Well when politicians are accused of "doing a 180" it doesn't mean they are continuing on their previous course.
This is because most politicians (I think all except the late Cyril Smith and Tony Benn) are former pro skateboarders. This is a fact.
This would lend them to think in terms of a 180 being a sort of political (metaphorical)
volta face.
However, if one looks at a roundabout as a sort of windrose, then to enter a roundabout and drive (or cycle) the full 360 degrees round it would involve one completing a full circle and using the opposing carriageway of one's ingress as an egress.
Were one to complete that single revolution (not in socio-political terms) and continue round the junction a further 180 degrees, the total number of degrees by which one has circumnavigated the junction may be described as 540 degrees.
However, as has been pointed out above, the manoevre I describe is selfish and wrong. Which is why I never do it and never have.
I hope I have helped. More than that, it is my passionate wish that I have helped. I feel that I have.