Grade separation

The composition of such transport axes does not have to be uniform; it can consist of a mixture of roads, footpaths, railways, canals, or airport runways.

Bridges (or overpasses, also called flyovers), tunnels (or underpasses), or a combination of both can be built at a junction to achieve the needed grade separation.

Grade-separated road junctions are typically space-intensive, complicated, and costly, due to the need for large physical structures such as tunnels, ramps, and bridges.

Their height can be obtrusive, and this, combined with the large traffic volumes that grade-separated roads attract, tend to make them unpopular to nearby landowners and residents.

Typically, large freeways, highways, motorways, or dual carriageways are chosen to be grade separated, through their entire length or for part of it.

For instance, it is extremely uncommon to find an at-grade junction on a British motorway; it is all but impossible on a U.S. Interstate Highway, though a few do exist.

Weaving can often cause side-on collisions on very fast roads with top speeds of up to 200 kilometres per hour, as well as the problem of blind spots.

Also in Britain, the Southern Railway later made extensive use of flying junctions on other parts of its busy former LSWR main line.

The most complex of these junctions, near Philadelphia Zoo, handles railway traffic for Amtrak, SEPTA, New Jersey Transit, Norfolk Southern, CSX Transportation, and Conrail.

Footbridges and subways (called underpasses in North America as well as in the United Kingdom when referring to roads) may be employed to allow pedestrians and cyclists to cross busy or fast streets.

Though introduced to Central Park in New York City in the 1860s, subways are far more common today in Europe, especially in countries such as the Netherlands and Denmark where cycling is strongly encouraged.