Junction 8/9 near Maidenhead, Berkshire, and High Wycombe, Buckinghamshire is the only one in the UK with dual numbers. This arose at the time when the M4 turned north near junction 8, where it met the A308, and headed for the original junction 9, where the motorway ended at a roundabout interchange with the A4.
Four level stack interchangesThe M4 has two of the three four-level stack interchanges in the UK, including the first UK example at junction 20, the Almondsbury Interchange with the M5. The other is junction 4b with the M25.
By October 2007, any hopes of junction 31 finally being built were dashed when Cardiff Council denied the planning application and renewed plans for the Thornhill interchange were abandoned.
At 231 miles (370km), the M6 is the UK's longest motorway. It runs from Catthorpe (junction 19 on the M1) to the Scottish Border. The M62 is the highest motorway in the UK. It reaches 1,220ft (372m) near the Pennine Way footbridge.