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Selecting Ramp Design Speeds, Volume 1: Guide (2021)

Chapter: Section 8. Abbreviations, Acronyms, Initialisms, and Symbols

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Page 127
Suggested Citation:"Section 8. Abbreviations, Acronyms, Initialisms, and Symbols." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Page 127
Page 128
Suggested Citation:"Section 8. Abbreviations, Acronyms, Initialisms, and Symbols." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
×
Page 128
Page 129
Suggested Citation:"Section 8. Abbreviations, Acronyms, Initialisms, and Symbols." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Page 129

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127 Section 8. Abbreviations, Acronyms, Initialisms, and Symbols Term Description Equation No. General Terms AASHTO American Association of State Highway and Transportation Officials NCHRP National Cooperative Highway Research Program ITE Institute of Transportation Engineers HSM Highway Safety Manual HCM Highway Capacity Manual VPI Vertical point of intersection SSD Stopping sight distance DSD Decision sight distance RSPM Ramp speed profile model FMRT Freeway mainline ramp terminal CRT Crossroad ramp terminal RP Ramp proper C-D Collector-distributor road PC Point of curvature (i.e., point at which the curve begins) MC Midpoint of a curve PT Point of tangent (i.e., point at which the curve ends) Gr Overall ramp grade Terms Associated with Speeds / Accelerations VDS Hwy Highway (or freeway) design speed VOS Hwy Average operating speed on the highway (or freeway) mainline 5,6,7,8,9,10,11, 12,14,15,16,17,24 VSL Hwy Regulator speed limit on the highway (or freeway) mainline 7,9,20 VAcc Length(i) Speed at the beginning of the acceleration length 2 VAcc Length(f) Speed at the end of the acceleration length 2 VMerge Predicted vehicle speed where the vehicle merges from the freeway speed-change lane onto the highway (or freeway) 12 VDec Length(i) Speed at the beginning of the deceleration length 3,4 VDiverge Predicted vehicle speed at the diverge location 18,19 VDec Length(f) Speed at the end of the deceleration length 3 Vi,j,NC Predicted vehicle speed for curve i at point j (j = PC, MC, or PT), by applying the model for tangents with no downstream curve 5 Vi,j,C Predicted vehicle speed for curve i at point j (j = PC, MC, or PT), by applying the model for tangents with a downstream curve 6 VXRoad Vehicle speed at the crossroad ramp terminal 5,6,11,14,24,25,26,27 VDS Curve Design speed of a curve 1 VDS Tan Design speed of tangent Va Speed after tn sec of deceleration without brakes 3,4 VMC Predicted vehicle speed at the MC 7,8,20 VPC Predicted vehicle speed at the beginning of a curve (i.e., initial point of horizontal curve – PC) 7,8,20,21 VPT Predicted vehicle speed at the end of a curve (PT) 8,21 VTan-End,C Predicted vehicle speed at the end of a tangent if the tangent is followed by a curve 9,22

128 Term Description Equation No. VTan-Begin Predicted vehicle speed at the beginning of a tangent. 9,10,22,23 VTan-End,NC Predicted vehicle speed at the end of a tangent if the tangent is not followed by a curve. 10,23 VG Predicted vehicle speed at the gore point. 11,12,19 VPT,f Predicted vehicle speed at the PT of final curve on the ramp proper. 11 a Acceleration rate. 2 tn Deceleration time without brakes (assumed to be 3 sec). 3,4 dn Deceleration rate without brakes. 3,4 dwb Deceleration rate with brakes. 3 aGap Acpt Acceleration rate along gap acceptance length. 12 PGap Acpt Proportion of gap acceptance length used by merging vehicles [default = 0.50, based on Torbic et al., (2012)]. 12 vmax,i Limiting speed for curve i. 13,15,17,25,27 vent,1 Average entry speed for curve 1. 14,15,24,25 vext,1 Average exit speed for curve 1. 15,25 vent,i Average entry speed for curve i (i = 2, 3, ...). 16,17,26,27 vext,i Average exit speed for curve i (i = 2, 3, ...). 16,17,26,27 ∆𝑉 Mean speed differential between freeway mainline speed and the initial speed of vehicles entering the speed- change lane. 18 aSCL Deceleration rate along the speed-change lane. 19 aavg Average acceleration rate. 28 Vi Vehicle speed at the beginning of the section. 28,29,30 Vf Vehicle speed at the end of the section. 28 aDesign Maximum design acceleration rate. 29,30 Terms Associated with Lengths, Mileposts, and Locations LAcc Length Acceleration length. 2 LAcc Length RP This length is that portion of the acceleration length measured upstream from the gore. LDec Length Deceleration length. 3 LDec Length RP That portion of the deceleration length measured downstream from the gore point. LGap Acpt Gap acceptance length. 12 LDiv Zone Divergence zone length. LSCL Length of speed-change lane. 19 PSCL Proportion of speed-change lane located upstream of the diverge point (default value is 0.10 based on Torbic et al. (2012)). 19 LTaper Taper length. Xj Milepost for point j. 5,6 LT Tangent length. 9,10,22 LC Curve length. 21 LC,i Length of horizontal curve i. 15,16,17,25,26,27 L Segment length. 28 XPT,f Milepost for the PT of final curve on the ramp proper. 11 XG Milepost for the gore point. 11 Xi Milepost of the point of change from tangent to curve (PC) for curve i. 14,16,24,26 XPC Milepost of the beginning of the curve. 20 XMC Milepost of the MC. 20 XXRoad Milepost at the crossroad ramp terminal.

129 Term Description Equation No. A Location of the beginning of the acceleration length (i.e., the boundary between the ramp proper and freeway mainline ramp terminal of an entrance ramp). D Location of the end of the deceleration length (i.e., the boundary between the freeway mainline ramp terminal and ramp proper of an exit ramp). LLength to Q Length from the beginning of the crossroad ramp terminal to the end of the storage length. LDecis Length 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 (see Figure 19). LMan Length Distance that a vehicle travels after the breaks are applied (see Figure 19). LQ Storage Stopping queue or storage length (see Figure 19). Terms Associated with Curve Elements and Parameters Rmin Minimum radius of curvature. 1 emax Maximum superelevation rate. 1 fmax Maximum side friction factor. 1 R Curve radius. 7,8,20,21 nc Number of curves on the ramp proper. 11 Ri Radius of curve i 13 IDN Indicator variable for downward slope (= 1 if the overall ramp slopes downward, 0 otherwise). 20 ILV Indicator variable for level slope (= 1 if the overall ramp is level, 0 otherwise). 20 IUP Indicator variable for upward slope (= 1 if the overall ramp slopes upward, 0 otherwise). 20 Rnext Radius of subsequent curve. 22 Rmax Maximum radius of curve. 22 Terms Associated with Tangent Elements and Parameters I55 Indicator variable for 55-mph regulatory speed limit on the highway (or freeway) mainline (= 1 if the regulatory speed limit is 55 mph, 0 otherwise). 6,9 I60 Indicator variable for 60-mph regulatory speed limit on the highway (or freeway) mainline (= 1 if the regulatory speed limit is 60 mph, 0 otherwise). 6,9 I65 Indicator variable for 65-mph regulatory speed limit on the highway (or freeway) mainline (= 1 if the regulatory speed limit is 65 mph or above, 0 otherwise). 6,9

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Selection of a design speed should be based upon the anticipated operating speed, topography, adjacent land use, modal mix, and functional classification of the roadway.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 313: Selecting Ramp Design Speeds, Volume 1: Guide provides further detail for selecting an appropriate ramp design speed than presented in the 2018 Green Book, to address several overarching challenges that may lead to confusion or inconsistent interpretation of existing AASHTO guidance for selecting an appropriate ramp design speed.

Supplemental to the document are NCHRP Web-Only Document 313: Selecting Ramp Design Speeds,Volume 2: Conduct of Research Report and Ramp Speed Profile Model worksheets.

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