The Great Barr Interchange is junction 7 of the M6, intersecting with the A34 between Walsall and Birmingham. Whilst at first glance it appears to be a 3-level stacked roundabout, the west facing sliproads are attached to the inside of the roundabout instead, probably because of the M5 nearby along with the lack of space between the westbound M6 and the northern end of the northbound exit sliproad from the A34. As a result, the interchange works slightly differently to a roundabout. Another unusual feature that is very rarely seen with 2-level roundabout interchanges, 3-level stacked roundabout interchanges or roundabouts in general, is the absence of stop lines and other give way lines on the roundabout with the exception of the southern end of the A34 southbound exit sliproad. Combined with the absence of traffic lights on the roundabout, this feature in effect makes this interchange almost fully free flowing with many movements catered for by a circular sequence of lane gain from and lane drop to the sliproads, all around the roundabout. Unfortunately the very short weaving spaces on the eastern side of the roundabout severely reduce the safety and effectiveness of this interchange.
Early design for the junction
Designs for this junction were compromised by its close proximity to the M5. The earliest known plan shows a standard roundabout interchange sited to the east of the present junction with a short spur connecting it to the A34. This idea of this layout is to maximise the distance between the west facing slips and junction 8
A slightly later design again shows a standard roundabout interchange but now it's situated in its present location which brings the west facing slip roads close to the M5. To overcome this the eastbound exit slip has been lengthened and leaves the M6 before the M5 joins. However there is no similar treatment for the equivalent westbound movements. This may have been a factor in the rejection of this layout.
Great Barr Interchange as it is today.