Selecting Ramp Design Speeds, Volume 1: Guide (2021) / Chapter Skim
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57 Section 4. Design Tool to Evaluate Ramp Designs for Consistency with the Selected Ramp Design Speed A tool to estimate vehicle speeds along interchange ramps could help designers and traffic engineers in conducting operational analyses and making design decisions.
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58 4.1 Input Data for the RSPM The RSPM contains two analysis worksheets – one for entrance ramps and one for exit ramps. The input cells on these worksheets are color-coded blue or orange.
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59 • General Characteristics - Ramp grade (percent)
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60 - Proportion of gap acceptance length used by merging vehicles, PGap Acpt [From a design perspective, PGap Acpt = 1.0 meaning merging vehicles are expected to use the entire gap acceptance length. However, field data show that many drivers use only a portion of the gap acceptance length prior to merging onto the freeway.
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61 Figure 21. Elements and Dimensions of Freeway Mainline Ramp Terminals Input into RSPM for Entrance Ramps RSPM Input Data – Exit Ramps Figure 22 provides a screen shot of the data input portion of the exit ramp analysis worksheet.
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62 Inputs for Exit Ramps: • Speeds - Freeway mainline design speed, VDS Hwy (mph)
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63 • Ramp Proper - Input for individual tangents: • Design speed of tangent, VDS Tan (mph)
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64 Figure 22. Input Data for RSPM for Exit Ramps
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65 Figure 23. Elements and Dimensions of Freeway Mainline Ramp Terminals Input into RSPM for Exit Ramps RSPM Input Data – Error Checks and Warning Messages The RSPM performs several checks of the input data to determine if any errors were made or inconsistent values entered into the spreadsheets.
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66 message will also appear to the right of the curve input data stating "Check milepost and length data," and a message will appear above the output graph stating "Check curve data." • If the analyst specifies a milepost for the gore point that is less than the previous curve's ending milepost, the analyst will receive a message box that reads "Invalid data entry. The last curve cannot extend past the gore point." A message will also appear to the right of the input data stating "Check milepost and length data" and "Check gore location data", and a message will appear above the output graph stating "Check location data for gore and last curve." On the exit ramp worksheet, the following error checks and warning messages are provided: • If any required data elements are not entered, a note will appear above the output graph to remind the analyst to "Provide all missing data elements and re-run the analysis." • If the analyst specifies a beginning milepost for a curve that is less than the previous curve's ending milepost, the analyst will receive a message box that reads "Invalid data entry.
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67 RSPM Output Data – Entrance Ramps Figure 24 shows the graph containing estimated vehicle speeds along an entrance ramp based on the inputs in Figure 20. In this example, the ramp consists of four tangents and three curves in alternating series, followed by the speed-change lane.
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68 Design speeds for the ramp components are plotted as follows: • The design speed for each curve is shown as the specified value for the curve. • The design speed for each tangent is shown as the specified value for the tangent, or for the next downstream curve if no value is specified, or the freeway mainline if there is no downstream curve.
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69 Table 11. Acceleration Rate Calculations for Sample Entrance Ramp RSPM Output Data – Exit Ramps Figure 25 shows the graph containing vehicle speed calculations for an exit ramp based on the inputs in Figure 22.
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70 Figure 25. Graphical Illustration of Vehicle Speeds for Sample Exit Ramp Design speeds for the ramp components are plotted as follows: • Along the speed-change lane and the first tangent, the design speed of the freeway mainline is shown as an upper limit.
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71 freeway mainline ramp terminal. The second portion begins where the diverge maneuver is complete and the vehicle is positioned within the speed-change lane.
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72 For example, the speed profile shown in Figure 25 is for an exit ramp with three curves (see input data in Figure 22)
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73 mainline ramp terminal. By definition, the speed-change lane includes the tapered area.
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74 Crossroad Ramp Terminal The vehicle speed at the beginning point of an entrance ramp at the crossroad ramp terminal, VXRoad, is estimated based on the traffic control at the crossroad ramp terminal or is input by the analyst. Default speeds are based on the traffic control type at the crossroad ramp terminal as follows: • Stop, yield, or signal: 15 mph.
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75 Equations 5 and 6 are the calibrated tangent speed models. By applying these models to the curve points, speeds are predicted along the ramp proper as if it is a tangent ramp.
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76 where: VTan-End,C = Predicted vehicle speed at the end of a tangent if the tangent is followed by a curve, mph. VTan-Begin = Predicted vehicle speed at the beginning of a tangent, mph.
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77 Figure 27. Primary Components of an Entrance Ramp near the Freeway Mainline Ramp Terminal and Associated Speed Points of Interest The speed at the gore point (VG)
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78 The speed calculations for Scenarios 2 and 3 apply to the tangent speed model described by Equation 11. In all cases, the upper bound for the speed at the gore point is the average operating speed on the highway (or freeway)
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79 65 3.18 2.57 2.36 1.96 70 2.62 2.49 2.05 75 2.76 2.60 2.12 Table 15. Entrance Ramp Acceleration Rates along Gap Acceptance Length, |Gr| ≥ 2.5 percent (adapted from Torbic et al., 2012)
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80 Table 16. Input Data for Ramp Curve Speed Prediction Procedures in ISATe (adapted from Bonneson et al., 2012)
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81 where, vext,1 = Average exit speed for Curve 1, ft/s. The boundary conditions of Equation 15 indicate that the value computed (vext,1)
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82 Figure 28. Graphical Illustration of Vehicle Speeds for Sample Entrance Ramp with Alternate Profile Speed Calculations for Exit Ramps Applicable speed calculations are presented in the order of the primary components of an exit ramp based on the direction of travel: • The freeway mainline ramp terminal.
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83 Figure 29. Primary Components of an Exit Ramp near the Freeway Mainline Ramp Terminal and Associated Speed Points of Interest The diverge location where the vehicle enters the speed-change lane is typically about one-tenth the length of the speed-change lane (Torbic et al., 2012)
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84 Vehicle speed at the gore point is estimated based on vehicle speed at the diverge location and field-measured deceleration rates (Torbic et al., 2012) as follows: 𝑉 = 36005280 52803600𝑉 + 2𝑎 (1 − 𝑃 )
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85 To obtain speed estimates at the midpoint and end of the curve, knowledge of vehicle speed at the beginning of the curve (PC) is required.
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86 Alternate Speed Prediction Models for Exit Ramps The RSPM spreadsheet tool also includes speed prediction models developed for use with the HSM safety analysis procedures for interchange ramps (Bonneson et al., 2012) that can be used to estimate vehicle speeds on exit ramps.
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87 Step 7 -- Calculate Speed on Successive Curves: The entry and exit speeds for Curve 3 and successive curves are computed by applying Steps 5 and 6 for each curve. This step is the same as Step 7 for the entrance ramp procedure.
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88 • Graphical illustration of estimated speeds to design speeds of individual ramp components. Check of Acceleration/Deceleration Rates After the speeds are estimated at the beginning and end of each ramp section, the average acceleration rate for the section is computed using basic kinematics as follows: 𝑎 = 𝑉 − 𝑉2𝐿 (28)
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89 Check of Merge Speed on Entrance Ramps For the design of an entrance ramp, the Green Book states that motorists should attain a merge speed within 5 mph of the speed of the highway (or freeway) by the time they reach the end of the gap acceptance length (LGap Acpt)
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90 default based on field observation. However, this value could be adjusted by the analyst so that the diverge location corresponds with the beginning of the divergence zone length and deceleration length.
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91 • The 85th percentile of the distribution of observed operating speeds is a frequently used measure associated with the design of geometric features. The speed prediction models incorporated in the RSPM yield estimates of average operating speeds, not the 85th percentile of operating speeds.
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