Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report (2021)

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99 Section 8. Abbreviations, Acronyms, Initialisms, and Symbols Term Description Equation No. AASHTO American Association of State Highway and Transportation Officials. NCHRP National Cooperative Highway Research Program. ITE Institute of Transportation Engineers. HSM Highway Safety Manual. SCL Speed-change lane. SPUI Single point urban interchanges. RSPM Ramp speed profile model. PC Point of curvature (i.e., point at which the curve begins). MC Midpoint of a curve. PT Point of tangency (i.e., point at which the curve ends). FI Fatal-and-injury severity levels. PDO Property-damage-only severity level. CMF Crash modification factor. SPF Safety performance function. N Predicted crash frequency per year on the ramp. 1 Lr Ramp length. 1 AADTr Average annual daily volume on the ramp. 1 a,b,c,d Regression coefficients. 1 Xi Milepost of the point of change from tangent to curve (PC) for curve i. 5,7,9 Ri Radius of curve i. 2 LC,i Length of horizontal curve i. 4,5,6,8,9,10 VFrwy Average traffic speed on freeway during off-peak periods of the typical day. 3,4,5,6,7 VXRoad Vehicle speed at the crossroad ramp terminal. 3,7,8,9,10 vmax,i Limiting speed for curve i. 2,4,6,8,10 vent,1 Average entry speed for Curve 1. 3,4,7,8 vext,1 Average exit speed for Curve 1. 4,8 vent,i Average entry speed for curve i (i = 2, 3, ...). 5,6,9,10 vext,i Average exit speed for curve i (i = 2, 3, ...). 5,6,9,10 vent,c,MC Average passenger car speed at the midpoint of the entrance ramp controlling curve. 11 vent,c,PT Average passenger car speed at the PT of the entrance ramp controlling curve. 12 vext,c,PC Average passenger car speed at the PC of the exit ramp controlling curve. 13 vext,c,MC Average passenger car speed at the midpoint of the exit ramp controlling curve. 14 Il2 Indicator variable for lane 2 (= 1 if predicting speed in the outside lane, 0 otherwise). 11,12 Il2-3 Indicator variable for lanes 2 and 3 (= 1 if predicting speed in the middle or outside lanes, 0 otherwise). 14 R Radius (measured to the inside of the traveled way). 11,12,13,14 Wl Lane width. 11 Wos Outside (left) shoulder width. 11,12,14

100 Term Description Equation No. Wis Inside (right) shoulder width. 11 Itk Indicator variable for trucks (= 1 if predicting truck speed, 0 otherwise). 11,12,13,14 Irs Indicator variable for curve radius type (= 1 if simple, 0 if compound). 14 Id Indicator variable for drop SCL (= 1 if present, 0 otherwise). 14 Ip Indicator variable for parallel SCL (= 1 if present, 0 otherwise). 14 Iw Indicator variable for weaving SCL (= 1 if present, 0 otherwise). 14 vc Average passenger car speed. 15 DC Degree of horizontal curvature. 15 Z Distance to the first downstream at-grade signalized or stop-controlled intersection. 15 vTZ Point speeds in the transition zone. 16 vFW Point speeds on the freeway mainline. 16 v85,DL 85th-percentile operating speeds on the deceleration lane. 17 v85,FW 85th-percentile operating speeds on the freeway mainline. 17,20 v85,UR 85th-percentile operating speeds on the upstream ramp curve. 18 v85,LR 85th-percentile operating speeds on the downstream ramp curve. 19 LDL Length of deceleration lane. 17,20 SLRP Speed limit on the ramp. 17 R1 Radius of the upstream ramp curve. 18,21,22 R2 Radius of the downstream ramp curve. 19,22 85VFW-DL 85th-percentile speed difference between freeway mainline and the deceleration lane. 20 85VDL-UR 85th-percentile speed difference between deceleration lane and the upstream ramp curve. 21 85VUR-LR 85th-percentile individual speed difference between upstream ramp curve and the downstream ramp curve. 22 D1 Deflection angle of the upstream ramp portion. 21 vc,PC Average passenger car speed at the PC of the exit ramp controlling curve. 23 vt,PC Average truck speed at the PC of the exit ramp controlling curve. 24 vc,MC Average passenger car speed at the midpoint of the exit ramp controlling curve. 25 vt,MC Average truck speed at the midpoint of the exit ramp controlling curve. 26 R Radius. 23,24,25,26  Longitudinal gradient. 25,26 va,u,l Average speed of unimpeded trucks at a level site. 27 D Distance from end of scale. 27

101 Term Description Equation No. v85Merge 85th-percentile speed of merging vehicles. 28,29,30 v85RL 85th-percentile speed of vehicles in the rightmost lane of the freeway mainline. 30 Θ Angle of convergence between entrance ramp and mainline. 28,29 L Entrance ramp length. 28,29,31 AADTSCL Annual average daily traffic volume on entrance ramp SCL. 28 QSCL Equivalent hourly traffic volume on entrance ramp SCL. 29 QRL Equivalent hourly traffic volume in rightmost freeway mainline lane. 30 D85Merge 85th-percentile passenger car merging distance. 31 CMFenr|agg Aggregated ramp entrance CMF. 32 Penr Proportion of freeway segment length adjacent to a ramp entrance. 32 Ienr Average ramp entrance length. 32 VMC Predicted vehicle speed at the MC. 33,35 VPC Vehicle speed at the beginning of a curve (i.e., initial point of horizontal curve – PC). 33,34,35,36 R Curve radius. 33,34,35,36 VSL Hwy Regulator speed limit on the highway (or freeway) mainline. 33,34,35 VPT Predicted vehicle speed at the end of a curve (PT). 34,36 IDN Indicator variable for downward slope (= 1 if the overall ramp slopes downward, 0 otherwise). 35 ILV Indicator variable for level slope (= 1 if the overall ramp is level, 0 otherwise). 35 IUP Indicator variable for upward slope (= 1 if the overall ramp slopes upward, 0 otherwise). 35 LC Curve length. 36 VTan-End,C Predicted vehicle speed at the end of a tangent if the tangent is followed by a curve. 37,39 VTan-End,NC Predicted vehicle speed at the end of a tangent if the tangent is not followed by a curve. 38,40 VTan-Begin Predicted vehicle speed at the beginning of a tangent. 37,39,40 LT Tangent length. 37,38,39 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). 37 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). 37 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). 37 Rnext Radius of subsequent curve. 39