The road system in Milton Keynes is based on the idea of major roads between districts and minor roads within them. The major roads (often dual carriageways) run almost all the way across the city but the minor roads are just for local use. Most of the major roads have a national speed limit: 60 MPH for single carriageway and 70MPH for dual carriageway - thus traffic passing through Milton Keynes can cross from one side to the other in a relatively short time. The major roads run either north/south or east/west, though not in straight lines. This network is called a grid: the major roads are called 'grid roads'. Roundabouts are used at the junctions of the grid: traffic lights are rare. 