01-30-2016, 12:21 PM
The core problem is when the input speed has a big delta compared to the roundabout speed, so you get bunching as people decelerate approaching the roundabout. Then there are no gaps between cars for the cross traffic to feed in to.
Ideally, everyone would have a gap, and everyone else would fit into that gap (every-other-car, zipper), but in practice typically I see "chunks" break off from the input lanes (3 or 4 cars) super close together and then zoom through leaving no room for the cross traffic to get in to.
If the roundabout speed were actually faster than the input lane speeds, it would induce this. In fact, I think that's maybe exactly what they've tried to do at Franklin/Sheldon, which has severe chicanes approaching the roundabout, mainly in the North/South direction (Franklin).
Ideally, everyone would have a gap, and everyone else would fit into that gap (every-other-car, zipper), but in practice typically I see "chunks" break off from the input lanes (3 or 4 cars) super close together and then zoom through leaving no room for the cross traffic to get in to.
If the roundabout speed were actually faster than the input lane speeds, it would induce this. In fact, I think that's maybe exactly what they've tried to do at Franklin/Sheldon, which has severe chicanes approaching the roundabout, mainly in the North/South direction (Franklin).