Simister Island is where the M60, M62 and M66 meet, just north of Heaton Park. It is junction 18 of the M60 and of the M62, and junction 4 of the M66. National Highways counts about 90,000 vehicles a day through it.
Its quirk is that the motorways do not run straight through in the way the names suggest. Coming round the M60 clockwise, the main carriageway carries on as the M62 to Leeds. To stay on the M60 you leave by the slip, go halfway round the roundabout and rejoin. Going the other way, the M60 from Middleton runs straight on into the M66.
Every movement that uses the roundabout is a right turn. Straight on and left turns all have their own roads. So each car on the roundabout rides two quarters of it: the first in the inner lanes, the second in the outer lanes, ready for its exit. The lane colours on the map follow that spiral, coloured by where each lane leads.
The cars follow the real lanes and slip roads from OpenStreetMap. The roundabout's signals run a simple two-stage plan: entries on opposite sides go together while the circulating traffic in front of them is held. Traffic levels and the split between destinations are made up to look plausible; they are not counts.
Drag the map to move it, and scroll or pinch to zoom. Click a car or a route to follow its way through. Space pauses the traffic.