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.

Overpasses and interchanges are very costly and are usually built as part of a freeway 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.

  • Reduction in intersection crash types.
  • Improved traffic flow.
  • Simplifies potentially complex movements typical at 'T' and 'X' intersections.
  • Removes the cost of running at-grade traffic control hardware.
  • A range of design options should be considered before an interchange layout is chosen.
  • An interchange should allow bicycle and pedestrian traffic to pass through the area.
  • Adding on-ramps and off-ramps to a freeway can increase high speed weaving and merging crashes.
  • Interchanges can negatively impact the appearance of an area.
  • Interchanges may separate communities due to their size.
  • Grade separating rail crossings can involve vertical realignment of a long length of rail track (because trains cannot travel on steep grades), which is very costly.

Benefits

  • Reduction in intersection crash types.
  • Improved traffic flow.
  • Simplifies potentially complex movements typical at 'T' and 'X' intersections.
  • Removes the cost of running at-grade traffic control hardware.

Implementation issues

  • A range of design options should be considered before an interchange layout is chosen.
  • An interchange should allow bicycle and pedestrian traffic to pass through the area.
  • Adding on-ramps and off-ramps to a freeway can increase high speed weaving and merging crashes.
  • Interchanges can negatively impact the appearance of an area.
  • Interchanges may separate communities due to their size.
  • Grade separating rail crossings can involve vertical realignment of a long length of rail track (because trains cannot travel on steep grades), which is very costly.

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Treatment Summary

Costs
High
Treatment life
20 years +
Effectiveness
25-40%

References

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