Safety Analysis Tools Highway Safety Manual Ashley Reinkemeyer, P.E. MoDOT October 17, 2012
Presentation Summary Module 1 Introduction and Overview Module 2 Providing MoDOT Better Information
MODULE 1 Introduction and Overview
Module 1 Summary 1 What is the HSM? 2 Why is the HSM needed? 3 What is covered by the HSM? 4 Who should use the HSM?
1. WHAT IS THE HSM?
WHAT IS THE HSM? Provide Best factual information Tools To facilitate explicit safety considerations for: Planning Design Operations Maintenance through Synthesis of validated highway research Adapted & integrated to practice Analytical tools for predicting impact on road safety
The HSM Contains Best Science & Research Synthesis of previous research New research commissioned by AASHTO and FHWA
The Vision of the HSM - A Document Akin To the HCM 1 Definitive; represents quantitative state-ofthe-art information 2 Widely accepted within professional practice of transportation engineering 3 Science-based; updated regularly to reflect research
WHAT THE HSM IS NOT vs The HSM does not set requirements or mandates The HSM is not a best practice document for design or operations. The HSM contains no warrants or standards and does not supersede other publications that do.
2. WHY IS THE HSM NEEDED?
Is This Road Safe or Unsafe?
Highway Safety Has Two Dimensions Nominal Safety Substantive Safety Examined in reference to compliance with standards, warrants, guidelines and sanctioned design procedures The expected or actual crash frequency and severity for a highway or roadway *Ezra Hauer, ITE Traffic Safety Toolbox Introduction, 1999
Substantive Safety May Vary When Nominal Safety Does Not Existing Conditions Alternative 1 Alternative 2 Alternative 3
3. WHAT IS COVERED BY THE HSM?
Outline of the HSM Part A Introduction, Human Factors, and Fundamentals Part B Roadway Safety Management Process Part C Predictive Method Part D Crash Modification Factors
PART A Introduction, Human Factors, & Fundamentals Chapter 1 Chapter 2 Chapter 3 Introduction and Overview Human Factors Fundamentals
4 5 6 7 8 9 Part B Roadway Safety Management Process 4 5 6 7 8 9
Part C Predictive Method Two-Lane Rural Roads Urban/ Suburban Arterial Highways Rural Multilane Highways Special Part C Common Procedures Methodology Applications Safety issues not explicitly addressed by the methodology Example problems References Calibration Combining predicted with observed crashes
Part C - Predictive Method N predicted = N SPF x (CMF 1 x CMF 2 x CMF 3 ) x C x Where: N SPF = Expected number of crashes from SPF for base conditions (outlined in HSM per facility type) CMF 1x = Crash Modification Factors Adjust for non-base conditions Lane width, shoulder width, horizontal curves, etc. C x = Calibration factor Adjust for local (regional) conditions Climate, drivers, animal population, etc.
Part D Crash Modification Factors CHAPTER 13 Roadway Segments CHAPTER 14 Intersections CHAPTER 15 Interchanges CHAPTER 16 Special facilities and Geometric Situations CHAPTER 17 Road Networks
4. WHO SHOULD USE THE HSM?
Who Should Use the HSM? System Planning Project Planning Preliminary Design, Final Design, & Construction Operations & Maintenance
MODULE 2 Providing MoDOT Better Information
Module 2 Summary 1 How does MoDOT use the manual? 2 Example Application
1. How does MoDOT use the manual?
MoDOT decided to: Focus on Part C (predicted and/or excepted number of crashes) Start with safety-related Design Exceptions MoDOT also uses the HSM for: Design Alternatives Safety Projects Network Screening Project Evaluation
Design Exceptions Safety-related issues Lane width Shoulder width Shoulder type Rumble strips Turn lanes Horizontal alignment Grade Median width Sideslopes Lighting
Design Exceptions Quantify the effects The HSM won t provide a yes or no. Engineering judgment! Better information More informed decisions Solid documentation
Design Exception Policy EPG 131.1 If the design exception request involves any features that are safety related, then sufficient accident data and history is attached to the request to support the reasons for justification. A summary report of the accident information is acceptable if the volume of the data is excessive. Examples of safety related features are included in, but not limited to, the following list: lane width, shoulder width, shoulder type, rumble strips, turn lanes, bridge width, bridge approach rail, horizontal alignment, vertical alignment, grade, horizontal clearance, vertical clearance, guardrail, etc. Any other items that may be perceived as a safety concern will also follow these requirements. In addition, if the design exception request involves safety related features that are adequately addressed in the AASHTO Highway Safety Manual, then documentation of the exception should include a safety analysis as described in the manual. In general, this safety analysis should compare the expected number of crashes for the facility with the design exception to the expected number of crashes of the facility without the design exception. Currently, not all safety related features are explicitly addressed in the Highway Safety Manual. A list of features currently addressed by the manual include: lane width, shoulder width, shoulder type, center line rumble strips, horizontal alignment (length, radius), grade, roadside hazard rating, fixed objects, driveway density, median width, sideslope, lighting, intersection skew angle and turn lanes. Not all features in the manual are addressed for each facility type.
2. Example Application
Design Exception Example Resurfacing job on a 2-lane, rural road Existing Conditions: Major road 12 lanes, 2 paved shoulders No rumbles AADT = 2,500 veh/day Length = 8.9 miles Major road expectations 4 paved shoulders with rumble stripes
Design Exception Example Assume adding 4 paved shoulders requires extensive grading and possible ROW purchase. What would be the effect on crashes if a design exception was allowed for 2 paved shoulders with rumble stripes vs. the standard major road design of 4 paved shoulders with rumble stripes? Compare standard design to proposed design exception.
Design Exception Example Standard design: 4 paved shoulders with rumbles Predicted crashes = N spf x CMF s x C Base model prediction (SPF) N spf = AADT x L x 365 x 10-6 x e (-0.312) N spf = 2,500 x 8.9 x 365 x 10-6 x e (-0.312) N spf = 5.945 crashes per year
Design Exception Example Standard design: 4 paved shoulders with rumbles Predicted crashes = N spf x CMF s x C Determine CMF s Shoulder width Other CMF s are applicable, but shoulder width is the only variant between the standard design and the proposed design exception (both designs will have rumble stripes).
Design Exception Example CMF for shoulder width CMF = (CMF wra x CMF tra 1.0) x p ra + 1.0 p ra = 0.574 (default) CMF wra = 1.15 (from Table 10-9) CMF tra = 1.0 (paved) CMF = (1.15 x 1.0 1.0) x 0.574 + 1.0 CMF = 1.09
Design Exception Example Standard design: 4 paved shoulders with rumbles Predicted crashes = N spf x CMF s x C Apply CMF s Predicted crashes = 5.945 x 1.09 x 1.0 Predicted crashes = 6.5 crashes per year
Design Exception Example Proposed design exception: 2 paved shoulders with rumbles Predicted crashes = N spf x CMF s x C Base model prediction (SPF) N spf = AADT x L x 365 x 10-6 x e (-0.312) N spf = 2,500 x 8.9 x 365 x 10-6 x e (-0.312) N spf = 5.945 crashes per year
Design Exception Example Proposed design exception: 2 paved shoulders with rumbles Predicted crashes = N spf x CMF s x C Determine CMF s Shoulder width Other CMF s are applicable, but shoulder width is the only variant between the standard design and the proposed design exception (both designs will have rumble stripes).
Design Exception Example CMF for shoulder width CMF = (CMF wra x CMF tra 1.0) x p ra + 1.0 p ra = 0.574 (default) CMF wra = 1.30 (from Table 10-9) CMF tra = 1.0 (paved) CMF = (1.30 x 1.0 1.0) x 0.574 + 1.0 CMF = 1.17
Design Exception Example Proposed design exception: 2 paved shoulders with rumbles Predicted crashes = N spf x CMF s x C Apply CMF s Predicted crashes = 5.945 x 1.17 x 1.0 Predicted crashes = 7 crashes per year
Design Exception Example Standard design = 6.5 crashes/year Proposed design exception = 7 crashes/year Decision time for the designer/engineer. Do savings in ROW and construction justify one additional crash every two years?
Thank You. Ashley Reinkemeyer, P.E. ashley.reinkemeyer@modot.mo.gov