The National Academies Press

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From page 23... ... 23 Section 2. Ramp Elements Related to Ramp Design Speed The definition of ramp design speed indicates a combination of contextual issues such as the type of intersecting highways, area type, ramp configuration, and site constraints that should be considered when selecting a ramp design speed. Read the entire page →
From page 24... ... 24 For situations where the design speeds of both intersecting freeways of a ramp are equivalent, the highest possible ramp design speed for the connecting ramp would be the design speed of the intersecting freeways. However, due to physical constraints and related right-of-way costs, in most cases it is not practical for the ramp design speed to be equivalent to the design speed of the two intersecting freeways. Read the entire page →
From page 25... ... 25 As a guide, high ramp design speeds generally correspond to values of 50 mph and above; intermediate ramp design speeds are in the range of 35 to 45 mph; and low ramp design speeds correspond to values of 30 mph or less. 2.2 Ramp Configuration When selecting the appropriate ramp design speed for a particular ramp configuration, the primary considerations are the intended purpose of the ramp and the controlling feature of the ramp. Read the entire page →
From page 26... ... 26 Loop ramps (see Figure 7) may be used in both system and service interchanges. Read the entire page →
From page 27... ... 27 Entrance Ramps On entrance ramps, vehicles travel from the crossroad ramp terminal to the ramp proper to the freeway mainline ramp terminal. At the crossroad ramp terminal vehicles speeds are low and increase along the ramp proper. Read the entire page →
From page 28... ... 28 acceleration lengths are equivalent for both taper- and parallel-type entrance ramps given the same initial speed (VAcc Length (i) Read the entire page →
From page 29... ... 29 a parallel-type exit, there is a greater likelihood that the horizontal alignment immediately downstream of the deceleration lane on the ramp proper will be curvilinear, affect the speed of vehicles, and be the controlling feature of the ramp. If the controlling feature of the ramp is the horizontal alignment immediately downstream of the deceleration lane on the ramp proper, the ramp design speed may be higher than if the operational characteristics of the crossroad ramp terminal are the controlling feature and ramp design speeds will be lower. Read the entire page →
From page 30... ... 30 stopped for the traffic control. Thus, rather than designing to accommodate vehicles traveling 15 mph at the intersection of the ramp and crossroad, the ramp design speed needs to be selected based on vehicles stopped at the end of the queue storage leading to the crossroad. Read the entire page →
From page 31... ... 31 • Rugged terrain: 6 to 8 percent. The lower values for each range of gradient are applicable to the higher ramp design speeds, particularly for major interchanges. Read the entire page →
From page 32... ... 32 higher design speeds than lower functional class facilities. For example, multilane loop ramps have higher capacities and typically consist of: • More significant movements. Read the entire page →
From page 33... ... 33 Table 3. Maximum Algebraic Difference in Cross Slope at Turning Roadway Terminals (AASHTO, 2018) Read the entire page →
From page 34... ... 34 minimum length requirements, superelevation transition of the pavement could become the controlling factor in the design of the adjoining sections of ramps. 2.11 Ramp Metering Ramp metering regulates the flow of vehicles at entrance ramps to reduce turbulence in the merge area to improve the efficiency of freeway operations and reduce crash frequency. Read the entire page →
From page 35... ... 35 • Physical site constraints such as topography, bodies of water (e.g., streams, lakes, and rivers) , existing infrastructure, buildings, and right-of-way limits. Read the entire page →
From page 36... ... 36 and location and the length and orientation of each ramp and directly impact the selection of the ramp design speed as part of the geometric design process. Collectively, these key ramp elements and contextual considerations factor into the selection of an appropriate ramp design speed for a given ramp. Read the entire page →
From page 37... ... 37 Table 4. Range of Guide Values for Ramp Design Speed as Related to Highway Design Speed, Interchange Type, Ramp Configuration, and Contextual Considerations Context Ramp configuration Highway design speed (mph) Read the entire page →
From page 38... ... 38 Figure 13. Primary Components and Controlling Curves at Entrance and Exit Ramps at Service Interchanges Figure 14. Read the entire page →
From page 39... ... 39 By providing a range of guide values for ramp design speeds, Table 4 provides designers with flexibility in determining the contextual and design considerations that apply for a given ramp (e.g., in determining whether a ramp is located in an unconstrained or constrained area) Read the entire page →

From page 23...

... 23 Section 2. Ramp Elements Related to Ramp Design Speed The definition of ramp design speed indicates a combination of contextual issues such as the type of intersecting highways, area type, ramp configuration, and site constraints that should be considered when selecting a ramp design speed.