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From page 92... ... 92 Section 5. Case Studies This section demonstrates through several case studies how to evaluate the consistency of a ramp design based on the selected ramp design speed. Read the entire page →
From page 93... ... 93 • Drivers tend to begin the merge, acceleration process only after gaining a clear view of the freeway right-traffic lane. If the driver's view from the ramp is obstructed, the acceleration length will not begin until near the ramp gore where the view of the freeway becomes unobstructed (Hunter and Machemehl, 1999) Read the entire page →
From page 94... ... 94 Interchange type: Service Conditions: Constrained Area type: Urban/Suburban Ramp configuration: Loop Freeway characteristics Design speed: 70 mph Operating speed: 65 mph Speed limit: 65 mph Crossroad characteristics Type of roadway: Primary/major rd Speed limit: 45 mph Type of traffic control at terminal: Signal Ramp Proper Ramp grade: 1 % downgrade Max superelevation (emax) : 12 % Curve 1 PC milepost: 0.044 mi Curve length: 0.076 mi Curve radius 182 ft Superelevation: 12 % Design speed 25 mph Curve 2 PC milepost: 0.120 mi Curve length: 0.036 mi Curve radius 224 ft Superelevation: 10 % Design speed 25 mph Curve 3 PC milepost: 0.223 mi Curve length: 0.032 mi Curve radius 3000 ft Superelevation: 4 % Design speed 50 mph Freeway Mainline Ramp Terminal Milepost of gore point 0.338 mi Acceleration length (upstream of the gore point) Read the entire page →
From page 95... ... 95 other sections and elements of the ramp should be designed consistent with this ramp design speed to provide appropriate speed transitions along the ramp. To evaluate the adequacy and consistency of the existing ramp design, the ramp proper should be divided into individual tangents and curves. Read the entire page →
From page 96... ... 96 Figure 2-33) Read the entire page →
From page 97... ... 97 • Design acceleration rate, computed using Equation 29, ft/s2. • Notes comparing the estimated average acceleration rates to design acceleration rates. Read the entire page →
From page 98... ... 98 Plotted Speed Data Acceleration Data Figure 33. Case Study No. Read the entire page →
From page 99... ... 99 A couple of points are worth noting from the speed profile illustrated in Figure 33: • The predicted operating speed at the end of the first tangent (28.75 mph) is greater than the design speed of Curves 1 and 2 (25 mph) Read the entire page →
From page 100... ... 100 Acpt was changed to 1.0. Figure 35 shows the revised speed profile with a merge speed of 60 mph at the end of the acceleration length and gap acceptance length (MP 0.614) Read the entire page →
From page 101... ... 101 Plotted Speed Data Acceleration Data Figure 35. Case Study No. Read the entire page →
From page 102... ... 102 Case Study No. 2: Diagonal Entrance Ramp in Rural Area Background Information This case study features a diagonal entrance ramp to a high-speed rural freeway. Read the entire page →
From page 103... ... 103 Interchange type: Service Conditions: Unconstrained Area type: Rural Ramp configuration: Diagonal Freeway characteristics Design speed: 85 mph Operating speed: 72 mph Speed limit: 85 mph Crossroad characteristics Type of roadway: Local/minor rd Speed limit: 45 mph Type of traffic control at terminal: Stop control Ramp Proper Ramp grade: 0.17 % Maximum superelevation (emax) : 6 % Curve 1 PC milepost: 0.123 mi Curve length: 0.055 mi Curve radius 8056 ft Superelevation: 3.51 % Design speed 80 mph Curve 2 PC milepost: 0.179 mi Curve length: 0.062 mi Curve radius 2497 ft Superelevation: 3.51 % Design speed 40 mph Curve 3 PC milepost: 0.272 mi Curve length: 0.078 mi Curve radius 2856 ft Superelevation: 2.50 % Design speed 35 mph Freeway Mainline Ramp Terminal Milepost of gore point 0.398 mi Acceleration length (upstream of the gore point) Read the entire page →
From page 104... ... 104 design guidelines presented herein, the footnote at the bottom of Table 4 reads where the horizontal alignment of a diagonal entrance ramp is relatively straight and has little impact on vehicle speeds, the ramp design speed is dependent on the operational characteristics of the freeway mainline ramp terminal. For this situation, it is recommended that the ramp design speed be within 15 to 20 mph of the highway design speed. Read the entire page →
From page 105... ... 105 Figure 37. Case Study No. Read the entire page →
From page 106... ... 106 Plotted Speed Data Acceleration Data Figure 38. Case Study No. Read the entire page →
From page 107... ... 107 proper (i.e., the gore point) is approximately 46 mph, which is less than the ramp design speed of 65 mph. Read the entire page →
From page 108... ... 108 actual acceleration length of 0.089 mi. Considering these inconsistencies between the predicted speed profile and the assumed initial and merge speeds from Table 6, this ramp should potentially be redesigned to allow for a longer acceleration length, gap acceptance length, and/or speed-change lane length. Read the entire page →
From page 109... ... 109 Plotted Speed Data Acceleration Data Figure 40. Case Study No. Read the entire page →
From page 110... ... 110 ramp terminal) or the downstream end of the ramp (i.e., associated with the crossroad ramp terminal) Read the entire page →
From page 111... ... 111 Interchange type: Service Conditions: Unconstrained Area type: Urban/Suburban Ramp configuration: Loop Freeway characteristics Design speed: 75 mph Operating speed: Unknown Speed limit: 70 mph Crossroad characteristics Type of roadway: Local/minor rd Speed limit: 35 mph Type of traffic control at terminal: Signal Queue storage length: 0.050 mi Milepost of crossroad ramp terminal 0.256 mi Ramp Proper Ramp grade: 1 % downgrade Max superelevation (emax) : 8 % Curve 1 PC milepost: 0.000 mi Curve length: 0.068 mi Curve radius 525 ft Superelevation: 7 % Design speed 30 mph Curve 2 PC milepost: 0.068 mi Curve length: 0.116 mi Curve radius 265 ft Superelevation: 6.6 % Design speed 25 mph Freeway Mainline Ramp Terminal Taper length 0.057 mi Divergence zone length 0.127 mi Deceleration length (downstream of the gore point) Read the entire page →
From page 112... ... 112 Design Consistency Assessment Based on the design guidelines presented herein, given the freeway design speed is 75 mph and the ramp is located in a suburban area, Table 4 indicates the ramp design speed for a loop ramp should be between 25 and 40 mph. Given the first curve encountered on the ramp proper that significantly affects vehicle speeds has a design speed of 30 mph, the ramp design speed would be 30 mph based on the design guidelines presented herein. Read the entire page →
From page 113... ... 113 Figure 42. Case Study No. Read the entire page →
From page 114... ... 114 Plotted Speed Data Acceleration Data Figure 43. Case Study No. Read the entire page →
From page 115... ... 115 Figure 43 shows that vehicles are predicted to exit the freeway mainline ramp terminal at the gore point (i.e., MP 0.000) and enter Curve 1 at a speed of 44 mph. Read the entire page →
From page 116... ... 116 Figure 44. Case Study No. Read the entire page →
From page 117... ... 117 • Based on a highway design speed of 75 mph and a design speed of 30 mph for the controlling feature on the ramp proper, AASHTO policy assumes an average running speed of 26 mph at the end of the deceleration length entering the ramp proper. Based on the given inputs describing this ramp, the RSPM predicts a speed of 44 mph at the gore point which corresponds to the end of the deceleration length. Read the entire page →
From page 118... ... 118 Interchange type: Service Conditions: Constrained Area type: Urban/Suburban Ramp configuration: Diagonal Freeway characteristics Design speed: 70 mph Operating speed: 62 mph Speed limit: 65 mph Crossroad characteristics Type of roadway: Primary/major rd Speed limit: 35 mph Type of traffic control at terminal: Signal Queue storage length: 0.060 mi Milepost of crossroad ramp terminal 0.368 mi Ramp Proper Ramp grade: 1 % downgrade Max superelevation (emax) : 12 % Curve 1 PC milepost: 0.114 mi Curve length: 0.124 mi Curve radius 850 ft Superelevation: 10 % Design speed 45 mph Curve 2 PC milepost: 0.302 mi Curve length: 0.035 mi Curve radius 335 ft Superelevation: 10 % Design speed 30 mph Freeway Mainline Ramp Terminal Taper length 0.028 mi Divergence zone length 0.006 mi Deceleration length (downstream of the gore point) Read the entire page →
From page 119... ... 119 To evaluate the adequacy and consistency of the existing ramp design, one of the first checks for an exit ramp is to assess whether the design speeds of contiguous segments along the ramp proper decrease sequentially in a stepwise manner. For this case study, the design speed of Curve 1 from MP 0.114 to MP 0.238 is 45 mph. Read the entire page →
From page 120... ... 120 considered to redesign the ramp by either changing the horizontal alignment or lengthening the ramp to reduce speeds in proximity to the crossroad ramp terminal. Another option to consider is providing signing (e.g., Signal Ahead) Read the entire page →
From page 121... ... 121 Plotted Speed Data Acceleration Data Figure 47. Case Study No. Read the entire page →

From page 92...

... 92 Section 5. Case Studies This section demonstrates through several case studies how to evaluate the consistency of a ramp design based on the selected ramp design speed.