Undivided highway

A single carriageway (British English) or undivided highway (American English) is a road with one, two or more lanes arranged within a single carriageway with no central reservation to separate opposing flows of traffic.

A two-lane road or two-lane highway is a single carriageway with one lane for each direction. A single-track road has a single lane with passing places for traffic in both directions. Road traffic safety is generally worse for high-speed single carriageways than for dual carriageways due to the lack of separation between traffic moving in opposing directions.

The term single carriageway is used for roads in the Republic of Ireland. Speed limits on single-carriageway roads vary depending on their classification: national primary roads and national secondary roads have a general speed limit of 100 km/h (62 mph), while regional roads and local roads have a general speed limit of 80 km/h (50 mph). In urban areas, the general speed limit is 50 km/h (31 mph).

The maximum UK speed limit for single-carriageway roads is lower than the maximum for dual-carriageway roads. The National Speed Limit, which is lower for built-up areas, only applies in places where a lower numeric speed limit is not in place. The UK has one major single-carriageway motorway, the A38(M), but a number of link roads at motorway interchanges are single-carriageway.

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Wikipedia