Example Curve Widening

Curve Widening:

This program determines the width that a horizontal curve pavement should be widened. This is covered by the Green Book (AASHTO 2018) sections 3.3.9 and 3.3.10. It uses the equations presented in the Green Book, not the tables. The equations used are as follows:


w = Widening (ft) = Wc - Wn

Wn = Width on Tangent (ft)

Wc = Width on Curve (ft) = N(U + C) + (N - 1)FA + Z

N = Number of lanes

U = Track width on curve (ft) = u + R - SQRT(R^2 - SUM(Li^2))

u = Track width on tanget (out to out of tires) (ft)

R = Radius of curve of turning roadway between first and second lane (ft)

Li = Wheelbase of design vehicle between consecutive axles and articulation points (ft)

C = Lateral clearance (ft)

FA = Width of front overhang (ft) = SQRT(R^2 + A(2L + A)) - R

A = Front overhang of inner lane vehicle (ft)

L = Wheelbase of single unit or tractor (ft)

Z = Extra width allowance (ft) = V/SQRT(R)

V = Design Speed (mph)

Theoretically, the value of Z also involves the number of lanes N and the curve radius R. But this hasn't been included in the equation for Z. This is compensated in the Green Book method by multiplying the two-lane widening width by 1.5 and 2.0 for 3-lane and 4-lane roads respectively. This results in "effective" widths for Z and thus Wc for roadways with more than 2 lanes.

Input variables:


Design Vehicle:

Select the Design Vehicle from the dropdown options

Design Speed:

Select the Design Speed from the dropdown options (mph)


Radius R - Radius of curve of turning roadway between first and second lane (ft)

Degree D (optional) - Delta angle of the curve between first and second lane in Decimal Degrees

Num Lanes - Number of turning lanes

Lane Width - Lane width (ft)>

emax - maximum superelevation>

Select the maximum superelevation from the dropdown options ('/')


Values less than 2.0 ft may be disregarded

The values for 3-lane roadways are 1.5 times the values for 2-lane roadways

The values for 4-lane roadways are 2.0 times the values for 2-lane roadways