Introduction aux courbes horizontales

The center lines of highways and railroads are made up of straight segments, or tangents, joined by horizontal curves. 

These curves allow vehicles to transition smoothly between tangents, minimizing speed reduction and reducing the risk of accidents.

A circular curve connects two tangents, starting at the Point of curvature (P.C.) and ending at the Point of tangency (P.T.), the tangents meet at the Point of intersection (P.I.).

The first tangent is the back tangent, and the second is the forward tangent. The angle between them, I, is the intersection angle. The curve's radius is R, and the tangent distance is T.

The external distance, E, is the shortest distance from the P.I. to the midpoint of the curve. L represents the actual curve length.

A compound curve combines two or more arcs with different radii, while a reverse curve consists of two arcs bending in opposite directions. 

A spiral or transition curve has a radius that changes continuously, starting flat and gradually increasing in curvature as it transitions into a circular arc.