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HSM Applications to Two-Lane Rural Highways

HSM Applications to Two-Lane Rural Highways. Predicting Crash Frequency and Applying CMF’s for Two-Lane Rural Highway Intersections. - Session #6. Describe the SPF Base Models for prediction of Intersection Crash Frequency

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HSM Applications to Two-Lane Rural Highways

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  1. HSM Applications to Two-Lane Rural Highways Predicting Crash Frequency and Applying CMF’s for Two-Lane Rural Highway Intersections - Session #6

  2. Describe the SPF Base Models for prediction of Intersection Crash Frequency • Calculate Predicted Crash Frequency for Rural Two-lane Highway Intersections • Describe CMF’s for Rural 2 Lane Intersections • Apply CMF’s to Predicted Crash Frequency Predicting Crash Frequency for Two-Lane Rural Highway Intersections Outcomes:

  3. Why Intersection Safety? • A small part of overall highway system, but - • In 2008 – 7,772 fatalities related to intersections • (21% of Total Highway Fatalities) • Each year more than 3.17 million intersection crashes occur (over 55% of all reported crashes)

  4. 2008 US Total Crash Characteristics 37% 42% Source: USDOT Traffic Safety Facts 2008 Early Edition, A Compilation of motor vehicle crash data from FARS and GES, Table 29, Page 52

  5. Physical vs Functional Area of an Intersection

  6. Functional Area of an Intersection • Decision Distance • Maneuver Distance • Queue-Storage Distance

  7. Process for Prediction of Crash Frequency and Application of Crash Modification Factors Three Steps: 1. Predict Crash Frequency - Safety Performance Functions (SPF) Equations 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 • Calibration, Cr or Ci - Accounts for local conditions/data

  8. Models to Predict Crash Frequency for Rural Two-Lane Highway Intersections • Three-Approach Stop Control (Stop of Stem of Tee) • Four-Approach Stop Control (2-way Stop) • Four-Approach Signal Control

  9. SPF Models for RURAL Two-Lane Intersections with Stop Control on Minor-Road Three-Leg Stop Controlled Intersection (3ST): Nspf-3ST=exp[-9.86 + 0.79 ln(AADTmaj) + 0.49 ln(AADTmin)] Four-Leg 2-Way Stop Controlled Intersection (4ST): Nspf-4ST=exp[-8.56 + 0.60 ln(AADTmaj) + 0.61 ln(AADTmin)] AADTmaj = Avg Annual Daily Volume on Major Road (veh/day) AADTmin = Avg Annual Daily Volume on Minor Road (veh/day)

  10. SPF Models for RURAL Signalized Intersections Four-Leg Signalized Intersection (4SG): Nspf-4SG = exp[-5.13 + 0.60 ln(AADTmaj) + 0.20 ln(AADTmin)] Nspf-4SG = estimate of intersection-related predicted average crash frequency for base conditions; AADTmaj = Avg Annual Daily Volume on Major Road (veh/day) AADTmin = Avg Annual Daily Volume on Minor Road (veh/day)

  11. Base Conditions for Rural Two-Lane Intersections: • Intersection Skew Angle: 0odegrees • Presence of Left-Turn Lanes: none • Presence of Right-Turn Lanes: none • Lighting: none

  12. SPF Model for RURAL Stop Controlled Intersection– Example: 1-Way STOP on Minor Approach to a “T” Intersection (3-leg): • For a 1-Way STOP with an AADT of 5000 across the top of the “T” on the main Road and 500 AADT on the minor road of the “T”, • What is the predicted # of Crashes? Discussion

  13. SPF Model for RURAL Stop Controlled Intersection– Example: Three-Leg Stop Controlled Intersection (3ST): Nspf-3ST = exp[-9.86 + 0.79 ln(AADTmaj) + 0.49 ln(AADTmin)] For AADTmaj = 5,000 and AADTmin = 500: Nspf-3ST = exp[-9.86 + 0.79 ln(5,000) + 0.49 ln(500)] = exp[-9.86 + 6.729 + 3.045] = exp[-0.086] = 0.917 crashes per year or 4.59 crashes in a 5 year period

  14. Safety Performance Function (SPF) Highway Safety Manual Approach: “one rate” Average Crash Rate

  15. “Is this a Higher Crash Frequency Site?” Highway Safety Manual Approach: “Substantive Crash Frequency” 6 crashes/yr “Difference” “Predicted Crash Frequency” 1.2 crashes/yr 0.5 crashes/yr

  16. SPF Base Model for RURAL Signalized Intersection - Exercise 4-Approach Signalized Intersection: • For a 4-Approach signalized intersection with AADT = 9,000 on the major road and AADT = 4,500 on the minor road, • What is the predicted # of Crashes? Discussion

  17. SPF Base Model for RURAL Signalized Intersection – Example: 4-Approach Signalized Intersection: Nspf-4SG = exp[-5.13 + 0.60 ln(AADTmaj) + 0.20 ln(AADTmin)] For range of AADTmaj from zero to 25,200 and AADTmin from zero to 12,500 For AADTmaj = 9,000 and AADTmin = 4,500: Nspf-4SG = exp[-5.13 + 0.60 ln(9,000) + 0.20 ln(4,500)] = 7.5 crashes per year

  18. Severity Index for all highways and streets Severity index (SI) is the ratio of crashes involving an injury or fatality to total crashes * From NCHRP 486 • ..however, Chapter 10 of the HSM provides “better” injury and fatal crash distribution by type of rural intersection control in Tables 10-5 and 10-6

  19. Crash Severity for Rural 2-Lane Intersections Table 10-5

  20. Table 10-6: Default Distribution for Collision Types and Manner of Collisions Default Distribution of Crash Types for Rural 2-Lane Intersections

  21. Applying Severity Index to Rural Two-Lane Highway Intersections Example: Two-way stop controlled 4-approach intersection with 9,000 AADT on Major and 4,500 AADT on minor; Fatal and Injury crashes are 5 of 9 total crashes • a. Compute the actual Severity Index (SI) • SI4st = Fatal + Injury Crashes = 5/9 = 0.55 • Total Crashes

  22. Applying Severity Index to Rural Two-Lane Highway Intersections b. Compute the Predicted Severity Index (SI) SI4st = Fatal + Injury Crashes = 43.1/100= 0.43 Total Crashes

  23. Applying Severity Index to Rural Two-Lane Highway Intersections Example: Two-way stop controlled 4-approach intersection with 9,000 AADT on Major and 4,500 AADT on minor; Fatal and Injury crashes are 5 of 9 total crashes • a. Actual Severity Index (SI) = ? • b. Predicted Severity Index (SI) = ? 0.55 0.43 - Is the Actual Severity Index higher or lower than the Predicted Severity Index? Higher ?

  24. Process for Prediction of Crash Frequency and Application of Crash Modification Factors Three Steps: 1. Predict Crash Frequency - Safety Performance Functions (SPF) Equations - Predict Crash Frequency 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 • Calibration, Cr or Ci - Accounts for local conditions/data

  25. HSM Crash Modification Factors for Rural Two-Lane Highway Intersections • Configuration - Number of Legs • Intersection Designs - Roundabouts • Angle of Intersection (Skew) • Left Turn Lanes • Right Turn Lanes • Lighting

  26. Comparison of 4-leg/3-leg Intersections Potential Conflict Points Cross intersection has 32 conflict points, Offset T has 22 points

  27. Number of Intersection Legs • Crash Frequency for intersections with only 3 approaches is lower • Crash Frequency for intersections with 4 approaches are greater than for those intersections with only 3 approaches • Collision rates for intersections with more than 4 approaches are 2 to 8 times greater than for 4 approach Intersections

  28. CMF for Rural Intersection Skew Angle @ 90 degrees Skew Angle • Some studies (McCoy, for example) show adverse effect of skew • Skews increase exposure time to crashes; increase difficulty of driver view at stopped approach Intersection Angle = 350 Skew = 900 – 350 = 550 SKEW = Intersection Skew Angle (degrees) as the absolute value of the difference between 90 degrees and the actual intersection angle

  29. CMF for Intersection Skew Angle (CMF1i) For 3- legged Stop Controlled Intersections: CMF1i = exp (0.0040 SKEW) For 4- legged Stop Controlled Intersections: CMF1i = exp (0.0054 SKEW) SKEW = Intersection Skew Angle (degrees) as the absolute value of the difference between 90 degrees and the actual intersection angle *NCHRP 500, Strategy 17.1 B16 – Realign Intersection Approaches

  30. CMF for Intersection Skew Angle (CMF1i) Intersection Skew from 90 degree side road for 4-leg Approaches Skew= 0 15 30 45 CMF = 1.00 1.08 1.18 1.28 • *Max skew of 15 degrees - Older Driver Handbook and ITE • Max skew of 30 degrees – 2004 Green Book *NCHRP 500, Strategy 17.1 B16 – Realign Intersection Approaches

  31. CMF for Intersection Skew Angle (CMF1i) Example: • For each of the four (4) intersections, calculate the safety effect of skew angle Skew = 15 CMF1i = e0.0040(15) =1.062 #1 –90 deg #2 –45 deg #3 –80 deg #4 –75 deg Skew = 0 CMF1i = 1.000 Skew = 45 CMF1i = e0.0054(45) =1.275 Skew = 10 CMF1i = e0.0040(10) =1.041

  32. Solutions to Skewed Intersections NewAlignment OldAlignment *NCHRP 500, Strategy 17.1 B16 – Realign Intersection Approaches

  33. Solutions to Skewed Intersections • Locate Intersection at Mid-Point of Curve *NCHRP 500, Strategy 17.1 B16 – Realign Intersection Approaches

  34. Left Turn Lanes in the Rural Highway Environment • Left turn lanes remove stopped traffic from through lanes • mitigate rear-end conflict • enable selection of safe gap warrants for turn lanes in the rural environment see NCHRP 457 • “Capacity” is generally not the issue *NCHRP 500, Strategy 17.1 B1 – Provide Left-Turn Lanes

  35. CMF for Left Turn Lanes (CMF2i) ____ NCHRP 500, Strategy 17.1 B1 – Provide Left-Turn Lanes

  36. Rural Left Turn By-Pass Lanes • Less cost than conventional left turn lane • At low volume intersections, may be just as effective • Minnesota study unable to conclude bypass lanes just as safe as left turn lanes *NCHRP 500, Strategy 17.1 B4 – Provide By-Pass Lanes

  37. CMF for Right Turn Lanes (CMF3i) • Right turn lanes remove slowing traffic from through lanes which are not stop controlled • “Capacity” is generally not the issue *NCHRP 500, Strategy 17.1 B6 – Provide Right-Turn Lanes

  38. CMF for Right Turn Lanes (CMF3i) ____ NCHRP 500, Strategy 17.1 B6 – Provide Right Turn Lanes

  39. CMF for Lighting of Rural 2-Lane Intersections (CMF4i) CMF4i= 1-0.38pni

  40. CMF for Lighting of Rural 2-Lane Intersections (CMF4i) – Example: For 4 approach Two-Way Stop Controlled rural intersection: CMF4i= 1-0.38pni = 1-0.38(0.244) = 0.907 NCHRP 500, Strategy 17.1 E2-Improve Visibility of Intersection by Providing Lighting (P)

  41. Additional CMF’s from Part D and Research Beyond the SPF’s and CMF’s detailed in Part C Chapter 10: • CMF’s for Roundabouts from Chapter 14 • CMF for 4-Way Stop • CMF for STOP AHEAD Pavement marking • CMF for STOP Beacons • CMF for driveways within 250 feet from TTI Research

  42. Roundabouts are Alternatives to conventional intersections • Number of conflicts is reduced • Severe conflicts (angle) are eliminated • Speed differentials are reduced or eliminated *NCHRP 500, Strategy 17.2 B5 – Construct Special Solutions – Roundabout Design

  43. CMF’s for Conversion of 2-Way Stop Intersection to Roundabout

  44. Roundabouts in the rural environment *NCHRP 500, Strategy 17.1 F3 – Provide Roundabouts Before After CMF (single lane) = 0.29 CMF (multi-lane) = 0.56 Converting Stop-Control to Roundabout

  45. Roundabouts in the rural environment • Single Lane Rural Roundabout: • Approach speed limits 45 mph, • 60 foot right of way Before Crash Info – 2 yrs: - 12 crashes with 4 F/Inj After Crash Info – 2 yrs: - 4 crashes with 0 F/Inj Summit County Ohio

  46. CMF’s for Conversion of 2-Way Stop to All-Way Stop Control

  47. CMF’s for STOP AHEAD Supplementary Pavement Marking

  48. CMF’s for Beacons Table 14-42 Four approach, STOP control, Two lane roads

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