Enhanced Safety Prediction Methodology and Analysis Tool for Freeways and Interchanges James A. Bonneson August 2012 NCHRP Project 17-45

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

Project Oversight NCHRP – 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

Freeway Inventory Freeway – Interstate miles 46,709 – Non-interstate miles13,384 60,093 Number of Interchanges – Interstate 17,800 – Non-interstate 6,900 24,700 U.S Freeway System for 2007

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 (VMT: 963,805 mvm) – Ramps 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

Freeway Safety Indicators Interchange Area Fatal Crash Rate – Fatality Analysis Reporting System (NHTSA) – fatal crashes/yr; 12.5 total crashes/yr

Freeway Segmentation Process – 2 speed-change lanes – 1 crossroad ramp terminal In Concept – Disaggregate road system into individual “sites” – Rules for identifying segments and intersections Application – 3 freeway segments – 5 C-D road segments – 5 ramp segments

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 – 7 configurations – Stop control, signal control Diagonal, Exit Diagonal, Entrance Diagonal, 4-Leg

Ramp Terminal Predictive Method SPFs – 7 configurations – Stop control, signal control 4-quad Parclo A 4-quad Parclo B 2-quad Parclo B 2-quad Parclo A

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

Implementation Activity – FHWA is implementing the predictive methods in the Interactive Highway Safety Design Model (IHSDM) Questions – Comments?

Predictive Model Model – Crash frequency, N p = N spf x CMF lw x CMF sw... x C Model Components – Safety performance function (SPF), N spf – Crash modification factors (CMF), CMF i – Calibration factor, C Volume Lane Width Predicted Crash Frequency

Freeway Predictive Method Safety Performance Function – N spf = a × (AADT) b × L – Coefficients “a” and “b” by... Area type Number of lanes Crash type Severity

Crash Modification Factor Lane Width CMF – CMF = Ratio of crash frequency Sites with X-ft lane width to sites with base lane width – Lane width base condition 12 ft lanes CMF base = 1.00 – Example Proposed roadway 11 ft lanes CMF 11 ft = 1.04