Extra travelled way width needed on horizontal curves so that the design vehicle keeps its tangent clearances: rear-wheel off-tracking, front overhang and an allowance for steering difficulty, per the AASHTO Green Book. Particularly useful for highway geometric design.
w = WC − WN, WC = N(U + C) + (N − 1)FA + Z
U = u + R − √(R² − ΣLi²) — track width of the design vehicle on the curve (off-tracking of the rear wheels).
FA = √(R² + A(2L + A)) − R — width taken by the front overhang of the inner-lane vehicle, L = L₁.
Z = 0.1·V / √R — extra width for the difficulty of driving on a curve (V in kph, R in m).
C = lateral clearance per vehicle; WN = N × lane width. All in metres.
For a two-lane road with the vehicle and lane width entered on the left, rounded up to 0.1 m. highlighted = current speed column. Greyed values are below 0.6 m and may be disregarded.
On curves without spirals, place the widening on the inside edge of the travelled way. Where spiral transitions are used it may be split equally between the inside and outside edges. Run the widening in gradually, normally over the superelevation runoff length, with part of it on the tangent in the same way as the runoff, so that the edge forms a smooth, natural-looking alignment with no kinks. Place the centreline marking at the middle of the widened travelled way. Widening below about 0.6 m may be disregarded. The method is for two-lane single carriageways: for a three-lane road multiply the widening by 1.5 and for a four-lane undivided road by 2. Curves on divided highways are usually flat enough for widening to be negligible, and single-lane ramps and turning roadways use the turning roadway width criteria instead.
On a curve the rear wheels of a truck follow a path inside the front wheels (off-tracking), the front overhang swings towards the next lane, and drivers find it harder to stay in the middle of the lane. Widening restores the clearances the design vehicle has on the tangent.
AASHTO gives w = WC − WN with WC = N(U + C) + (N − 1)FA + Z. U is the track width of the design vehicle on the curve, C the lateral clearance per vehicle, FA the width of the front overhang and Z an allowance for steering difficulty. The tool shows each term.
Use the largest vehicle expected to use the road regularly — usually a WB-20 or WB-19 semitrailer on highways and a single-unit truck or bus on local roads. The dimensions filled in are typical values; edit them to match the design vehicle adopted by your road authority.
AASHTO suggests that widening of less than about 0.6 m may be disregarded. Widening is most often needed on sharp curves, narrow lanes and roads with a high proportion of large trucks.
The method follows A Policy on Geometric Design of Highways and Streets, 7th Edition (2018), traveled way widening on horizontal curves. Always confirm against the edition adopted by your road authority.