Roadway Cross-Slope Edge Drop
Roadway Cross-Slope Edge Drop
Every travel lane is built with a slight cross slope so rainwater sheds toward the shoulder instead of pooling in the wheel paths. That cross slope, multiplied across the width of the lane, produces a small but real elevation drop from the crown of the road to its edge.This edge drop matters for drainage design, for setting curb and gutter elevations, and for checking that a paving overlay does not create an unsafe lip at a lane edge — small percentages become tens of millimeters once multiplied across a full lane width.
The elevation drop is y = slope × width, the cross slope times the lane width. where slope is the cross-slope ratio and width is the lane width over which it is measured.
Inputs
slope_x := 0.02=0.02
w_lane := 3.6 m=3.6 m
Multiplying the cross-slope ratio directly by the lane width gives the physical elevation change from crown to edge.
y_drop := slope_x·w_lane=72 mm
Results
A drop of roughly 70 mm across a standard 3.6 m lane at a typical 2% cross slope is normal and imperceptible to drivers, but it is exactly the kind of number pavement overlay crews must track so successive lift thicknesses do not distort the drainage cross slope. Steeper cross slopes (used on superelevated curves) produce proportionally larger edge drops.
Assumptions & limitations
- Straight, planar cross slope (no crown break within the lane).
- Standard 2% slope used for tangent sections; superelevated curves differ.
- Pavement overlay thickness effects not included.