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Ramp Design Speed Selection

Description:

Selecting a design speed for interchange ramps can greatly influence project costs, right-of-way needs, and the potential project environmental and social impacts. This is particularly true for system interchanges and service interchanges with loop ramps or flyover type ramps. General ramp design considerations are presented in Chapter 10 of the American Association of State Highway and Transportation Officials (AASHTO) Green Book, including considerations for selecting ramp design speeds. The ramp design speed is established as a fixed percentage of the connected highways (Green Book Table 10-1). For example, the “lower” range values are set at approximately 50% of the connected highway design speed. The ramp design speed is designated to apply to the controlling alignment element on the ramp proper regardless of whether the ramp is a free-flowing system connection or a service-interchange ramp that terminates at an intersection. There are many contextual factors that should be considered in selecting an appropriate ramp design speed. The rationale, and perhaps oversimplification, of the existing guidance is frequently called into question. Additionally, the application of the design speed along all portions of a ramp and across the spectrum of interchange forms is questionable and does not sufficiently take into account the inherent speed profiles of different ramp types. Due to the lack of substantial research in establishing this rationale, design practitioners find it difficult to implement flexibility in the design.

Objective:

The objective of this research is to develop enhanced guidance on selecting an appropriate ramp design speed based on a combination of contextual considerations and quantitative information as well as to examine the definition of ramp design speed, specifically the portion(s) of the ramp to which design speed should be applicable.

Related Research:

NCHRP Report 730, Design Guidance for Freeway Mainline Ramp Terminals

NCHRP 17-45 – Enhanced Safety Prediction Methodology and Analysis Tool for Freeways and Interchanges

Freeway and Interchange Geometric Design Handbook. Institute of Transportation Engineers, Joel Leisch (ed.), Washington, DC, 2005.

NCHRP 08-88-Guidelines for Ramp and Interchange Spacing. Kittelson & Associates, Inc. Jan. 2010.

FHWA-NHI-380073 – Fundamentals of Planning, Design and Approval of Interchange Improvements to the Interstate System

NCHRP 03-105, Design Guidance for Interchange Loop Ramps.

Schurr, Karen and Townsend, Devin. “Acceleration Ramps Along high Operating Speed Roadways”, Nebraska Transportation Center, 2010.

Hunter, Michael; Machemehl, Randy; and Tysganov, Alexei. “Reevaluation of Ramp Design Speed Criteria”, University of Texas at Austin, 1999.

Bauer, K.M., and Hawood, D.W. “Statistical Models of Accidents on Interchange Ramps and Speed-Change Lanes”, MRI / FHWA. 1997.

Hunter, Michael and Machemehl, Randy. “Re-Evaluation of Ramp Design Speed Criteria: Review of Practice and Data Collection Plan” University of Texas Austin. 1999.

Tasks:

An evaluation framework, including appropriate performance measures must be established. The following contextual factors that may have significant influence of the selection of an appropriate ramp design speed should be considered. These factors may include:

  • Differences for system vs. service interchanges

  • Differences for entrance vs. exit ramps

  • Degree of constraint, impact and right-of-way cost

  • Effects of ramp grade

  • Use of transition curves into the sharpest (controlling) ramp curve

  • Presence of ramp meters

  • Differences in ramp access design (tapered vs. paralleled, presence of auxiliary lane, different types of traffic control at ramp terminals, etc.)

The research should examine the safety and operational aspects of ramp design speed selection over a full range of interchange forms, ramp types, and area environments (rural vs. urban). The geometric design dimensions resulting from the suggested ramp design speeds (such as minimum radius of curvature) must also consider driver expectations and behaviors over a range of traffic conditions and the functional classification of the two interchanging roadways. Development of this enhanced guidance will require the examination of field data and site observations supplemented with safety modeling considerations and traffic operational simulations.

The research should examine the performance and feasibility aspects related to the applicability of design speed on various portions of ramps and connections, taking into account the different characteristics and speed profiles exhibited by different ramp types in different interchange forms. Right-of-way footprint and construction cost implications associated with current and potential alternate policies should be investigated as they might apply to representative interchange sites. Speed transitions within ramps – particularly those inherent to certain service-interchange circumstances but also those that may exist on system connectors – should be studied for performance and policy implications.

The issues associated with selecting an appropriate ramp design speed are complex and need research to offer modern perspectives and insights on safety performance and economic trade-offs associated with this important design choice. The final report should include enhanced guidance to design practitioners and propose appropriate changes to AASHTO policy if the results support a change.

Implementation:

The AASHTO Technical Committee on Geometric Design prioritized this research topic as the #2 need at the July 2013 meeting. There is urgency in advancing this research so that revised guidance could be included in the next edition of the AASHTO Green Book.

Relevance:

This research need has been identified as a high priority by the AASHTO Technical Committee on Geometric Design, the TRB Committee on Geometric Design, and the TRB Committee on Operational Effects of Geometrics at their July 2013 combined meeting. The research is needed to provide enhanced guidance that will directly impact future ramp design practices for use nationally in the design of interchanges. The potential payoff is the selection of more cost efficient, safer, and contextually appropriate interchange designs.

Sponsoring Committee:AKD10, Performance Effects on Geometric Design
Research Period:12 - 24 months
Research Priority:High
RNS Developer:James Rosenow, Reza Amini, Mark Leiferman, David McDonald, Brian Toombs, Barbara De Ste. Croix, Hasmukh Patel, Zhongren Wang, Mark Doctor
Source Info:Problem statement developed as a result of the Safety Effects of Geometric Design Decisions Workshop at the 2013 mid-year meeting of TRB Committees AFB10 (Geometric Design) and AHB65 (Operational Effects of Geometrics), in conjunction with the AASHTO Technical Committee on Geometric Design. Problem Statement # 2015-C-08 on the NCHRP/AASHTO SCOR list.
Date Posted:02/07/2014
Date Modified:02/10/2014
Index Terms:Interchanges and intersections, Highway design, Ramps (Interchanges), Design speed, Traffic flow, Right of way (Traffic),
Cosponsoring Committees: 
Subjects    
Highways
Design
Operations and Traffic Management
Safety and Human Factors

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