NCHRP Project 17-45
Enhanced Safety Prediction Methodology
and Analysis Tool for Freeways and
Interchanges
James A. Bonneson
Texas
Transportation
Institute
August 2012
Project Objectives
• Objectives
– Develop safety prediction methodology
• Freeways
• Interchanges
– Develop a software tool that implements the
methodology
– Document in a chapter for a future HSM
• NCHRP
Project Oversight
– Chuck Niessner, Mark Bush
• TRB
– Rick Pain
• Panel
–
–
–
–
–
–
–
John Milton, Chair, Washington DOT
James Allen, FHWA, Region 2
Geni Bahar, NAVIGATS Inc.
Michael Matzke, FHWA, Office of Infrastructure
Johnson Owusu-Amoako, Maryland DOT
Dan Turner, Univ. of Alabama
Clayton Chen, FHWA, Office of Safety R&D
Research Team
• Texas Transportation Institute
– J. Bonneson , S. Geedipally, D. Lord, M. Pratt
• CH2M-Hill
– J. Moller, T. Neuman
Project Status
• Schedule
– Begin: May 2009
– End: August 2012
• Status
– Final report and products submitted
Overview
• Background
• From System to Sites (Segmentation Process)
• Freeway Predictive Method
– Freeway segments
– Freeway speed-change lanes
• Ramp Predictive Method
– Ramp and C-D road segments
– Crossroad ramp terminals
• Method Scope
HSM Safety Evaluation Coverage
• 2010 HSM Part C Predictive Methods (52%)
– Rural Two-Lane Highways (VMT: 362, 088 mvm)
– Rural Multilane Highways (VMT: 356,810 mvm)
– Urban and Suburban Arterials (VMT: 831,710 mvm)
• NCHRP Project 17-45: Freeways (33%)
– Freeways
– Ramps
(VMT: 963,805 mvm)
• HSM Coverage (85%)
– U.S. Streets and Highways
(VMT: 2,966,506 mvm)
Freeway Safety Indicators
• Freeway Fatal Crash Rate
– Fatality Analysis Reporting System (NHTSA)
– U.S. highway system: 1.02 fatal cr/100 mvm
Fatal Crashes/100 mvm
0.9
0.8
Rural Freeways
0.7
0.6
0.5
0.4
Urban Freeways
0.3
0.2
0.1
0.0
2007
2008
2009
Year
2010
Freeway Segmentation Process
• Concept
– Disaggregate road system into individual “sites”
– Rules for identifying segments and intersections
• Application
– 2 speed-change lanes
– 1 crossroad ramp
terminal
– 3 freeway segments
– 5 C-D road segments
– 5 ramp segments
PLAN VIEW
In
In
Ramp Entrance
Length, Len
Ren1
CD1
Fr1
CD2
Rex5
Crossroad
Rex4
CD3
Fr2
Ren2
Ramp Exit
Length, Lex
Rex3
CD4
CD5
Fr3
Freeway Predictive Method
• Safety Performance Functions (SPFs)
– Lanes: 4, 6, 8, 10 (9, 10 urban areas only)
– Guidance for segments with 5, 7, 9 lanes
• Crash Modification Factors (CMFs)
– Horizontal curve (one roadbed, both roadbeds)
– Lane width
– Inside shoulder width
– Median width
Freeway Predictive Method
• CMFs
– Median barrier (continuous and pieces)
– High volume (portion of vehicles in congestion)
– Lane change (weaving)
– Outside shoulder width
– Shoulder rumble strip
– Outside clearance (clear zone width)
– Outside barrier (continuous and pieces)
Ramp Segment Predictive Method
• Segment SPFs
– Lanes: 1, 2 (2 urban areas only)
– Ramps, C-D Roads
• CMFs
– Horizontal curve
– Lane width
– Right shoulder width
– Left shoulder width
Ramp Segment Predictive Method
• CMFs
– Right side barrier
– Left side barrier
– Lane add, lane drop
– Ramp speed-change lane
– Weaving section (C-D road only)
• Supplemental Models
– Ramp curve speed prediction model
Ramp Terminal Predictive Method
• SPFs
Ramp
Diagonal, Entrance
Freeway
Diagonal, Exit
Type: D3en
Crossroad
Diagonal, 4-Leg
Type: D4
Ramp
Freeway
Type: D3ex
Ramp
Freeway
– 7 configurations
– Stop control, signal control
Ramp Terminal Predictive Method
• SPFs
– 7 configurations
– Stop control, signal control
4-quad Parclo B
Ramp
Freeway
Freeway
4-quad Parclo A
Ramp
Type: A4
Freeway
2-quad Parclo A
Ramp
Ramp
Ramp
2-quad Parclo B
Type: B2
Type: A2
Ramp
Freeway
Type: B4
Ramp Terminal Predictive Method
• CMFs
– Exit ramp capacity (ramp lanes and control mode)
– Crossroad left-turn lane or bay
– Crossroad right-turn lane or bay
– Access point frequency (driveway count)
– Segment length (distance to next intersection)
– Median width
Ramp Terminal Predictive Method
• CMFs for Signal-Controlled Terminals
– Protected-only left-turn operation
– Channelized right turn on crossroad
– Channelized right turn on exit ramp
– Non-ramp public street leg
• CMFs for One-Way Stop-Controlled Terminals
– Skew angle
Method Scope
• Methods Do Not Address the Following
– Ramp meters
– HOV by-pass lanes on ramps
– Frontage road segments or terminals
– Freeways with buffer-separated HOV/HOT lanes
• Note: can evaluate general-purpose lanes with barrier
separation
Buffer
Barrier
(ok)
Method Scope
• Freeway Evaluation Scenarios
– Interchange ramp spacing
– Weaving section length
– Reallocation of cross section width
• Interchange Scenarios
– System interchange configuration
– Service interchange configuration (parclo, diamond)
• Analysis Scale
– Single year analysis or design life analysis
– Freeway system analysis or individual section
Project Deliverables
• Draft final report
– Main body: documents development of methods
– Appendix: Draft HSM Freeways Chapter
– Appendix: Draft HSM Ramps Chapter
– Appendix: Draft HSM Appendix A for Part C
– Appendix: Draft algorithm description
Project Deliverables
• Enhanced Interchange Safety Analysis Tool (ISATe)
– Excel spreadsheet
– User manual
• Workshop Training Materials
– PowerPoint slides
– Course notes
– Instructor guide
ISATe
Questions – Comments?
• Implementation Activity
– FHWA is implementing the predictive methods in the
Interactive Highway Safety Design Model (IHSDM)