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1 Summary In the American Association of State Highway and Transportation Officialsâ (AASHTO) A Policy on Geometric Design of Highways and Streets, commonly referred to as the Green Book, design speed is defined as a selected speed used to determine the various geometric design features of the roadway (AASHTO, 2018). Design speed is used to determine all pertinent highway features to obtain a balanced design. Over an extended length of highway, it is desirable to select a uniform design speed. Where high-speed facilities meet low-speed facilities, drivers are expected to reduce their speeds suitable to the roadway environment they are entering. This change in speed should be achieved over an extended distance; but in the case of an interchange ramp, this is not possible. Application of the design speed concept and selection of an appropriate ramp design speed as presented in the AASHTO Green Book is overly simplistic and does not consider many factors. This document provides guidelines for selecting appropriate ramp design speeds for ramps. The guidelines facilitate the selection of an appropriate ramp design speed based on a combination of contextual considerations and quantitative information. The guidelines define ramp design speed and all terms and elements related to its application. The guidelines address the following topics related to selecting an appropriate ramp design speed: ⢠System and service interchanges. ⢠Ramp configuration. ⢠Entrance and exit ramps. ⢠Freeway mainline ramp terminal configuration (i.e., taper vs. parallel). ⢠Type of crossroad ramp terminal (i.e., stop control, signal control, free-flow). ⢠Ramp grade. ⢠Type of horizontal curvature (i.e., simple, spiral, compound, and reverse curves). ⢠Number of lanes (i.e., single vs. multilane ramps). ⢠Crossover crown line. ⢠Superelevation and superelevation transition. ⢠Ramp metering. ⢠Collector-distributor (C-D) and frontage roads. ⢠Physical site constraints, environmental and social impacts, and right-of-way costs. Ramp design speed is defined as the selected speed used to determine the various geometric design features of a ramp. The selected ramp design speed should be a logical one with respect to the type of intersecting highways, area type (i.e., urban or rural), ramp configuration, and site constraints (including physical, environmental, and social). Guide values for ramp design speeds are presented for system and service interchanges. The guide values are presented as a range of values given the context, ramp configuration, and type of interchange. The guide values were developed to be relatively consistent with the guide values presented in the 2018 edition of the Green Book. Where the horizontal alignment of the ramp is curvilinear, the ramp design speed applies to the controlling curve on the ramp proper. For an entrance ramp, the last curve encountered along the ramp proper that significantly affects vehicle speed is considered the controlling curve. For an exit ramp, the controlling curve is the first curve encountered along the ramp proper that
2 significantly affects vehicle speed. For direct connection and outer connection ramps, the sharpest curve on the ramp proper is considered the controlling curve. The freeway mainline ramp terminal, the crossroad ramp terminal, and the adjoining sections of the ramp proper that connect to the controlling curve should be designed to allow for appropriate speed transitions, consistent with driver behavior and expectations and vehicle performance capabilities, for the selected ramp design speed. Where the horizontal alignment of the ramp is relatively straight and has little or no influence on vehicle speeds, the ramp design speed is based on the operational characteristics of the freeway mainline ramp terminal for an entrance ramp and the operational characteristics of the crossroad ramp terminal for an exit ramp and applies to the tangential section of the ramp proper. Each individual section of the ramp proper (i.e., tangent and curve) has its own design speed. The design speed of the controlling feature of the ramp (i.e., the ramp design speed) is selected first, and then the design speeds of the other elements or segments of the ramp proper should be determined accordingly. It is desirable (but not essential) that the design speeds of the contiguous segments along the ramp proper increase or decrease in a stepwise manner. The change in design speed between adjoining sections should be limited to no more than 10 to 15 mph. For entrance ramps, this means that the design speeds of the contiguous segments along the ramp proper where practical should increase in a stepwise manner; and for exit ramps the design speeds of the contiguous segments along the ramp proper should decrease in a stepwise manner. The design speed of each individual curve and tangent along the ramp proper should be consistent with the speeds that drivers are likely to be traveling along the respective segment. The freeway mainline ramp terminal portion of a ramp does not have an associated design speed. However, the operational and design conditions of the freeway mainline ramp terminal are related to and should be coordinated with the operational and design conditions of the adjoining sections of the ramp proper and freeway. Each component of the ramp should be designed such that a vehicle has sufficient distance to accelerate or decelerate within the performance capabilities of the vehicle and driver expectations given the design speeds and features of the individual components of the ramp. Several design controls and criteria can be used to design ramps so that the individual components and features of the ramps are designed in a consistent manner, accounting for sequential speed transitions from one component or section to the next. A spreadsheet-based tool was developed to estimate vehicle speeds along interchange ramps to help designers and traffic engineers conduct operational analyses and make design decisions. The spreadsheet tool is applicable to entrance and exit ramps at service interchanges including diagonal ramps, loop ramps, and outer connection ramps. The spreadsheet-based tool incorporates several speed prediction models based on vehicle speeds observed on ramps. With the capability to adjust several input parameters, the spreadsheet-based tool can be used to predict vehicle speeds based upon anticipated driver behaviors and design conditions. Output from the tool can be interpreted to help design an interchange ramp in a consistent manner that meets driver expectations.
