Selecting Ramp Design Speeds, Volume 1: Guide (2021) / Chapter Skim
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Pages 40-56
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From page 40... ...
40 Section 3. Guidelines for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed Interchange design and the design and orientation of each individual interchange ramp is an art form that requires consideration of a wide range of factors including: • Number of intersecting legs.
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41 The design speed of the controlling feature of the ramp (i.e., the ramp design speed) is first selected, and then the design speeds of the other elements or segments of the ramp proper should be determined accordingly.
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42 appropriate in order of the primary components of an entrance ramp based on the direction of travel. As such, applicable design controls and criteria are first discussed for the crossroad ramp terminal, then the ramp proper, and finally the freeway mainline ramp terminal.
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43 Table 5. Minimum Lengths for Tangents and Curves on Ramp Proper of Entrance Ramps Speed Exiting the Tangent or Curve on the Ramp Proper (Final Speed)
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44 edition of the Green Book to be used in the design of higher speed facilities. The new design values (shown in red text in Table 6)
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45 Table 6. Minimum Acceleration Lengths for Entrance Terminals with Flat Grades of Less Than 3 Percent (adapted from AASHTO, 2018)
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46 VAcc Length(f) = merge speed (i.e., the speed at which a driver merges with through traffic at the end of the acceleration length)
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47 the right lane of the freeway, the beginning of the acceleration length may be located somewhere along the curve. This means the beginning of the acceleration length may be located upstream of the gore point.
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48 Guidelines for Exit Ramps With exit ramps, a primary design control is vehicle deceleration. Individual components and features of the ramp should be designed with sufficient length such that vehicles decelerate in a controlled manner from an initial to final speed over the length of the section and do not exceed an assumed maximum deceleration.
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49 Table 7. Minimum Deceleration Lengths for Exit Terminals with Flat Grades of Two Percent or Less (adapted from AASHTO, 2018)
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50 Figure 17. Freeway Mainline Ramp Terminal Elements of an Exit Ramp AASHTO policy uses a basic two-step process for establishing the minimum deceleration length criteria for exit ramps.
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52 on vehicle speeds, then conceptually the freeway mainline ramp terminal may extend beyond the gore point all the way to the beginning of the functional area of the crossroad ramp terminal. No design criteria exist for specifying minimum length requirements of the individual tangent and curve sections of the ramp beyond the gore point; but the deceleration distances depicted in Green Book Figure 2-34 (Deceleration Distances for Passenger Vehicles Approaching Intersections)
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53 Table 8. Deceleration Distances for Passenger Vehicles Approaching Intersections based on Steady Braking Initial Speeds (mph)
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54 Similar to the brake reaction time used for stopping sight distance (SSD) , the decision length is the distance covered by a vehicle from the instant that the driver recognizes the presence of a queue of vehicles stopped ahead until the instant that the driver actually applies the breaks.
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55 speed of the curve should be consistent with the speeds that drivers are likely to be traveling at the midpoint of the curve. Guidelines for Freeway-to-Freeway Ramps The same design controls and criteria for freeway mainline ramp terminals and the ramp proper for entrance and exit ramps are also applicable to freeway-to-freeway ramps.
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56 • Realign the ramp to increase the distance from the gore point to the controlling curve. • Supplement the standard advisory speed signs to make the signing more conspicuous, increase the distance from the signing to the controlling curve and draw attention of truck drivers to the signing.
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