Design of Interchange Loop Ramps and Pavement/Shoulder Cross-Slope Breaks (2017) / Chapter Skim
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Part 1: Design Guidance for Interchange Loop Ramps
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1 - ii ACKNOWLEDGMENT The research reported herein was performed under NCHRP Project 3-105, "Design Guidance for Interchange Loop Ramps" as Phases I and II. This report was prepared by Dr.
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1 - iii Abstract The objective of this research was to develop improved design guidance for interchange loop ramps. An observational field study was conducted to investigate the relationship between speed and lane position of vehicles and key design elements of the ramp proper, and the difference in performance on the ramp proper between single-lane and multi-lane loop ramps.
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1 - iv Part 1 Contents ACKNOWLEDGEMENT .............................................................................................................. ii Abstract .................................................................................................................................
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1 - v 6.1 Conclusions .......................................................................................................100 6.2 Recommendations for Future Research ............................................................103 Section 7. References .................................................................................................................105 Appendices Appendix A -- Sites Included in Observational Field Study of Loop Ramps Appendix B -- Recommended Changes for Consideration in the Next Edition of the Green Book
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1 - vi Figures Figure 1. General Types of Ramps ..................................................................................................2 Figure 2.
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1 - vii Tables Table 1. Guide Values for Ramp Design Speed as Related to Highway Design Speed ................11 Table 2.
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1 - viii Table 32. Descriptive Statistics for California Urban Exit Ramps ................................................75 Table 33.
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1 - ix Summary Interchange projects are among the most complex and expensive projects constructed by highway agencies. Interchanges are comprised of individual ramps.
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1 - x lane width and shoulder widths on speeds, for a given incremental increase in width (e.g., 1 ft) , shoulder widths have a greater influence on speeds than travel lane widths.
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1 - xi Key roadway and cross-sectional design elements that significantly influence lane position at the midpoint of the controlling curve include lane width and type of freeway mainline ramp terminal. As lane width increases, vehicles tend to move farther away from the inside lane line (which is expected)
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1 - xii Based upon speeds and lane positions of vehicles operating in the outside lane of multilane loop ramps, no special design considerations are necessary for the design of multilane loop ramps to accommodate large differentials in speeds of vehicles traveling in the outside lane compared to the inside lane or to accommodate vehicles in the outside lane that significantly gravitate to the inside and encroach on the inside travel lane. Outside lane widths of 12-ft for multi-lane entrance loop ramps and 14-ft for multi-lane exit loop ramps are sufficient to accommodate traffic comprised primarily of passenger vehicles.
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1 - 1 Section 1. Introduction 1.1 Background Interchange projects are typically among the most complex and expensive projects constructed by highway agencies.
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Figure 1. General Typ 1 - 2 es of Ramps (AASHTO, 2011)
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1 - 3 Full Cloverleaf Parclo-A Parclo-B Parclo-AB Parclo-A/4 Quad Parclo-B/4 Quad Figure 2. Expanded Interchange Configurations with Loop Ramps
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1 - 4 Parclo-AB/4 Quad Full Diamond With One Loop Directional With Two Loops Figure 2. Expanded Interchange Configurations with Loop Ramps (Continued)
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1 - 5 The relationship between speed and lane position of vehicles and key design elements of the ramp proper The impact of key design elements on safety of loop ramps The difference in performance on the ramp proper of single-lane and multi-lane loop ramps While the findings and recommendations of this research are generally applicable to loop ramps on both service interchanges (i.e., interchanges of a freeway with a surface street [arterial or collector]
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1 - 6 Section 2. Summary of Literature Review This section summarizes the state of the art and state of the practice in loop ramp design based on a review of the literature including research studies, guidance documents, and design manuals.
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1 - 7 2.1.2 Horizontal Alignment (Green Book Section 3.3) The objective in designing a horizontal curve is to balance the forces acting on a vehicle for safe and comfortable operation at speeds appropriate for the general conditions of the roadway.
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1 - 8 speed increases, lower design side friction factors are specified. Maximum side friction factors range from 0.38 for a design speed of 10 mph to 0.08 for a design speed of 80 mph.
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1 - 9 Compound curves can create desirable turning roadway shapes for interchange ramps. When the design speed of the turning roadway is 45 mph or less, compound curvature can be used to form the entire alignment of the turning roadway.
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1 - 10 Recommended roadway widths are further defined based upon traffic conditions as follows: Traffic Condition A consists predominantly of passenger vehicles, but some consideration is also given to single-unit trucks (e.g., SU-30)
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1 - 11 values in Green Book Table 10-1 for freeway design speeds of 70 mph require a loop ramp design speed of 35 mph. Values in Green Book Table 10-1 apply to the sharpest, or controlling, ramp curve, usually on the ramp proper.
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recomme and shou When an by weavi suggested interchan radii and ramp is d Fi A speedchange in lanes are at a flat a 10-3 (Tab terminals Table 3) grades ar Book Tab ndations are ld be checke entrance ram ng consider in Green B ges, the dist roadway an esirable.
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1 - 13 Table 2. Minimum Acceleration Lengths for Entrance Terminals with Flat Grades of Two Percent or Less (AASHTO, 2011)
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1 - 14 2.2 ITE Freeway and Interchange Geometric Design Handbook The following discussion details the design guidance most applicable to loop ramps in the ITE Freeway and Interchange Geometric Design Handbook (Leisch, 2006)
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2.3 Hi In Nation (2012) de methodo volume e Method f models fo Maine.
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1 - 16 The crash prediction models use a SPF in the following form to account for the effect of ramp traffic volume on safety: N = Lr × exp [a + b× ln(c AADTr)
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1 - 17 Unlike previous studies, Bonneson et al. did not model loop ramps differently from other ramp configurations but developed a detailed methodology accounting for the effect of ramp curvature on crash frequency.
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1 - 18 Level of service criterion for freeway mainline ramp terminals (both entrances and exits) is defined in terms of density.
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1 - 19 Table 7. Summary of State Policies Related to Design Speed, Curve Radius, and Widths for Loop Ramps State Design speed (DS)
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1 - 20 In general, there is no difference between crash rates for trucks and crash rates for all vehicles at either entrance or exit ramps. The exit ramp configurations where truck crash rates exceed the overall crash rates by the greatest amount include parclo loop, free-flow loop, and "other" ramp configurations (Torbic et al., 2012)
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1 - 21 highways. In addition, the speed prediction models only incorporate a few key geometric features including curve length and radius and type of crossroad terminal, so a better understanding of how key geometric design features along the ramp proper of a loop ramp influence vehicle speeds is desirable.
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Secti Obse This sect the ramp designed ramps, na elements single-lan 3.1 St Observat 13 exit ra in urban perspecti included Figur on 3. rvation ion describe proper of si to address t mely: (a)
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1 - 23 Site characteristic data for the ramps were collected through a combination of reviewing plans, profile sheets, and aerial images, while other data were collected in the field or estimated based on available information. The following site characteristics were collected for the ramp proper: Area type (urban, rural)
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1 - 24 Recall that the Green Book states that for highway design speeds above 50 mph, the loop ramp design speed should preferably be no less than 25 mph. Of the 28 ramps included in the observational study, one loop ramp (Ramp ID 27)
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1 - 25 Table 8. General Characteristics of Study Locations (Entrance Ramps)
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1 - 26 Table 9. General Characteristics of Study Locations (Exit Ramps)
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F igure 6. Crossroad Ramp Termin 1 - 27 al Types/C lassifications (AASHT O, 2014)
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1 - 28 Table 10. Characteristics of Ramp Proper (Entrance Ramps)
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1 - 29 Table 11. Characteristics of Ramp Proper (Exit Ramps)
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1 - 30 3.2 Field Data Collection Procedures The general field data collection procedures conducted along the ramp proper of freeway entrance and exit loop ramps are described below. For entrance loop ramps, the primary measures of interest collected in the field included: Speeds of vehicles at the midpoint of the controlling curve on the ramp Speeds of vehicles at the end [i.e., point of tangency (PT)
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the opera radio to t vehicle b when it c vehicle a for an ex passenge unit and t At the m inches ap A video r reference of the veh A video r decelerat investiga shoulder tor of the fir he operator eing tracked ame into vie t the midpoi it ramp of th r vehicles (i ractor-semi Figure 7. idpoint of th art in the tra ecorder was markers.
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1 - 32 this was assessed to be the most likely portion of the ramp where critical driving behavior would occur. Videos from the recorders were viewed in the office for post-processing to record lane position data and unusual or critical driving behavior.
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1 - 33 (passenger side) tire that was closest to the inside lane line (when the vehicle was completely positioned within the travel lane)
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F igure 8. Sample Images from Vide Single 1 - 34 o to Docum -Lane Ram ent Vehicl p e Lane Position on a
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1 - 35 3.3.3 Exiting Maneuver Data Video recordings were viewed in the office to document unusual or critical behaviors (e.g., swerving, severe braking) of exiting vehicles that occurred along the first several hundred feet of the ramp proper of the exit loop ramp.
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Figure 9. Sample Images from Video on E 1 - 36 to Documen xit Ramps t Unusual or Critical Maneuvers
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1 - 37 3.4 Analysis of Observational Data This section describes the analysis approach and presents descriptive statistics and the analysis results. Three types of observational data from the field studies were analyzed: Vehicle speed Lane position Exiting maneuver 3.4.1 Vehicle Speed The objective of this analysis was to develop speed prediction models to assess the effects of key design elements on vehicle speeds on loop ramps.
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1 - 38 of issues of collinearity and confounding. Inclusion of nonrelevant or highly correlated independent variables may render other variables in the model statistically non-significant or lead to counterintuitive signs for some of the estimated model coefficients.
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1 - 39 for maximum superelevation rates ranging from 4 to 12 percent based on Method 5. Corresponding Green Book Tables 3-8 to 3-12 show minimum values of curve radii for various combinations of superelevation and design speeds for maximum superelevation rates ranging from 4 to 12 percent for a full range of common design conditions.
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1 - 40 Table 12. Descriptive Statistics of Vehicle Speeds on Ramp Proper (Entrance Ramps)
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1 - 41 Table 13. Descriptive Statistics of Vehicle Speeds on Ramp Proper (Exit Ramps)
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1 - 42 Table 14. Summary Statistics for Speed Data Entrance ramps Exit ramps Variable Min Max Avg Std Dev Min Max Avg Std Dev Speed 1 10 46 27.60 5.85 15 69 35.78 6.98 Speed 2 11 47 29.69 5.84 12 47 29.06 5.28 Speed 2- Speed 1 -12 15 2.09 3.49 -34 11 -6.82 5.92 Site characteristics (continuous variables)
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3.4.1.2 This sect elements entrance further ex speeds ba the study estimated Effect of Before de speeds, it which va correlatio F Entrance After rev goodness the ramp data for s normal" Analysis R ion first pre of loop ram and exit ram plained. Fin sed on regr locations to and predict Key Design veloping re is importan riables shou n among va igure 10.
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1 - 44 correlations of the coefficient for lane width (CC_LW) and two other predictors.
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1 - 45 resulting model using data for single-lane and multi-lane ramps combined. Interestingly, only the radius (CCRadius)
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1 - 46 speeds of passenger vehicles. Vehicles speeds at the midpoint of the controlling curve on an exit loop ramp designed with a simple curve are estimated to be approximately 3.6 mph faster than on loop ramps designed with a compound curve, and the type of speed-change lane significantly impacts vehicle speeds at the midpoint of the controlling curve on an exit loop ramp.
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1 - 47 Table 18. Speed Prediction Model to Estimate Speed at the Beginning of the Controlling Curve on the Ramp Proper of an Exit Loop Ramp Speed Prediction Procedures Incorporated in ISATe ISATe (Bonneson et al., 2010)
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1 - 48 Step 2 - Compute Limiting Curve Speed: The limiting curve speed is computed for each curve on the ramp using Equation 2. vmax,i = 3.24 (32.2 Ri)
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1 - 49 Step 7 - Calculate Speed on Successive Curves: The entry and exit speeds for curve 3 and successive curves are computed by applying Steps 5 and 6 for each curve. Exit Ramp Procedure The procedure for exit ramps involves seven steps.
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1 - 50 Comparison of Estimated Speeds Output by ISATe, Predicted Speeds from Regression Models, and Measured Speeds In this section, the estimated speeds output by ISATe, the predicted speeds based on regression models developed as part of this research, and the measured speeds at the study locations are compared to gain a better understanding of the accuracy and reliability of the speed prediction procedures incorporated in ISATe and that of the regression models developed as part of this research (results shown in Table 15 through Table 18)
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1 - 51 The ISATe speed prediction procedure for entrance ramps resulted in the same estimated speeds for the midpoint and PT of the controlling curve. The ISATe speed prediction procedure for entrance ramps provides reasonably accurate estimates of speeds at the midpoint and PT of the controlling curves.
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1 - 52 Table 20. Comparison of Speeds Estimated by ISATe, Predicted by Model, and Measured in Field (Entrance Ramps)
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1 - 53 Table 21. Comparison of Speeds Estimated by ISATe, Predicted by Model, and Measured in Field (Exit Ramps)
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1 - 54 3.4.1.3 Key Findings In summary the key findings from analyses of the speed data for entrance and exit loop ramps are as follows: General findings - The design speeds of the loop ramps were calculated using Method 5 and Method 2 of the Green Book for distributing superelevation and side friction. At all study locations, the estimated design speeds using Method 5 are 10 to 15 mph lower than the estimated design speeds using Method 2, and the observed speeds are higher than the estimated design speeds using Method 5 and are much closer to the estimated design speeds using Method 2.
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1 - 55 - The ISATe speed prediction procedure for exit ramps is not very accurate in estimating the speeds of vehicles at the beginning of the controlling curve, at least for ramps where the distance from the gore point to the controlling curve is relatively short. - The predicted speeds at the PC of the controlling curve of an exit loop ramp based on regressions models developed as part of this research are considerably more accurate than the estimated speeds output from ISATe.
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1 - 56 Ramp length Design speed of controlling curve Radius of controlling curve Superelevation of controlling curve Length of controlling curve Inside shoulder width of controlling curve Lane width of controlling curve Outside shoulder width of controlling curve Freeway speed limit Crossroad speed limit Vertical profile (grade, up or down) Type of speed-change lane Type of horizontal curvature (simple or compound curve)
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1 - 57 Table 22. Descriptive Lane Position Statistics on Ramp Proper (Entrance Ramps)
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1 - 58 Table 23. Descriptive Lane Position Statistics on Ramp Proper (Exit Ramps)
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1 - 59 Table 24. Summary Statistics for Lane Position Data Entrance ramps Exit ramps Variable Min Max Avg Std Dev Min Max Avg Std Dev Lane position (in)
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3.4.2.2 This sect loop ram effect of assessed represent the corre Figu Entrance After rev goodness of the con some cor freedom, coefficien percent, h Analysis R ion presents ps on lane p key design e to identify w ation of the lation.
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1 - 61 Table 25. Lane Position Model for Entrance Ramps This lane position prediction model can be interpreted as follows.
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1 - 62 (p-value of 0.1193)
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1 - 63 - Vehicles in the outside lane of a multi-lane loop ramp are positioned approximately 10 inches farther away from the inside lane line than vehicles traveling in the inside lane. - There is no significant difference in the lane positions between passenger vehicles and trucks.
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1 - 64 Table 27. Summary Statistics of Encroachment and Critical Maneuvers Observed on Exit Ramps Ramp ID Number of Obs Truck count Percent trucks Time reviewed Number of encroachments and/or critical maneuvers observed Percent encroachments and/or critical maneuvers 16 574 42 7.3% 1:31:01 45 7.8% 17 136 1 0.7% 1:01:00 32 23.5% 18 60 4 6.7% 0:53:04 1 1.7% 20 162 6 3.7% 0:30:00 23 14.2% 21 109 18 16.5% 0:52:40 7 6.4% 24 141 13 9.2% 0:30:00 7 5.0% 25 137 17 12.4% 0:31:45 4 2.9% 26 53 4 7.5% 0:30:20 2 3.8% 27 19 0 0.0% 0:52:28 18 94.7% All 1391 105 7.5% 7:12:18 139 10.0% As shown in Table 28, most of the maneuvers recorded were encroachments onto the inside or outside shoulders.
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1 - 65 3.4.3.2 Analysis Results Of the 139 encroachments and critical maneuvers observed, only three involved swerving or severe braking; the remaining 136 maneuvers involved encroachments on the shoulder (51 on the inside shoulder and 85 on the outside shoulder)
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1 - 66 Several observations related to the patterns of encroachment and/or critical maneuvers that occurred along the first several hundred feet of the ramp proper at the nine exit loop ramps studied can be made as follows: It is likely that encroachments and/or critical maneuvers are directly related to the speeds of exiting vehicles, as some of the ramps with the highest percent encroachments and/or critical maneuvers observed also have the highest measured speeds of vehicles at the beginning of the controlling curve. It is likely that encroachments and/or critical maneuvers are directly related to the lane width of ramp proper.
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1 - 67 Section 4. Application of the HSM Ramp Crash Prediction Method to Loop Ramps This section of the report presents the results of applying the HSM ramp crash prediction method to loop ramps, as well as to a contrasting ramp configuration, diamond ramps.
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1 - 68 The SPFs for single-vehicle crashes use the following functional form: Nspf, sv = Lr × exp[a + b×ln(c×AADTr)
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1 - 69 The CMF for horizontal curves on ramps is computed as: ܥܯܨଵ ൌ 1.0 a ൈ ଵ,ଷଶ.ଶ ሾ∑ ሺ ௧, ோ ሻ ଶ ൈ ܲ,ୀଵ ሿ (13) where: ܥܯܨଵ = crash modification factor for horizontal curvature on a ramp segment m = number of horizontal curves in the ramp segment ܸ௧, = average entry speed for curve i (ft/s)
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1 - 70 5. From highway agency records, obtain the actual observed crash frequency, by severity level, for the "ramp proper" area, for a five-year period 6.
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1 - 71 Criteria for the selection of a ramp as a study site included: Complete data on area type, ramp configuration, ramp type, and AADT available in HSIS data files Meets the definition of one of the 12 ramp classifications listed above No C-D roads or ramp-to-ramp junctions present No lane additions or lane drops (except at ramp terminals) No ramp metering present No unusual or atypical features present The site selection used a quota-sampling method in which ramps were reviewed and selected until a maximum of 30 ramps of any particular type in each state were selected.
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1 - 72 1, and the proportion of any curve on a ramp segment, ܲ, in Equation 13, was always either 0 or 1. Crash history data were obtained from the HSIS data files for a five-year period from 2007 to 2011, inclusive.
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1 - 73 Table 30. Descriptive Statistics for California Rural Exit Ramps Parameter N Min Max Mean Median Diamond ramps (N = 30)
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1 - 74 Table 31. Descriptive Statistics for California Rural Entrance Ramps Parameter N Min Max Mean Median Diamond ramps (N = 30)
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1 - 75 Table 32. Descriptive Statistics for California Urban Exit Ramps Parameter N Min Max Mean Median Diamond ramps (N = 30)
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1 - 76 Table 33. Descriptive Statistics for California Urban Entrance Ramps Parameter N Min Max Mean Median Diamond ramps (N = 24)
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1 - 77 Table 34. Descriptive Statistics for Washington Rural Exit Ramps Parameter N Min Max Mean Median Diamond ramps (N = 26)
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1 - 78 Table 35. Descriptive Statistics for Washington Rural Entrance Ramps Parameter N Min Max Mean Median Diamond ramps (N = 26)
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1 - 79 Table 36. Descriptive Statistics for Washington Urban Exit Ramps Parameter N Min Max Mean Median Diamond ramps (N = 22)
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1 - 80 Table 37. Descriptive Statistics for Washington Urban Entrance Ramps Parameter N Min Max Mean Median Diamond ramps (N = 22)
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1 - 81 frequencies addressed only the "ramp proper" portion of each ramp, and not crashes attributed to either the freeway or crossroad ramps terminals. 4.8 Summary Comparison of Predicted and Observed Crash Frequencies Table 38 (total crashes)
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1 - 82 Table 38. Total Crash Statistics by State and Ramp Classification State Ramp classification Number of ramps MVMT per year Predicted Observed Crash rate ratios Area type Ramp type Ramp configuration Number of total crashes (per year)
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1 - 83 Table 39. Fatal-and-Injury Crash Statistics by State and Ramp Classification State Ramp classification Number of ramps MVMT per year Predicted Observed Crash rate ratios Area type Ramp type Ramp configuration Number of total crashes (per year)
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1 - 84 4.9 Statistical Comparison of Predicted and Observed Crash Frequencies This section describes the statistical analysis conducted to compare the crash frequencies predicted using the HSM ramp crash prediction method to the crashes observed at the 439 ramps in the study database. Key issues addressed in this analysis include: How accurately does the HSM ramp crash prediction method predict crashes for diamond ramps and loop ramps?
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1 - 85 Backward elimination was used to identify which factors and interactions were statistically significant at the 5-percent level. Since the objective is to estimate the ratios of predicted over observed crash rates separately for each ramp configuration, ramp configuration, the predicted/observed indicator variable, and their interaction were kept in the models, whether they were statistically significant at the 5-percent level or not.
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1 - 86 confidence interval of the ratio estimates includes 1. In other words, if the interval includes 1, then there is not enough evidence to conclude that the predicted crash rate is statistically significantly different from the observed crash rate at the 95-percent confidence level.
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1 - 87 4.10 Discussion of Results The analysis was conducted with models that predict crash frequencies (or crash counts per year)
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1 - 88 In summary, the analysis results indicate that the HSM ramp crash prediction models can be applied to both diamond and loop ramps, but that separate calibration for diamond and loop ramps is necessary to make accurate comparisons between the safety performances of these different ramp types.
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1 - 89 Section 5. Design Guidance The primary components of a ramp include the freeway mainline ramp terminal (i.e., a speedchange lane)
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1 - 90 and less the current approach to horizontal curve design generally overestimates the margins of safety against skidding and rollover for all vehicle types. Considering that the three primary components of a ramp (the freeway mainline ramp terminal, the ramp proper, and the crossroad ramp terminal)
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1 - 91 controlling curve of the loop ramp. It is recommended that the same, or similar, lane and shoulder widths will be used for all curves along the ramp proper.
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1 - 92 every combination that came out of Step 1 for a given design speed and curve radius produced similar safety estimates. The third step in the sensitivity analysis involved inputting the design conditions into the lane position model developed as part of this research (see Table 25)
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1 - 93 Table 43. Recommended Lane and Shoulder Width Combinations for Controlling Curve on Entrance Loop Ramps Design speed (mph)
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1 - 94 The sum of lane and shoulder widths in Table 43 differ from those presented in the Green Book Table 3-29 for two main reasons. First, the values in Green Book Table 3-29 are specified for turning roadways at intersections, such as channelized-right turn lanes, and are referenced for use in loop ramp design, rather than being presented specifically for loop ramp design.
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1 - 95 Combinations were evaluated for passenger vehicles on a single-lane ramp or the inside of a multi-lane ramp. The range of values used for each design element is consistent with the range used in developing the model and with guidance specified in the Green Book (AASHTO, 2011)
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1 - 96 from 100 ft to 300 ft and for design speeds from 20 to 35 mph. For exit loop ramps, the minimum recommended lane width is 16 ft.
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1 - 97 3. Apply the lane position model to remove alternatives that result in vehicles encroaching on the shoulder or adjacent lane Table 43 through Table 45 are presented for ease of application; however, designers can make use of the speed and lane position prediction models developed as part of this research and ISATe in a similar fashion to evaluate and assess different design alternatives in more detail.
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1 - 98 5.3 Multi-Lane Ramps It was found that vehicles travel approximately 1 to 2 mph faster in the outside lane of a multilane ramp compared to the inside lane. This is logical as vehicles traveling in the outside lane must travel faster to cover slightly longer travel distances and keep pace with vehicles on the inside lane of the ramp.
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1 - 99 methodology does a better job of predicting diamond ramp crashes than predicting loop ramp crashes. Therefore, separate calibration factors should be calculated for diamond ramps and loop ramps.
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1 - 100 Section 6. Conclusions and Recommendations for Future Research The objective of this research was to develop improved design guidance for interchange loop ramps.
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1 - 101 width. It is logical that the outside shoulder width influences vehicle speeds as drivers approach the end of the curve and begin looking for gaps in freeway traffic.
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1 - 102 lane or weave area. This may be due to fewer lane-changing maneuvers in the immediate vicinity of a freeway ramp that follows a lane drop.
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1 - 103 ramp are sufficient to accommodate traffic comprised primarily of passenger vehicles. If the outside lane is expected to accommodate a moderate to high volume of trucks, the outside lane width should be increased.
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1 - 104 at the midpoint of the controlling curve for a loop ramp. Development of a more detailed speed profile model that predicts the speed of vehicles along the entire length of a loop ramp may be helpful to better inform the design of loop ramps.
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1 - 105 Section 7. References American Association of State Highway and Transportation Officials (AASHTO)
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1 - 106 Iowa Department of Transportation (Iowa DOT) , Design Manual, 2010.
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1 - 107 Yates, J
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1A - 1 Appendix A Sites Included in Observational Field Study of Loop Ramps
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1A - 2 This appendix provides aerial views of each loop ramp included in the observational study described in Section 3 of this report. More than one study site may be illustrated in a single figure.
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1B-1 Appendix B Recommended Changes for Consideration in the Next Edition of the Green Book
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1B-2 This appendix provides recommended changes to the 2011 edition of the Green Book for consideration in the next edition of the Green Book. The recommendations are based on the findings and conclusions of this research.
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1B-3 (Page 10-50) Partial Cloverleaf Ramp Arrangements … Figure 10-29 illustrates the manner in which the turning movements are made for various two- and three-quadrant cloverleaf arrangements.
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1B-4 For a given radius (measured to the inside of the traveled way) and ramp design speed, the recommended lane and shoulder widths are expected to result in similar levels of safety, and vehicles are expected to stay within their intended travel lane.
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1B-5 4.8 0.6 1.5 – 3.0 0.9 0.9 – 2.7 1.2 1.2 – 2.4 1.5 1.5 – 2.1 1.8 1.8 5.4 0.6 1.2 – 2.4 0.9 0.9 – 2.1 1.2 1.2 – 1.8 1.5 1.5 6.0 0.6 0.9 – 1.8 0.9 0.9 – 1.5 1.2 1.2 Table 10-X1b. Recommended Lane and Shoulder Widths for Controlling Curve on Entrance Loop Ramps U.S.
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1B-6 4 4 - 9 5 5 - 8 16 2 5 - 10 3 3 - 9 4 4 - 8 5 5 - 7 6 6 18 2 4 - 8 3 3 - 7 4 4 - 6 5 5 20 2 3 - 6 3 3 - 5 4 4 Table 10-X2a. Recommended Lane and Shoulder Widths for Controlling Curve on Exit Loop Ramps – Simple Curves Metric Design speed (km/h)
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1B-7 18 6 - 8 20 4 - 6 25 200 16 2 8 - 10 18 6 - 8 20 4 - 6 30 200 16 2 8 - 10 18 6 - 8 20 4 - 6 16 3 7 - 9 18 5 - 7 20 3 - 5 30 250 16 2 8 - 10 18 6 - 8 20 4 - 6 35 300 16 2 8 - 10 18 6 - 8 20 4 - 6 16 3 7 - 9 18 5 - 7 20 3 - 5 Table 10-X3a. Recommended Lane and Shoulder Widths for Controlling Curves on Exit Loop Ramps – Compound Curves Metric Design speed (km/h)
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From page 137... ...
1B-8 60 90 4.8 0.9 2.1 - 2.7 5.4 1.5 - 2.1 6.0 0.9 - 1.5 4.8 1.2 1.8 – 2.4 5.4 1.2 – 1.8 6.0 1.2 4.8 1.5 1.5 - 2.1 5.4 1.5 Table 10-X3b. Recommended Lane and Shoulder Widths for Controlling Curves on Exit Loop Ramps – Compound Curves U.S.
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From page 138... ...
1B-9 Design widths of ramp traveled ways for turning roadways under various conditions are also given in Table 3-29, which may be used to determine widths for ramps with design characteristics not shown in Tables 10-X1, 10-X2, and 10-X3. Values in Table 3-29 are shown for three general design traffic conditions, as follows: Traffic Condition A -- predominantly P vehicles, but some consideration for SU trucks Traffic Condition B -- sufficient SU vehicles to govern design, but some consideration for semitrailer vehicles Traffic Condition C -- sufficient buses and combination trucks to govern design Traffic conditions A, B, and C are described in broad terms because design traffic volume data for each type of vehicle are not available to define these traffic conditions with precision in relation to traveled way width.
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From page 139... ...
1B-10 Ramps should have a lateral offset on the right outside of the edge of the traveled way of at least 1.8 m [6 ft] , and preferably 2.4 to 3.0 m [8 to 10 ft]
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Key Terms
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information on Chapter Skim is available.