High Quality Content by WIKIPEDIA articles! A dual carriageway (North American English: divided highway) is a class of highway with two carriageways for traffic travelling in opposite directions separated by a central reservation (North American English: median). Roads with two or more carriageways which are designed to higher standards with controlled access are generally classed as motorways, freeways, etc., rather than dual carriageways. A road without a central reservation is a single carriageway (North American English: undivided highway) regardless of the number of lanes. Dual carriageways have improved road traffic safety over single carriageways and typically have higher speed limits as a result. In some places, express lanes and local/collector lanes are used within a local-express-lane system to provide more capacity and to smooth traffic flows for longer-distance travel.