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

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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

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….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

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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

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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

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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

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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 = D L DL V t 4-12

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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

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

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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

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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

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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

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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

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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

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Safety Effects for Horizontal Curves (CMF 3r ) CMF 3r = 1.55 L c + (80.2/R) * 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

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Safety Effects of Horizontal Curves (CMF 3r ): Example with no Spiral present For:L c = 480 feet = 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 =

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For:L c = 480 feet = 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 * 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 = ? = *

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Crash Modification Function for Horizontal Curves: Superelevation Example: Design e = 4%, Actual e = 2% CMF 4r = ( ) = (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 = (SV-0.01) For SV > 0.02: CMF 4r = (SV-0.02) SV = 0.04 – 0.02 =

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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

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Crash Prediction for Roadway Segment for Existing Conditions – Example Calculation: Two-Lane Rural Roadway, CR 123 Anywhere, USA (MP – 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 ): L c = miles, R = 650’; with no spiral transition Grade = 4.5% Superelevation Variance =.02 ►Tangent Section on Grade (MP ): L = 0.55 miles; Grade = -6.3% 4-25

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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 – TangentN/A2.0%85 *12.00 – % TangentN/A3.0% TangentN/A- 6.3% TangentN/A- 3.0%

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Where: AADT = 3,500 vpd (current year) Length = miles N spf-rs = (AADT n ) (L) (365) (10 -6 ) e N spf-rs = (3,500) (0.186) (365) (10 -6 ) e = (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 ): Horizontal Curve on a 4.5% Grade 4-27

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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 = Segment 2: 11 foot wide lane: CMF 1r = (CMF ra - 1.0) p ra = ( ) * = (0.05) (0.574) = 1.03 From Table 10-8: CMF ra =

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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 = Segment 2: 2 ft wide gravel shoulder: CMF 2r = (CMF wra CMF tra - 1.0) p ra = ((1.30)(1.01) - 1.0) * = (0.313) (0.574) = 1.18 CMF wra = 1.30 (Table10-9) and CMF tra = 1.01 (Table10-10) 4-29

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CMF for Horizontal Curve (CMF 3r ): Calculation For:L c = 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 = 1.43 Segment 2: Horizontal Curve 4-30

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

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

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CMF Roadside Design (CMF10r): Example Calculation Segment 2: RHR = 5 = 1.14 CMF 10r = e ( (0.0668xRHR)) /e = e ( (0.0668x5)) /e

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Applying CMFs to the SPF Base Prediction Model Segment 2: SPF and CMF Values: AADT = 3,500 vpd, Length = 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 =

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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 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

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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 Total:

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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

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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

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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

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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

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

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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 4-42

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

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Signing Countermeasure for Horizontal Curves: *CRF = 35% Chevrons Signs *CMF Clearinghouse CMF =

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Safety Effects of Installing RPM’s 4-45

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

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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

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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

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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

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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

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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

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Questions and Discussion: 4-52

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