The A30 begins at Henlys Roundabout, where the route stems from the A4 near Hounslow. It runs south of the Southern Perimeter Road, Heathrow Airport and north of Ashford and Staines-upon-Thames, before reaching the M25 motorway orbital motorway. This first section is entirely dual carriageway. Taken with the A4, its natural continuation which nearby becomes non-dualled towards the M25, the section constitutes one of five routes into the southern half of London which reach Inner London with at least a dual-carriageway, the others being the A3 (M), the M3, the M20 and A2, however approximately one mile before reaching Inner London it is combined with the London variants of the M3 and M4 approaches.