The case for 2 + 1 layouts

Overtaking is amongst the most dangerous manoeuvres which drivers undertake and, on single-carriageway roads, it becomes progressively less likely that safe overtaking opportunities will arise as traffic volumes increase. Dual-carriageway roads offer the safest conditions for overtaking, but they are only likely to be an economically justifiable design solution at high traffic flows. This led to the National Roads Directorate of the Scottish Office Industry Department (SOID) commissioning a study of wide single carriageways.