The Patcham Interchange forms the junction of two of the south coast's most important routes, the A27 and the A23. It was built in the mid-1990s as part of the A27 Brighton and Hove bypass. The design of this junction is somewhat unusual due to the tight geographical constraints within which is resides. It consists of a typical dumbbell above the A27, with a D3 spur heading down to the Mill Road Roundabout on the A23. Additionally there is a free-flow slip from the A27 eastbound to A23 northbound. Heading in the opposite direction (A23 S to A27 W) is also free flow, but via a left hand turn bypass of the two roundabouts. Both these movements also offer lane gains/drops to both roads. The Mill Road Roundabout existed before the building of the Brighton and Hove bypass but only for access to local roads and as an end to what was then the S4 section of the A23.