What it means: how much slower traffic moves than it would on empty roads, as average seconds lost for each mile driven on England's local A roads, which carry most local journeys. Over a 10-mile trip, 45 seconds a mile adds about 7½ minutes. The chart is a rolling 12-month average to smooth out seasons. A change of one second or more counts as a change in status.

Source: DfT, average delay on local 'A' roads, table CGN0504a, from in-vehicle GPS data.