U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
REPORT 
This report is an archived publication and may contain dated technical, contact, and link information 

Publication Number: FHWAHRT17098 Date: January 2018 
Publication Number: FHWAHRT17098 Date: January 2018 
*©ASCE. Reproduced with the express written authority of the American Society of Civil Engineers (ASCE). Excerpted from the ASCE publication
Journal of Transportation Engineering, 131(4), ASCE, April 2005, pp. 261269.
†©ASCE. Reproduced with the express written authority of the American Society of Civil Engineers (ASCE). Excerpted from the ASCE publication
Journal of Transportation Engineering, 131(4), ASCE, April 2005, pp. 408418.
‡©TRB.
—No information.
FF_{85} = 85thpercentile freeflow operating speed (mph (km/h));
SL = posted speed limit (mph (km/h));
AD = access density;
park = indicator variable for the presence of parking;
V_{85MC} = 85thpercentile speed at middle of curve (mph (km/h));
Δ_{85}V = 85thpercentile speed differential calculated as 85thpercentile value of speed differentials of individual drivers;
V_{T} = approach tangent speed (mph (km/h));
DFC = deflection angle of circular curve (degrees);
SW = shoulder width (ft (m));
curve  dir = curve direction;
drv_{flag} = driveway flag; EV_{85} = estimated 85thpercentile speed (mph (km/h));
PSL = posted speed limit (mph (km/h));
V_{85, midpoint} = 85thpercentile speed at freeflow passenger cars at the curve midpoint (mph (km/h));
Δ = deflection or intersection angle of horizontal curve (degrees);
LC = length of horizontal curve (ft (m));
G_{1} = approach grade (percent);
V_{85, approach} = 85thpercentile speed at the approach location (mph (km/h));
V_{p} = posted speed limit (mph (km/h));
T_{ADT} = average daily traffic (vehicles per day);
V_{95, midpoint} = 95thpercentile operating speed at the midpoint of the horizontal curve (mph (km/h));
V_{95, approach} = 95thpercentile operating speed on the approach tangent to the horizontal curve (mph (km/h));
K = rate of vertical curvature (ft/percent (m/percent));
SP = posted speed limit (mph (km/h));
GM_{L} = geometric measure for long tangents between horizontal curves (ft^{2} (m^{2}));
GM_{S} = geometric measure for short tangents between horizontal curves (ft^{2} (m^{2}));
D = degree of curvature;
I = deflection or intersection angle of horizontal curve (degrees);
LW = lane width (ft (m));
CCR = curvature change rate (degrees per half mile);
V_{F} = desired speed of the 85thpercentile car (mph (km/h));
V_{85, tan} = estimated 85thpercentile operating speed on tangent (mph (km/h));
V_{85, curve} = 85thpercentile speed on the curve (mph (km/h));
V_{T85} = estimated 85thpercentile operating speed on tangent (mph (km/h));
PS = posted speed limit (mph (km/h));
R_{p} = radius of preceding horizontal curve (ft (m));
RHR = roadside hazard rating;
L_{T} = length of approach tangent (ft (m));
V_{C85} = estimated 85thpercentile operating speed on the curve (mph (km/h)).