Is that due to all the roundabouts switching direction relative to the Coriolis effect? Hence the relevance of the UK?

Presumably without roundabouts it's all random directions and balanced?

Or am I barking up the wrong tree?

It was indeed all about the roundabouts. I forget the details but I do know it took us quite a while to get there(!)

Those tutorials filled me with dread at the time, but with hindsight they were - how can I put this - a fairly formative experience.

Watching your tutor use paper and pencil - and estimation - to calculate something like that was actually quite inspiring. That was, once it stopped being terrifying.