HSM Applications to Rural Multilane Intersections

1. HSM Applications to Rural Multilane Intersections - Session #7 Prediction of Crash Frequency and Application of CMFs for Rural Multilane Intersections

2. Predicting Crash Frequency and Application of CMFS for Rural Multilane Intersections • Describe the models to Predict Crash Frequency for Rural Multilane Intersections • Calculate Predicted Crash Frequency for Rural Multilane Intersections • Describe CMFs for Rural Multilane Intersections • Apply CMFs to Crash Frequency for Rural Multilane Intersections Learning Outcomes:

3. 2008 US Total Intersection Crash Characteristics 55% 57%

4. Expressway Two-Way Stop Control

5. Safety Performance of Rural Multilane Expressway Intersections *NCHRP 650

6. Safety Performance of Rural Multilane Expressway Intersections *NCHRP 650

7. Safety Performance of Rural Multilane Expressway Intersections *NCHRP 650

8. Safety Performance of Rural Multilane Expressway Intersections 1. 87% of the right-angle crashes were due to the inability of minor road drivers to recognize oncoming expressway traffic and/or select safe gaps in the expressway traffic stream; 2. 78% of the right-angle crashes were “far-side” collisions [i.e., right-angle crashes involving left-turning or crossing minor road vehicles that successfully cross the first (near-side) set of expressway lanes, but collide with expressway traffic in the second (far-side) set of lanes after traversing through the median (the concept of near and far-side intersections is illustrated in Figure 8) *NCHRP 650

9. Safety Performance of Rural Multilane Expressway Intersections 3. Intersection recognition (i.e., running of the STOP sign) by drivers on the minor, stop-controlled approaches was not a contributing factor in any of the right-angle crashes at these intersections. Similarly, Burchett and Maze (14) found that the ratio of far-side to near-side collisions at 30 TWSC rural expressway intersections with the highest crash severity indices in Iowa was 62% to 38%; however, at 7 of these intersections where horizontal curves were present along the expressway, far-side and near-side collisions were nearly equally distributed at 51% and 49%, respectively. Therefore, horizontal curves on the mainline seem to create a unique hazard for minor road *NCHRP 650

10. Safety Performance of Rural Multilane Expressway Intersections *NCHRP 650

11. Defining Rural Multilane Highways • Methodology applies to four-lane undivided and divided rural highways. • “Rural”: • Defined per AASHTO (2004) Guidelines • Places outside the boundaries of urban places where the population is less than 5,000 inhabitants. • Any highway located outside the city limits of an urban agglomeration above 5,000 inhabitants is considered rural. • The boundary delimitating rural and urban areas can at times be difficult to determine, especially since most multilane rural highways are located on the outskirts of urban agglomerations.

12. Defining Multilane Highways • Multilane Facilities: • Have four through lanes. • May be divided with a rigid or flexible barrier, paved or landscaped median • Should not have access and egress limited by grade-separated interchanges (i.e., not freeways). • May have occasional grade-separated interchanges, but these should not be the primary form of access and egress

13. Limitations of Methodology • Methodology incorporates the effects on safety of many -but not all- geometric and traffic control features. • Only includes geometric design elements: • whose relationship to safety are well understood • Associated data is available for • The Statistical Model: • treats the effects of individual geometric design element and traffic control features as independent of each other • Ignores any potential interactions between them.

14. Predicting Crash Frequency of Rural Multilane Highways Separate Safety Performance Functions (SPFs) for: • Homogeneous highway segments • Intersections • Sum of Individual Intersection Calculations

15. Definition of Segments and Intersections A - All crashes that occur within this region are classified as intersection crashes B – Crashes in this region may be segment or intersection related, depending on the characteristics of the crash

16. Definition of Intersections: “the general area where two or more roadways join or cross, including the roadway and roadside facilities for traffic movements within the area.” Intersections may be: • signalized, • stop controlled, and • roundabouts

17. Definition of Intersections: • An at-grade intersection is defined by both: • its physical and • functional areas”

18. Definition of an Intersection The functional area “extends both upstream and downstream from the physical intersection area and includes any auxiliary lanes and their associated channelization.” • the functional area on each approach to an intersection consists of three basic elements: • Decision distance; • Maneuver distance; and, • Queue-storage distance.

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

20. Predicting Crash Frequency for Rural Multilane At-Grade Intersections Procedure for safety prediction for At- Grade Intersections: 1stApply SPF Model for base conditions, 2ndApply CMFs and calibration factor Nspf int = exp(a + b ln(AADTmaj) + c ln(AADTmin)) Npredicted int = Nspf int x(CMF1i x CMF2i x …. CMFni)Ci

21. Predicting Crash Frequency for Rural Multilane At-Grade Intersections SPF Models and Adjustment Factors addresses three types of Intersections: Three-leg intersections with STOP control on the minor road approach (3ST) Four-leg intersections with STOP control on the minor-road approaches (4ST) Four-leg signalized intersection (4SG) • Used for both divided and undivided rural four-lane highways

22. Predicting Crash Frequency for Rural Multilane At-Grade Intersections SPF Model for Rural Multilane Intersections (Applies to BOTH Divided and Undivided): Where: • Nspf int = expected number of intersection-related crashes per year for base conditions • AADTmaj = average daily traffic volume for the major road (vpd) • AADTmin = average daily traffic volume for the minor road (vpd) • a, b, and c = regression coefficients from Table 11-7 Nspf int = exp(a + b ln(AADTmaj) + c ln(AADTmin))

23. Base Conditions for Rural Multilane Intersections Stop-Controlled Intersections: • Intersection Skew Angle: 0odegrees • Presence of Left-Turn Lanes: none • Presence of Right-Turn Lanes: none • Lighting: none

24. Predicting Crash Frequency for Rural Multilane Stop-Controlled Intersections Nspf int = exp(a + b ln(AADTmaj) + c ln(AADTmin))

25. Predicting Crash Frequency for Rural Multilane Signalized Intersections Nspf int = exp(a + b ln(AADTmaj) + c ln(AADTmin))

26. Safety Prediction for a Rural Multilane Intersection: EXAMPLE • Four-Leg Stop-Controlled Intersection: • 10,000 AADT and 2,500 AADT • From Table 11-7: a = -10.008, b=0.848, c=0.448 Nspf int = exp(a + b ln(AADTmaj) + c ln(AADTmin)) N = exp(-10.008 +0.848ln(10,000)) + 0.448ln(2,500) = exp(-10.008 + 7.810 + 3.505) = exp(1.3075) = 3.7 crashes per year (base conditions)

27. Severity of Rural Multilane Intersections: • For example, for Rural 4-approach Signalized intersection with AADTs of 37,000 and 16,100: • Predicted Total Crashes = 39.52 crashes/yr • Predicted Injury + Fatal Crashes = 13.0 crashes/yr • Predicted Severity Index = 13.0/ 39.52 = 32.9%

28. Predicting Crash Frequency for Rural Multilane At-Grade Intersections Procedure for safety prediction for At- Grade Intersections: 1stApply SPF Model for base conditions, 2ndApply CMFs and calibration factor Nspf int = exp(a + b ln(AADTmaj) + c ln(AADTmin)) NEXT: Npredicted int = Nspf int x(CMF1i x CMF2i x …. CMFni)Ci

29. CMF’s for Rural Multilane Intersections

30. Effect of Angle or Skew Skew Angle @ 90 degrees • studies show adverse effect of skew • Skews increase exposure time to crashes; increase difficulty of driver view at stopped approach • SKEW = Intersection Angle (degrees) as difference (absolute value) between 90 degrees and actual intersection angle

31. Rural Multilane Intersection CMF for Intersection Skew Angle 3- legged Intersections (Stop-Control) on Minor Approach: CMF1i = 1 + (0.016 x Skew) (11-18) (0.98 + 0.16 x Skew) 4- legged Intersections (Stop –Control) on Minor Approach: CMF1i = 1 + (0.053 x Skew) (11-19) (1.43 + 0.53 x Skew) • CMF1i = CMF for the effect of intersection skew on total crashes • SKEW = Intersection Angle (degrees) as difference (absolute value) between 90 degrees and actual intersection angle

32. Safety Prediction for Intersection Skew Angle at a Rural Multilane Intersection: EXAMPLE • 3-Leg Stop-Controlled Intersection: • Skew Angle = 35 degrees CMF1i = 1 + [(0.016 x Skew)/(0.98 + 0.16 x Skew)] CMF1i = 1 + [(0.016)(35)/(0.98 + (0.16)(35) = 1 + (0.56/6.58) = 1 + 0.0851 = 1.085

33. Left Turn Lanes for Multilane Highways • Left turn lanes remove stopped traffic from through lanes • mitigate rear-end conflict • enable selection of safe gap • “Capacity” is generally not the issue *NCHRP 500, Strategy 17.1 B1 – Provide Left-Turn Lanes

34. CMF2i for Left-Turn Lanes at Rural Multilane Intersections:

35. CMF3i for Right-Turn Lanes at Rural Multilane Intersections:

36. CMF4i for Lighting at Rural Multilane Intersections: CMF4i = 1 - 0.38Pni Where: CMF4i = CMF for the effect of lighting on total crashes Pni = proportion of total crashes for unlighted intersections that occur at night

37. CMF4i for Lighting at Rural Multilane Intersections: Replace default values with local values if available *Note the lack of a value for 4SG signalized intersection

38. CMF4i for Lighting at a Rural 2-Way Stop Multilane Intersection: EXAMPLE CMF4i = 1 - 0.38Pni CMF4i = 1 - 0.38 x 0.273 = 1 – 0.10374 = 0.896

39. Safety Prediction for a Rural Multilane Intersection: Example • 4-Leg Rural Unsignalized Intersection: • 10,000 AADT and 2,500 AADT, • 35 Deg Skew, • left-turn lane • Right-turn lane on major road, • lighting NPredicted int = Nspf int (CMF1i x CMF2i x …. CMFni) Ci

40. Safety Prediction for a Rural Multilane Intersection: Example • 4-Leg Rural Unsignalized Intersection: • 10,000 AADT and 2,500 AADT, • 35 Deg Skew, left-turn lanes + right-turn lanes on major road, lighting Nspf int = 3.70 CMF2i(lt-trn) = ? 0.520 CMF1i(skew) = ? 1.092 CMF4i(lighting)= ? 0.896 CMF3i(rt-trn) = ? 0.740 NPredicted int = Nspf int (CMF1i x CMF2i x …. CMFni) Ci = 3.70 x (1.092 x 0.52 x 0.74 x 0.896) = 1.39

41. CMF’s for Rural Multilane Intersections • There are no CMF’s in the 1st edition of the HSM for Signalized Rural Multilane Intersections included in Chapter 11 of Part C

42. Rural Multilane Intersections • Additional CMF’s: • Increase median opening width • Convert minor road Stop control to All-Way Stop • Convert Stop Control to Signal Control

43. CMFs for Increasing Median Opening Width

44. CMFs for Increasing Median Opening Width

45. CMFs for Increasing Median Opening Width

46. CMFs for Converting Minor-Road Stop Control to All-Way Stop Control:

47. CMFs for Converting Stop Control to Traffic Signal Control:

48. Rural Multilane Intersections • Additional CMF’s beyond the HSM 1st Edition: • Positive Offset Left Turn Lanes • Access Density • “J” Turns July 2010

49. Rural Multilane Intersections • Additional CMF’s beyond the HSM 1st Edition: • Positive Offset Left Turn Lanes NCHRP 650: Before and After studies of crashes identified up to a 100% reduction in left turn crashes; one study found an increase in rear-end crashes; typical crash reduction in left turn crashes is 70%.

50. Offset Left-Turn Lane Geometry