Presentation is loading. Please wait.

Presentation is loading. Please wait.

Predicting Highway Safety for Curves on Two-Lane Rural Highway - Session #4 4-1 HSM Practitioner’s Guide for Two-Lane Rural Highways Workshop.

Similar presentations


Presentation on theme: "Predicting Highway Safety for Curves on Two-Lane Rural Highway - Session #4 4-1 HSM Practitioner’s Guide for Two-Lane Rural Highways Workshop."— Presentation transcript:

1 Predicting Highway Safety for Curves on Two-Lane Rural Highway - Session #4 4-1 HSM Practitioner’s Guide for Two-Lane Rural Highways Workshop

2 Predicting Highway Safety for Curves on Two-Lane Rural Highways Learning Outcomes: ► Describe the crash prediction method for Crash Performance on Horizontal Curves ► Identify low-cost safety improvements for horizontal curves 4-2

3 ….Curves present particular safety problems to designers The risk of a reported crash is about three times greater on a curve than on a tangent CRASH RATES (Crashes per 1 km segment--3 year timeframe) Tangent segments Segments w/curve Curved portion only (Curve plus transitions) Source: Glennon, et al, 1985 study for FHWA Crash Rate 4-3

4 4-4

5 4-5

6 4-6

7 4-7

8 4-8

9 Actual Driver Operations on Curves  Drivers ‘overshoot’ the curve (track a path sharper than the radius)  Path is a spiral  Path overshoot behavior is independent of speed Source: Bonneson, NCHRP 439 and Glennon et al (FHWA) Driver tracks a ‘critical radius’ sharper than that of the curve just past the PC 4-9

10 Driver “overshoot” behavior on curves (from Glennon, et al) Example -- a 1000-ft radius curve is driven by a 95th percentile driver at about a 700 ft radius at some point in the curve 700 4-10

11 Research confirms differences in actual operations versus AASHTO assumptions Drivers’ selected speed behavior does not match design assumptions Sharper curves (<80 km/h or 50 mph) are driven faster (drivers are more comfortable) Curves driven faster than Policy assumption Curves driven slower than Policy assumption 4-11

12 Speed Prediction Model for Horizontal Curves (Otteson and Krammes) Where V 85 = 85th percentile speed on the curve D = degree of curve L = length of curve (mi) V t = 85th percentile approach speed (mph)* *this should be measured in the field V 85 = 41.62 - 1.29D + 0.0049L - 0.12DL + 0.95 V t 4-12

13 A ‘risk assessment’ tool for speed profiles V 85 - V design = V delta Higher risk curves may be those with V delta high (i.e., operating speeds significantly greater than design speed)  V delta > 12 mph (20 km/h); high risk  6 mph (10 km/h) < V delta < 12 mph (20 km/h); caution 4-13

14 FHWA’s IHSDM Speed Consistency Model Addresses Continuous Speed Behavior 4-14

15 Truck operations on curves may in some cases be critical (Harwood and Mason) Under certain conditions, trucks will roll over before they skid  Trucks with high centers of gravity overturn before losing control due to skidding  Margin of safety for ‘f’ is therefore lower for trucks Trucks on downgrade curves generate greater lateral friction (superelevation is not as effective) 4-15

16 Summary of Research on Superelevation and Transition Design Studies confirm small but significant effect of superelevation on crashes FHWA (Zegeer) study noted 5 to 10% greater crashes when superelevation is “deficient” 1987 study of fatal crash sites on curves noted “deficiencies in available superelevation” 4-16

17 Research confirms benefits of spirals and recommends optimal transition design Zegeer et al found safety benefits in HSIS study of Washington Bonneson confirmed operational benefits noted by Glennon, etal Source: NCHRP Report 439 Spirals provides e transition leading into the curve Radius (m) 4-17

18 Zegeer et al. FHWA Study “Cost-Effective Geometric Improvements for Safety Upgrading of Horizontal Curves” (1991) Data Bases 10,900 Curves in Washington State 7-state data base of 5000 mi 78 curves in New York State Glennon 4-state data base of 3277 curve segments Statistical Analysis and Model Development Identified as key effort in TRB SR 214, recent NCHRP review by BMI, and key reference for IHSDM 4-18

19 Summary of findings from Zegeer study Features related to crashes include: Degree and length of curve Width through the curve Superelevation and, Spiral presence For typical volumes on 2-lane highways, expect 1 to 3 crashes per 5 years on a curve 4-19

20 Safety Effects for Horizontal Curves (CMF 3r ) CMF 3r = 1.55 L c + (80.2/R) - 0.012 * S 1.55L c Where: L c = Length of Curve including spirals, (mi) R = Radius of Curve (ft) S = 1 if spiral transition is present, 0 if not present 4-20

21 Safety Effects of Horizontal Curves (CMF 3r ): Example with no Spiral present For:L c = 480 feet = 0.091 miles R = 350’; no spiral transition CMF 3r = {1.55 L c + (80.2/R) – 0.012S } / 1.55L c = (1.55 x 0.091) + (80.2/350) – 0.012x0 1.55x 0.091 = 2.62 4-21

22 For:L c = 480 feet = 0.091 miles R = 350’; with spiral transition CMF 3r = {1.55 L c + (80.2/R) – 0.012S } / 1.55L c = (1.55 x 0.091) + (80.2/350) – 0.012x1 1.55x 0.091 * Without spiral CMF 3r = 2.62, with spiral CMF 3r = 2.54, Difference = 8% potential for fewer crashes with a spiral transition in this segment. Safety Effects of Horizontal Curves (CMF 3r ): Example with Spiral Transition = ? = *2.54 4-22

23 Crash Modification Function for Horizontal Curves: Superelevation Example: Design e = 4%, Actual e = 2% CMF 4r = 1.06 + 3(0.02-0.02) = 1.06 + 3(0.0) = 1.06 CMF 4r is based on “Superelevation variance” or SV  For SV less than 0.01: CMF 4r = 1.00  For 0.01 < SV < 0.02: CMF 4r = 1.00 + 6(SV-0.01)  For SV > 0.02: CMF 4r = 1.06 + 3(SV-0.02) SV = 0.04 – 0.02 = 0.02 4-23

24 HSM Applications to Two-Lane Rural Highway Segments HSM Crash Prediction Method for Two- Lane Rural Highway Segments: Applying SPF and CMFs Example Problem 4-24

25 Crash Prediction for Roadway Segment for Existing Conditions – Example Calculation: Two-Lane Rural Roadway, CR 123 Anywhere, USA (MP 10.00 – 15.02) ► AADT = 3,500 vpd for the current year ► Length = 26,485 feet = 5.02 miles Lane Width = 11.0 ft Shoulder Width = 2 ft; Shoulder Type = Gravel ► Horizontal Curve on Grade (MP 12.00-12.186): L c = 0.186 miles, R = 650’; with no spiral transition Grade = 4.5% Superelevation Variance =.02 ►Tangent Section on Grade (MP 13.45-14.00): L = 0.55 miles; Grade = -6.3% 4-25

26 Crash Prediction for Roadway Segment for Existing Conditions – Example: ► Divide Two-Lane Rural Roadway into Individual Segments: SegmentLength (miles) Horizontal Curve Radius (ft) Super- elevation Variance Grade (%) Driveway Density (per mile) RHR 10.00 – 12.00 2.000TangentN/A2.0%85 *12.00 – 12.186 0.186650.024.5%05 12.186 - 13.45 1.264TangentN/A3.0%45 13.45- 14.00 0.550TangentN/A- 6.3%05 1400- 15.02 1.020TangentN/A- 3.0%65 4-26

27 Where: AADT = 3,500 vpd (current year) Length = 0.186 miles N spf-rs = (AADT n ) (L) (365) (10 -6 ) e -0.312 N spf-rs = (3,500) (0.186) (365) (10 -6 ) e -0.312 = (3,500) (0.186) (365) (10 -6 ) (0.7320) = 0.17 crashes per year Safety Performance Function (SPF) for Base Conditions: Example Calculation Segment 2 (MP 12.00-12.186): Horizontal Curve on a 4.5% Grade 4-27

28 CMF for Lane Width (CMF 1r ): Calculation ►Adjustment for lane width and shoulder width related crashes (Run off Road + Head-on + Sideswipes) to obtain total crashes using default value for p ra = 0.574 Segment 2: 11 foot wide lane: CMF 1r = (CMF ra - 1.0) p ra + 1.0 = (1.05 - 1.0) * 0.574 + 1.0 = (0.05) (0.574) + 1.0 = 1.03 From Table 10-8: CMF ra = 1.05 4-28

29 CMF or Shoulder Width and Type (CMF 2r ): Calculation ►Adjustment from crashes related to lane and shoulder width (Run off Road + Head-on + Sideswipes) to total crashes using default value for p ra = 0.574 Segment 2: 2 ft wide gravel shoulder: CMF 2r = (CMF wra CMF tra - 1.0) p ra + 1.0 = ((1.30)(1.01) - 1.0) * 0.574 + 1.0 = (0.313) (0.574) + 1.0 = 1.18 CMF wra = 1.30 (Table10-9) and CMF tra = 1.01 (Table10-10) 4-29

30 CMF for Horizontal Curve (CMF 3r ): Calculation For:L c = 0.186 miles R = 650’; with no spiral transition CMF 3r = {1.55 L c + (80.2/R) – 0.012S } / 1.55L c = (1.55 x 0.186) + (80.2/650) – 0.012x0 1.55x 0.186 = 1.43 Segment 2: Horizontal Curve 4-30

31 CMF for Superelevation on Horizontal Curves (CMF 4r ) CMF 4r = 1.06 + 3(0.02-0.02) = 1.06 + 3(0.0) = 1.06 ►For SV > 0.02: CMF 4r = 1.06 + 3(SV-0.02) Segment 2: Horizontal Curve Superelevation Variance = 0.02 4-31

32 CMF for Percent (%) Grade on Roadway Segments (CMF 5r ) Segment 2: 4.5% Grade CMF 5r = 1.10 4-32

33 CMF Roadside Design (CMF10r): Example Calculation Segment 2: RHR = 5 = 1.14 CMF 10r = e (-0.6869 + (0.0668xRHR)) /e -0.4865 = e (-0.6869 + (0.0668x5)) /e -0.4865 4-33

34 Applying CMFs to the SPF Base Prediction Model Segment 2: SPF and CMF Values: AADT = 3,500 vpd, Length = 0.186 mi Radius = 650 ft N spf-rs = 0.17 crashes per year CMF total = 2.31 CRASH MODIFCATION FACTORS Lane Width = 11 ftCMF 1r = 1.03 Shoulder Width = 2 ft gravelCMF 2r = 1.18 Horizontal CurveCMF 3r = 1.43 Superelevation Variance (0.02)CMF 4r = 1.06 Percent Grade = 4.5%CMF 5r = 1.10 Driveway Density, NoneCMF 6r = 1.00 Centerline Rumble, NoneCMF 7r = 1.00 Passing/Climbing Lanes, NoneCMF 8r = 1.00 TWLTLs, NoneCMF 9r = 1.00 Roadside Design, RHR = 5CMF 10r = 1.14 Lighting, NoneCMF 11r = 1.00 Automated Enforcement, NoneCMF 12r = 1.00 4-34

35 N predicted-rs = N spf-rs x (CMF 1r … CMF 12r ) C r Applying CMFs to the SPF Base Prediction Model 0.17 x (1.03 x 1.18 x 1.43 x 1.06 x 1.10 x 1.00 x 1.00 x 1.00 x 1.00 x 1.14 x 1.000 x 1.00) x 1.00 N predicted-rs = = 0.17 x 2.31 x 1.00 = 0.4 crashes per year, 1 crash every 2.5 yrs Segment 2: Apply CMFs to SPF for Base Conditions: (letting C r = 1.0) 4-35

36 Crash Prediction for Roadway Segment for Existing Conditions – Example Calculation: For each Two-Lane Rural Roadway Segment: Table with SPF predicted crahses, CMFs, and Adjusted Total Crashes CRASH PREDICTION METHOD – TOTAL CRASHES Seg No. SPF base CMF 1r LW CMF 2r SW&ST CMF 3r ST CMF 4r e CMF 5r Grade CMF 6r DD CMF 7r CLRS CMF 8r PassLn CMF 9r TWLTL CMF 10 r RD CMF 11r Light CMF 12 r Spd Enf Total CMF Total Adjusted Crashes 11.871.031.181.00 1.071.00 1.141.00 1.492.8 20.171.031.181.431.061.101.00 1.141.00 2.310.40 31.271.031.181.00 1.141.00 1.391.8 40.511.031.181.00 1.161.00 1.141.00 1.610.8 50.951.031.181.00 1.021.00 1.141.00 1.421.4 Total:7.2 4-36

37 Predicting Crash Frequency Performance N total crashes = ∑N predicted-rs + ∑ N predicted-int Total Predicted Crash Frequency within the limits of the roadway being analyzed: N total crashes = 7.2 crashes/yr + ∑ N predicted-int 4-37

38 Overview of Good Alignment Design Practice (suggested by safety and operational research) ►Curves and grades are necessary features of alignment design (reflect the topography, terrain, and “context”) ►Pay particular attention to roadside design adjacent to curves ►Avoid long, sharp curves ►Adjust alignment design to reflect expected speeds on curves 4-38

39 Overview of Good Alignment Design Practice (continued) ►Avoid minimum radius designs where  actual speeds will be higher than design speeds  truck volumes will be substantial  combined with steep grades ►Use spiral transition curves, particularly for higher speed roads and sharper curves 4-39

40 Overview of Good Alignment Design Practice (continued) ► Minimize grades within terrain context ► Widen lanes and shoulders through curves ► Pay attention to access points related to horizontal and vertical curve locations 4-40

41 Low and Lower Cost Safety Improvements for Horizontal Curves ► Signing► Shoulders ► Lighting 4-41

42 Low Cost Intersection Safety Measures – Signing Countermeasures Injury Crashes CMF = 0.87 CRF = 13% PDO Crashes CMF = 0.71 CRF = 29% Advance Warning With Speed Advisory *CMF Clearinghouse http://www.cmfclearinghouse.org 4-42

43 Safety Effects of Installing Combination Horizontal Alignment Warning + Advisory Speed Signs 4-43

44 Signing Countermeasure for Horizontal Curves: *CRF = 35% Chevrons Signs *CMF Clearinghouse http://www.cmfclearinghouse.org CMF = 0.65 4-44

45 Safety Effects of Installing RPM’s 4-45

46 Low Cost Intersection Safety Measures – Signing Countermeasures Double Up Advance Warning Signs CRF = 31% CMF = 0.69 4-46

47 Low Cost Intersection Safety Measures – Signing Countermeasures  Radar activated flasher when speed is fast for 10mph curve Sharp 10 mph curve to right just over hill Activated Warning Beacon 4-47

48 Examples of Improving Safety of Existing Curves Widen 2’ Shoulder to 6’ Shoulder – NY Rte 82 north of Millbrook 6’ 2’ Widen Shoulders 4-48

49 Examples of Improving Safety of Existing Curves Widening on Inside of Curves NCHRP 500, Strategy 15.2 A11– Widening in Curves Widen Shoulder on Inside of Tight Curve 4-49

50 Route 376 near Poughkeepsie, NY Low Cost Intersection Safety Measures – Signing Countermeasures CRF = 28% for injury crashes highway lighting 9. Illumination of Rural Curves 4-50

51 Predicting Highway Safety for Curves on Two-Lane Rural Highways Learning Outcomes: ► Described the equation for prediction of Crash Performance on Horizontal Curves ►Identified low-cost safety improvements for horizontal curves 4-51

52 Questions and Discussion: 4-52


Download ppt "Predicting Highway Safety for Curves on Two-Lane Rural Highway - Session #4 4-1 HSM Practitioner’s Guide for Two-Lane Rural Highways Workshop."

Similar presentations


Ads by Google