A change in the vertical alignment of a road is often implicated in head-on, intersection and overtaking crashes.

Vertical road realignment may be used to:

• reduce grade (travelling down steep grades can cause brake failure, for heavy vehicles in particular, while travelling up steep grades must be done slowly in some vehicles, and can result in overtaking crashes and poor traffic flow).
• increase the radius of a crest for adequate sight distance (by “shaving” the top off of the crest)
• minimise vertical acceleration changes (for example, sag curves can be very uncomfortable for vehicle occupants
• address drainage problems (water can collect in sag curves, causing a safety problem).

Intersection approaches may be realigned vertically (or horizontally, or both) to improve sight distance. Realigning the road is costly and time consuming because it usually involves rebuilding a section of the road.