Design of Interchange Loop Ramps and Pavement/Shoulder Cross-Slope Breaks (2017) / Chapter Skim
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Part 2: Assessment of Design Criteria for Pavement/Shoulder Cross-Slope Breaks
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2 - ii ACKNOWLEDGMENT The research reported herein was performed under NCHRP Project 3-105, "Design Guidance for Interchange Loop Ramps" as Phase III – Assessment of Pavement/Shoulder Cross-Slope Breaks. While this assessment was conducted as part of a larger research project that focused on the design of interchange loop ramps, the assessment of pavement/shoulder cross-slope breaks concerned the design of cross-slope breaks on roadways in general and was not limited just to interchange loop ramps.
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2 - iii ABSTRACT The objectives of this research were to 1) assess AASHTO's current design policy for pavement/shoulder cross-slope breaks to determine whether updates in design criteria are needed, and 2)
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2 - v LIST OF FIGURES AND TABLES Figures Figure 1. Typical Superelevated Cross-Sections with Pavement/Shoulder Cross-Slope Breaks ...........................................................................................................................2 Figure 2.
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2 - vi Tables Table 1. Primary Vehicle Input Parameters Used in TruckSim .................................................16 Table 2.
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2 - vii SUMMARY In 2011, the National Transportation Safety Board (NTSB) completed a detailed investigation of a crash involving a tractor/tanker-trailer truck that overturned and caught fire while negotiating a curved interchange ramp.
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2 - viii fully-loaded trailer condition. Since the dynamic effects of liquid sloshing in a tractor/tankertrailer truck could not be simulated in this research, the tractor/tanker-trailer truck was simulated by modifying the center of gravity of the tractor/single-van-trailer truck to represent the performance of a tractor/tanker-trailer truck within the simulation model.
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2 - ix moderate departure vehicle trajectories were selected as the primary maneuvers of interest for assessing the design criteria for pavement/shoulder cross-slope breaks and drawing conclusions related to the need to change AASHTO's current design policy for pavement/shoulder crossslope breaks. Based upon the simulation results for the tractor/single-van-trailer truck, the tractor/double-vantrailer truck, and the fully-loaded tractor/tanker-trailer truck for the partial and full traversal moderate departure vehicle trajectories, there is no evidence to suggest the need to reduce the threshold value of 8 percent as the maximum recommended cross-slope break.
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2 - x Based on the results from the vehicle dynamics simulation modeling, it was concluded that there is no need to recommend a change to AASHTO's current design policy for pavement/shoulder cross-slope breaks on the outside of horizontal curves. There is some evidence to suggest that the recommended maximum cross-slope break could be increased to 10 percent.
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2 - 1 SECTION 1. Introduction 1.1 Background On October 22, 2009, a tractor/tanker-trailer truck overturned and caught fire while negotiating a semi-direct connection ramp between two interstate highways near Indianapolis, Indiana.
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Figur As report criterion Highway designs b There is passenge dynamics unit com 1970s an substanti e 1. Typica ed by the N appears to b -Vehicle-Ob y testing the a natural con r cars; furth simulation bination)
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2 - 3 NTSB's recommendation to conduct a detailed investigation of the cross-slope break design criterion to determine whether the existing policy is appropriate for the current fleet of passenger cars and trucks. 1.2 Research Objective The objectives of this research were to 1)
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2 - 4 1.4 Outline of Report This report presents an overview of research conducted to assess the need to update AASHTO's design policy for pavement/shoulder cross-slope breaks on the outside of horizontal curves. The remainder of this report is organized as follows: Section 2 -- Summary of Literature Review and Current Practice Section 3 -- Vehicle Dynamics Simulation Modeling Section 4 -- Crash-Based Safety Analysis Section 5 -- Conclusions and Future Research Section 6 -- References
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2 - 5 SECTION 2. Summary of Literature Review and Current Practice This section summarizes the results of a literature review and a survey to gather background information on pavement/shoulder cross-slope break issues that helped guide the direction of this research.
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2 - 6 Preliminary runs were conducted to establish the controlling details of the lane departure maneuver. These runs indicated that a 4-wheel departure produced more extreme responses than a 2-wheel departure and that the entry to the cross-slope break was more extreme than the exit.
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maneuve Similar to Figu the emerg three driv of travel. simulatio 2.2.3 Te The Texa recomme (Texas D D t D y B y y a Other sta handbook r was simula the Maeda re 2.
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2 - 8 2.3 Critical Design Elements and Variables Several documents that provided insight into key design elements and variables were reviewed. 2.3.1 Review of Truck Characteristics as Factors in Roadway Design In NCHRP Report 505, Review of Truck Characteristics as Factors in Roadway Design, Harwood et al.
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2 - 9 For passenger cars, mean maximum wet-tire friction values ranged from approximately 0.91 to 0.82, and mean skidding wet-tire friction values ranged from approximately 0.67 to 0.58 in the longitudinal (braking) direction.
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2 - 10 difficulties in construction than do planar sections. An alternate method to the convex shoulder consists of a planar shoulder section with multiple breaks in the cross slope.
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2 - 11 electronically to all 50 U.S. state highway agencies; highway agencies from 27 states responded to the survey.
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2 - 12 4. What is your agency's typical shoulder cross-slope on tangent sections on rural highways?
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2 - 13 9. Are you aware of any crashes on highways/roadways within your state that might be related to maximum pavement/shoulder cross-slope break issues?
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2 - 14 NO (please explain)
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2 - 16 3.1.1 Vehicle Types and Models Three vehicles were selected for use in the simulations: a tractor/single-van-trailer truck, a tractor/tanker-trailer truck, and a tractor/double-van-trailer truck. The basic input parameters for the vehicles were selected based on vehicle dimensions from the AASHTO Green Book, and mass and center of gravity values from the AASHTO Manual for Assessing Safety Hardware (MASH)
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2 - 17 for evaluation of pavement/shoulder cross-slope breaks as it was assumed that the dynamic effects of liquid sloshing should be minimal for the fully-loaded trailer condition. (Note: The dynamic effects of liquid sloshing in a tractor/tanker-trailer truck could not be simulated in this research.
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represent results fr making d maneuve Fig The path Glennon where: R R While tra travel alo moderate constant of the veh trajectori the pavem This met Partial an type, spe simulated ed the most om the full t ecisions reg r was consid ure 6. Vehic of moderate et al.
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Fig Fig In summ slope bre P t ure 7.
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2 - 22 tires of the vehicle traverse the cross-slope break and encroach onto the shoulder before the vehicle is steered back to the travel lane. This vehicle trajectory represented the mildest departure scenario simulated as part of this research.
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Figure 10. (0% Sup Figure 11.
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Figure Table 4 s tractor/si curvature values of under wh cross-slo shows gr Figure 14 influence maneuve Table 12. Illustrat hows the pr ngle-van-tra for design 0, 4, and 6 ich the vehi pe break of aphs of the r shows the of the cross r.
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F F igure 13. R igure 14.
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Figure 1 Table 5 s tractor/si curvature and 70 m Table Table 6 s tractor/si curvature superelev encounte Table 6 Several i combinat V d p t 5. Illustrat TruckSim hows the pr ngle-van-tra for design ph, the non5.
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2 - 28 further define the final combinations of simulation scenarios for testing in this research. It is anticipated that the less severe vehicle departure trajectories will have less of an impact on the vehicle dynamics when testing the effect of cross-slope break on vehicle stability.
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2 - 29 Vehicle Type - Tractor/single-van-trailer truck (~80,000 lb) , CG of ballast at 73 inches - Tractor/tanker-trailer truck (~80,000 lb)
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2 - 30 Table 7. Tractor/Single-Van-Trailer Truck with Partial Traversal Moderate Departure CSB 0% 4% 6% 8% 10% Speed (mph)
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2 - 31 Table 8 displays the recovery results for the tractor/single-van-trailer truck with a full traversal moderate departure vehicle trajectory. The largest maximum roll angle for the tractor/single-vantrailer truck with a full traversal moderate departure was 8.5 degrees for 4 percent superelevation, 10 percent cross-slope break, and 30 mph.
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2 - 32 Table 9. Tractor/Tanker-Trailer Truck with Partial Traversal Moderate Departure (Full Tanker Trailer)
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2 - 33 only the full traversal moderate departure vehicle trajectory scenarios were considered in the simulation combinations. Table 11 shows the recovery results for the tractor/double-van-trailer truck for a full traversal moderate departure vehicle trajectory.
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2 - 34 Table 11. Tractor/Double-Van-Trailer Truck with Full Traversal Moderate Departure CSB 0% 4% 6% 8% 10% Speed (mph)
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2 - 35 the fully-loaded trailer condition. Also, because existing vehicle dynamics simulation models do not have the capability to simulate the dynamic effects of liquid sloshing in a tanker trailer, the dynamic effects of a tractor/tanker-trailer truck were not truly simulated in this research, but rather the characteristics of the tractor/single-van-trailer truck were modified and simulated to represent situations analogous to a tractor/tanker-trailer truck.
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2 - 36 SECTION 4. Crash-Based Safety Analysis This section presents the crash-based safety analysis intended to investigate the extent to which large cross-slope breaks may contribute to crashes near superelevated horizontal curves.
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2 - 37 data because the NTSB study that recommended a review of the AASHTO policy on pavement/shoulder cross-slope break was an outcome of the investigation of a fatal tanker truck rollover crash. The data in Table 12 show that: 0.24 percent of all fatal crashes and 2.1 percent of fatal truck crashes are classified as untripped truck-related rollover (or overturning)
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2 - 38 where larger cross-slope breaks are more likely to occur, suggests that the role that cross-slope break plays in these crashes should be further investigated. Table 13.
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2 - 39 A combination of two approaches was used to identify horizontal curve sites for consideration in the analysis. The first approach was to identify potential sites based on their site characteristics, using the following criteria: Routes should contain horizontal curves that likely have a wide range of values for superelevation.
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2 - 40 4.2.2 Descriptive Statistics -- Site Characteristics and Crash Counts A total of 64 curves on rural two-lane highways and 44 curves on rural freeways were ultimately used in the analysis. Table 14 shows the range of site characteristics of the horizontal curves, separately for each state and roadway type.
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2 - 41 Table 14. Range of Site Characteristics of Horizontal Curves Included in Crash-Based Safety Analysis State Number of Curves Total Curve Length (ft)
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2 - 43 Table 15. Crash Counts by Crash Type, Roadway Type, and State Crash Type Roadway Type (Number of Sites)
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2 - 44 4.2.3 Analysis Approach The safety effect of cross-slope break on curves was estimated separately for rural two-lane highways and rural freeways and the following remaining three crash types: Total crashes Single-vehicle crashes Single-vehicle rollover crashes The six models were each estimated using a generalized linear model approach assuming a negative binomial (NB) distribution of crash counts using the combined crash data and selected roadway geometrics.
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2 - 45 If either degree of curvature or superelevation were not significant at the 10-percent level in the model including all three factors, they were removed from the model using backward stepwise elimination (i.e., the least significant of the two factors was excluded and the model rerun with the remaining factors, etc.)
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2 - 46 However, review of these results points to the fact that the sample sizes are too small and the datasets from the various states too disparate to draw meaningful conclusions. In particular, the effect of degree of curvature (equivalent to radius of curvature)
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2 - 47 severity. While no statistically significant effect of pavement/shoulder cross-slope breaks on any of the three crash measures could be found, the available sample size was simply too small to draw meaningful conclusions.
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2 - 48 SECTION 5. Conclusions and Future Research Needs The objective of this research was to assess AASHTO's current design policy for pavement/ shoulder cross-slope breaks on the outside of horizontal curves to determine whether updates in design criteria are recommended.
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2 - 49 Results from the detailed analysis of crashes for 64 curves on rural two-lane highways and 44 curves on rural freeways in four states provide insufficient evidence to show that cross-slope break has a significant effect on total crashes, single-vehicle, or single-vehicle rollover crashes. The results from the detailed crash-based safety analysis indicate that the available sample size is too small to determine whether pavement/shoulder cross-slope breaks result in more frequent or more severe crashes.
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2 - 50 SECTION 6. References American Association State Highway and Transportation Officials (AASHTO)
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2 - 51 Maeda, T., N
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APPE Simul Vehic This appe departure simulatio combinat departure in the tra all tires o vehicle is collision no pavem reasonab maneuve simulatio pavemen basis for departure of the de Table Ain TruckS tractor/si vehicle (A on the W determin The dyna because e effects of considere trailer tru represent TruckSim Four com to learn m a partiall character partiallyWhile Tr some rea combinat NDIX A ation Re le Trajec ndix presen vehicle traj n model to v ions of simu , the steerin vel lane (i.e f the vehicle steered bac avoidance m ent/shoulde le basis for h r simulated ns were not t/shoulder c determining and full tra sign criteria 1 presents th im based o ngle-van-tra ASHTO, 2 B-20D desig ed from Tab mic effects xisting veh liquid slosh d within the cks with dif situations a .
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2A-2 Table A-1. Primary Vehicle Input Parameters Used in TruckSim Vehicle Inputs Tractor/SingleVan-Trailer Truck Tractor/Tanker-Trailer Truck Tractor/DoubleVan-Trailer Truck Full Tanker Lower Half Tanker Upper Half Tanker Right Half Tanker Tractor type Sleeper Cab Sleeper Cab Sleeper Cab Sleeper Cab Sleeper Cab Day Cab Total mass of vehicle1 (lb)
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2A-3 Table A-2 provides simulation results for the tractor/single-van-trailer truck for the full traversal severe departure vehicle trajectory. Table A-3 through Table A-6 provide simulation results for the four combinations of tractor/tanker-trailer trucks modeled for the full traversal severe departure vehicle trajectory.
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2A-4 Tablel A-2. Tractor/Single-Van-Trailer Truck: Full traversal severe departure Superelevation 4% Cross-Slope Break 0% 4% 6% 8% Speed (mph)
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2A-5 Tablel A-3. Fully-Loaded Tractor/Tanker-Trailer Truck: Full traversal severe departure Superelevation 4% Cross-Slope Break 0% 6% 8% Speed (mph)
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2A-6 Tablel A-4. Partially-Loaded Tractor/Tanker-Trailer Truck (Lower Half)
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2A-7 Tablel A-5. Partially-Loaded Tractor/Tanker-Trailer Truck (Upper Half)
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2A-8 Tablel A-6. Partially-Loaded Tractor/Tanker-Trailer Truck (Right Half)
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APPE Simul Trajec This appe of pavem conclusio shoulder P t t t m F t t s t w p c d Simulatio and a trac trajectori single-va B-2 prov moderate tractor/ta Table Btraversal tractor/do NDIX B ation Re tories ndix provid ent/shoulde ns related to cross-slope.
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2B-2 Tractor/Single-Van-Trailer Truck Table B-1. Tractor/Single-Van-Trailer Truck: Partial traversal moderate departure Superelevation 4% Cross-Slope Break 0% 6% 8% 10% Speed (mph)
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2B-3 Table B-2. Tractor/Single-Van-Trailer Truck: Full traversal moderate departure Superelevation 4% Cross-Slope Break 0% 6% 8% 10% Speed (mph)
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2B-4 Tractor/Tanker-Trailer Truck Table B-3. Fully-Loaded Tractor/Tanker-Trailer Truck: Partial traversal moderate departure Superelevation 4% Cross-Slope Break 6% 8% Speed (mph)
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2B-5 Tractor/Double-Van-Trailer Truck Table B-5. Tractor/Double-Van-Trailer Truck: Full traversal moderate departure Superelevation 4% Cross-Slope Break 8% 10% Speed (mph)
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