Geometric road design: the alignment, the grade line and the typical cross-section

Designing a road looks like drawing, but it is a chain of linked decisions. As soon as you choose the category and the design speed, you have also determined the minimum radius, the maximum gradient and the sight distance. Every later deviation "to make it more economical" affects safety, not just cost.

The horizontal alignment: why arcs are not enough

A direct transition from a straight to an arc means an instantaneous change of curvature — and consequently of lateral acceleration. At high speed this is felt as a push sideways and forces the driver to correct suddenly. The clothoid, the spiral that increases curvature gradually, solves this and at the same time provides the length needed to develop the superelevation.

The grade line: the limits are double

The longitudinal profile requires a balance between the maximum gradient, the length of vertical curves and stopping sight distance. A convex curve is limited by visibility; a concave one by comfort and by the reach of headlights at night. The minimum gradient also matters, because without it water does not leave the surface.

The typical cross-section determines the cost

The width of the carriageway is the visible part; the part that weighs on the budget is what lies below and to the sides: the pavement layers, the embankment slopes, the ditches, the retaining walls. Balancing cut with fill along the alignment is often the difference between a buildable project and one that doubles the earth-haulage item.

How AlbGIS helps

• AlbGIS Infra — our desktop application for the horizontal alignment with clothoids, the grade line with vertical curves and typical cross-sections with all elements of the road body.
• Culverts — sizing of water crossings under roads according to HEC-22 / HDS-5.
• DEM to Contours — preparation of the existing terrain on which the project sits.