1. 1 The Highway Safety Manual Predictive Methods

2. 2 New Highway Safety Manual of 2010 ►Methodology is like that for assessing and assuring the adequacy of Capacity ►HSM allows the transportation professional to understand and quantify highway safety performance for informed and balanced decision making

3. Akin to the HCM, but for safety… 1 2 3 What is the HSM? 3

4. ►A set of tools for Prediction of Crash Frequency and for Analysis of safety ►Facilitates explicit consideration of safety throughout the project development process ►Provides a synthesis of validated highway safety research ►Provides analytical tools for predicting the impact of decisions on road safety What is the HSM? 4

5. 5 Why should we use the HSM? We need quantitative estimates of safety performance for many planning and project development decisions. More reliable estimates lead to more safety cost-effective decision making. The estimation methods in the HSM are based upon good science/research and improve upon much of current practice.

6. 6 Predict Safety Performance of Geometric Design Features? What Can the HSM Do for You? Quantitatively calculate Safety Effects? Predict Safety Performance of Intersections? YES Definitely YES Better Decisions for Safety?YES

7. 7 WHAT IS COVERED BY THE HSM?

8. Part C Common Procedures Safety Performance Functions Crash Modification Factors Calibration for local conditions Chapter 10 Chapter 11 Chapter 12 PART C: PREDICTIVE METHODS 8

9. 9 Chapters 10, 11 & 12: Safety Performance Function (SPF) Models for Roadway Segments and Intersections Used to determine crash frequency for a set of base geometric and traffic control conditions (crashes/yr) Crash Modification Factors (CMF) Adjusts prediction for base conditions to actual or proposed conditions PART C: PREDICTIVE METHODS

10. What are Safety Performance Functions? -Mathematical Regression Models for Roadway Segments and Intersections: Developed from data for a number of similar sites Developed for specific site types and “base conditions” Function of only a few variables, primarily AADT Used to calculate the expected crash frequency (crashes/year) for a set of base geometric and traffic control conditions PART C: PREDICTIVE METHODS 10

11. What are Crash Modification Factors? Adjusts the calculated SPF predicted value for base conditions to actual or proposed conditions Accounts for the difference between base conditions and site specific conditions PART C: PREDICTIVE METHODS 11

12. 12 HSM can be used to: Identify sites with the most potential for reducing crash frequency Identify contributing factors to crashes Calculate the safety effect of various design alternatives Estimate potential crash frequency on highways Estimate the potential effects on crash frequency of planning, design, operations, and policy decisions INTRODUCTION TO PREDICTION MODELS AND CRASH MODIFICATION FACTORS

13. 13 Homogeneous (like) highway segments Intersections – Sum of Individual Intersection Crashes Organizing information for Safety Analysis:  Separate project lengths (and crashes) into:  Identify project as rural or urban: Urban = location inside urban boundaries where population > 5,000 person INTRODUCTION TO PREDICTION MODELS AND CRASH MODIFICATION FACTORS

14. 14 Three Basic Steps: 1. Apply Safety Performance Functions (SPF) Equations  Predict safety performance for base conditions 2.Apply Appropriate Crash Modification Factors (CMFs)  Adjust predicted safety performance from base conditions to existing/proposed conditions  Are greater or less than 1: < 1.0 -- lower crash frequency > 1.0 -- increased crash frequency 3.Calibration, C r or C i  Accounts for local conditions/data INTRODUCTION TO PREDICTION MODELS AND CRASH MODIFICATION FACTORS

15. 15 Safety Performance Function and Crash Modification Factors for Rural Two-Lane Roadway Segments INTRODUCTION TO PREDICTION MODELS AND CRASH MODIFICATION FACTORS

16. 16 Rural Two-Lane Roadway Segments SPF Prediction Model for Base Conditions: N spf-rs = AADT x L x 365x10 -6 x e (-0.312) N spf-rs = predicted total crash frequency for roadway segment base conditions AADT = average annual daily traffic volume (vpd) L = length of roadway segment (miles) INTRODUCTION TO PREDICTION MODELS AND CRASH MODIFICATION FACTORS

17. 17 Base Conditions for Rural Two-Lane Roadway Segments Lane Width: 12 feet Shoulder Width: 6 feet Shoulder Type: Paved Roadside Hazard Rating: 3 Driveway Density: 5 driveways/mile Grade: None, level Horizontal Curvature: None Vertical Curvature: None Centerline rumble strips: None TWLTL, climbing, or passing lanes: None Lighting: None Automated Speed Enforcement: None INTRODUCTION TO PREDICTION MODELS AND CRASH MODIFICATION FACTORS

18. Crash Modification Factors - CMFs CMFs are used to adjust the calculated SPF predicted value for base conditions to actual or proposed conditions CMF = 1.0: Meets base conditions or the treatment has no effect on the expected crash frequency CMF < 1.0: The treatment reduces the expected crash frequency CMF > 1.0: The treatment increases the expected crash frequency PART D: CRASH MODIFICATION FACTORS 18

19. 19 N predicted-rs = N spf-rs x (CMF 1r … CMF xr ) C r Apply CMFs to the SPF Base Model Where: N predicted-rs = predicted average crash frequency for an individual roadway for a specific year (crashes per year) N spf-rs = predicted average crash frequency for base conditions for an individual roadway segment (crashes per year) CMF 1r... CMF xr = Crash Modification Factors for individual design elements C r = calibration factor INTRODUCTION TO PREDICTION MODELS AND CRASH MODIFICATION FACTORS

20. 20 CMFs for Rural Two- Lane Highway Segments

21. Predict Potential for Crash Frequency Performance of Shoulder Designs What Can the HSM Do? 21

22. N spf-rs = (ADT n ) (L) (365) (10 -6 ) e -0.312 Example: NY County Route 2,500 ADT 14.8 miles HSM prediction is N spf-rs = 9.9 crashes/year Crash Frequency for 2-Ln Rural Hwys 22

23. Example: NY County Route 2,500 ADT 10’ lane 10’ wide lanes NO shoulder Lane Width = 10’ CMF ra = 1.30 from HSM Table 10-8 Example: Crash Frequency for 10’ Lane + NO Shoulder CMF 1r = (1.30 - 1.0)x 0.574 + 1.0 = 1.172 Crash Frequency for 2-Ln Rural Hwys 23

24. Shoulder Width = 0’ CMF wra = 1.50 from HSM Table 10-9 CMF 2r = (((1.50x1.00)-1.0)x 0.574)+ 1.0 = 1.287 Example: NY County Route 2,500 ADT 10’ lane 10’ wide lanes NO shoulder Example: Crash Frequency for 10’ Lane + NO Shoulder Crash Frequency for 2-Ln Rural Hwys 24

25. Total CMF 1r&2r = 1.172x 1.287 = 1.508 50.8% more crashes than for a 12’ lane with a 6’ paved shoulder Example: Crash Frequency for 10’ Lane + NO Shoulder Crash Frequency for 2-Ln Rural Hwys 25

26. N spf-rs = (ADT n ) (L) (365) (10 -6 ) e -0.312 Example: IL State Route 2,500 ADT 14.8 miles HSM prediction is N spf-rs = 9.9 crashes/year Crash Frequency for 2-Ln Rural Hwys 26

27. Predict the Crash Reduction Effect of a Combination Shoulder For a 12 foot wide lane with a 2’ paved + 6 foot aggregate shoulder, what is the effect? 8 ft 2’6’ 12’ Crash Frequency for 2-Ln Rural Hwys 27

28. 8 ft 2’6’ 12’ Shoulder Width = 2’ + 6’ = 8’ CMF wra = 0.87 Combination Shoulder Surface CMF tra = (2’/8’)x1.00 + (6’/8’) x 1.02 = 1.015 CMF 2r =((0.87x1.015) -1.0) (0.574) + 1.0 = 0.933 - Crash Frequency less than for a 6’ paved shoulder Reduction in Crash Frequency of 13% Increase in Crash Frequency of 1.5% Reduction in Crash Frequency of 6.7% Crash Frequency for 2-Ln Rural Hwys 28

29. 29 Safety Performance Functions And Crash Modification Factors for Rural Two-Lane Intersections INTRODUCTION TO PREDICTION MODELS AND CRASH MODIFICATION FACTORS

30. 30 SPF Models for Rural Two-Lane Intersections: N spf-3ST =exp[-9.86 + 0.79 ln(AADT maj ) + 0.49 ln(AADT min )] 1) 3-Leg Stop Controlled Intersection (3ST): 2) 4-Leg 2-Way Stop Controlled Intersection (4ST): N spf-4ST =exp[-8.56 + 0.60 ln(AADT maj ) + 0.61 ln(AADT min )] INTRODUCTION TO PREDICTION MODELS AND CRASH MODIFICATION FACTORS 3) 4-Leg Signalized Intersection (4SG): N spf-4SG = exp[-5.13 + 0.60 ln(AADT maj ) + 0.20 ln(AADT min )]

31. 31 Base Conditions for Rural Two-Lane Intersections: Intersection Skew Angle: 0 degrees Presence of Left-Turn Lanes: None Presence of Right-Turn Lanes: None Lighting: None INTRODUCTION TO PREDICTION MODELS AND CRASH MODIFICATION FACTORS

32. 32 N predicted-int = N spf-int x (CMF 1i … CMF xi ) C i Apply CMFs to the SPF Base Model Where: N predicted-int = predicted average crash frequency for an individual intersection for a specific year (crashes per year) N spf-int = predicted average crash frequency for an intersection with base conditions(crashes per year) CMF 1i... CMF xi = crash modification factors for individual design elements C i = calibration factor for local conditions INTRODUCTION TO PREDICTION MODELS AND CRASH MODIFICATION FACTORS

33. 33 Safety Performance Functions And Crash Modification Factors for Urban and Suburban Multilane Intersections INTRODUCTION TO PREDICTION MODELS AND CRASH MODIFICATION FACTORS

34. 34 SPF Models for Urban/Suburban Multilane Intersections: 1) 3-Leg Stop Controlled Intersection (3ST): 2) 4-Leg 2-Way Stop Controlled Intersection (4ST): INTRODUCTION TO PREDICTION MODELS AND CRASH MODIFICATION FACTORS 3) 4-Leg Signalized Intersection (4SG):

35. 35 SUBSTANTIVE SAFETY MAY VARY Even when meet NOMINAL Geometric Requirements Existing Conditions Alternative 1 Alternative 2 Alternative 3

36. 36 Existing Conditions Alternative 1 Alternative 2 Alternative 3 At 20,000 ADT 17.2 crashes/mile 26.3 crashes/mile 8.6 crashes/mile4.2 crashes/mile SUBSTANTIVE SAFETY MAY VARY Even when meet NOMINAL Geometric Requirements

37. 37 Predict Safety Performance of Geometric Design Features? What Can the Highway Safety Manual Do for You? YES Quantitatively calculate Safety Effects? Predict Safety Performance of Intersections? YES Better Decisions for Safety?YES

38. 38 Questions and Discussion: The Highway Safety Manual