Science Fair Project Encyclopedia
In the U.K., although the term dual carriageway applies to any road with separated lanes, it is frequently used as a descriptive term for major routes built in this style. Such major dual carriageways usually have two lanes of traffic in each direction, with the lane nearest the centre being reserved for overtaking. Occasionally dual carriageways have only one lane in each direction, or more than two lanes each way (usually to permit easier overtaking of slower uphill traffic). Different speed limits apply on dual carriageway sections than apply on single carriageway sections of the same class of road, except in cities and built-up areas where the dual carriageway is more of a safety measure, often intended to prevent pedestrians from crossing a busy road.
Turning right (that is, across the line of traffic heading in the opposite direction) is usually only permitted at specific locations. Often the driver will be required to turn left (away from the dual carriageway) in order to loop around to an access road that permits crossing the major road. Roundabouts on dual carriageways are relatively common, especially in cities or where the cost of a grade-separated junction would be prohibitive.
A long-distance dual carriageway with grade-separated junctions and which meets other requirements may be upgraded to motorway standard, denoted as an (M) added after the road number (eg. "A1(M)").
The national speed limits on dual carriageways not in built-up areas are as follows. Local speed limits, where indicated by signs, take precedence over these.
|Type of vehicle||Speed limit|
|Car up to 2 tonnes/motorcycle||70 mph (about 112 km/h)|
|Car with caravan or trailer||60 mph (about 96 km/h)|
|Bus or coach up to 12 m long||60 mph|
|Goods vehicle up to 7.5 t||60 mph|
|Goods vehicle over 7.5 t||50 mph (about 80 km/h)|
In the U.S. this type of road is called a divided highway and has a median strip between the traffic directions.
Junctions may be at-grade or grade-separated, and there may be gaps in the median strip to allow turning and crossing.
Swiss dual carriageways are referred to as Autostrasse. There may or may not be a central reservation.
A very early example (perhaps the first) of a dual carriageway was the Via Portuensis , built in the 1st century by the Roman emperor Claudius between Rome and its port Ostia at the mouth of the Tiber.
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