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Intersection - Grade Separation​

Most crashes happen at intersections. The best way of stopping conflicting intersection movements is placing the intersecting roads at different levels, or grade separating them. This can be done with overpasses or interchanges.

An overpass is a simple grade separation of two roads whereby there is no actual link between them and hence no exchange of traffic is possible. Overpasses are typically used when a minor road crosses a major road, and where a rail line crosses a road.

An interchange allows traffic to move between two or more roads which are grade separated. Interchanges vary from simple arrangements with ramps and intersections at the minor road to complex layouts where two or more freeways (major highways or motorways) connect.

Compact interchanges have a tighter layout and are economic solutions for divided roads of lower design speeds. These interchanges require a single overpassing bridge and may be integrated with the road network of neighbouring urban developments.

Grade-separated intersections generally perform well in road safety since conflicting movements are removed. However, safety problems could arise at merging and diverging areas, and along any sharp curves on connecting roads.

Grade-separated intersections should be simple in layout and changes in design speeds should be obvious to drivers and riders. Successive merging or diverging points should be introduced in steps with ample separation to simplify the driving task. Safety barriers and parapets also require elaborated design since grade-separation is associated with diverge gores and frequent embankments, drops, underpasses or bridges.

Overpasses and interchanges are very costly and are usually built as part of a motorway system where large traffic flows justify the cost. Occasionally, interchanges and rail overpasses are built on busy urban highways when justified by road safety and traffic flow improvements.

The Star Rating Demonstrator is a freely available tool with the iRAP online software, ViDA. With the Star Rating Demonstrator, it is possible to explore the impact that this Safer Roads Treatment has on risk.

Treatment Summary



Treatment life

20 years +

Potential casualty reduction


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