Three-Level Stacked Roundabout Interchange
Cheaper than a row of run-down terraces in the path of a planned motorway, and about as cheerful, the stacked roundabout is the blight of four-way junctions everywhere. Everything about it screams "we spent money elsewhere", and there are only a notable few locations where these things can actually handle the traffic flows. They work best where a motorway crosses a fairly important A-road, but whoever first placed one to connect two motorways needs a good slap. They are, of course, the bigger brother of the standard Roundabout Interchange - basically both directions get a flyover. The contrast between the success of the two types is marked.
Where to spot them
Look for the traffic jams.
- In comparison to any other four-way fully grade-separated junction, cheap and easy to build.
- Has minimal land-take outside the roads that are crossing.
- Easy to navigate and to correct navigational errors.
- Low capacity - when both roads are dualled, the amount of traffic interchanging here is likely to be quite high.
- A complete pain to upgrade, other than adding traffic lights. See M60 J18 or M1 J42 for the type of struggle involved in increasing capacity.
- Excessively long name.
- Strange siren-like quality that causes otherwise level-headed planners to build these at major intersections.
Most variations that are found on this type of junction are through efforts to increase the capacity of the damned things.
The usual start point is to add traffic lights at every entry point. If (when) this fails to halt the problem, the usual strategy is to add left-turn lanes bypassing the roundabout, and in desperate cases, widen the circulatory carriageway (expensive because of bridge widenings). Simster Interchange's roundabout has reached six lanes now, which is double the width of any carriageway passing through.
A few desperate cases require more action. Lofthouse Interchange has had new sliproads tunneled underneath it at great expense; the A2/M25 junction will be having new elevated sliproads in two directions added in the near future.
The most exciting variant is at Great Barr, which is just mad.