0.371)		 0.504
(0.248)		 0.651
(0.273)		 n/a n/a n/a
     
      s.e means standard error n/a means not applicable
 
 
Safety Effects For Segments
Grouping Total (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.)
All 0.764
(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 lanes 0.964
(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 lanes 0.684
(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 lanes 0.599
(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 lanes 0.862
(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

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