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.
Upgrade of Junction 25, which is the main access point to Taunton and provides a critical link to the second strategic route into the South West via the A358 and A303.
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.