One big difference between UK/IE and California that took a while to get used to (and I'm not even sure I'm used to it yet) is the prevalence of roads with dividers and what it means...
Lots of roads here are very wide, and have lots of lanes. Four lanes in each direction isn't uncommon, three is commonplace. And there's usually a divider along the middle of the road.
One thing this means is that crossing the road on foot is a Big Deal. People just don't cross the road at arbitrary places: they go to junctions. (Here in California drivers really stop for people. They seem to be really serious about it. But anyway...) This is totally different to UK/IE where folk cross the road wherever they want - usually at junctions, but really wherever it suits. But that's because the roads are *so* much narrower.
Another implication of the wide roads with dividers is that if can see your destination on the other side of the road ... you ain't there yet! If there isn't a wee path in the central reservation to cross into the place your going to's car park (and *everywhere* has its own car park, except maybe in The City!) then you have to drive on to the next junction and do a U-turn.
Which leads me to another implication of these wide roads: U-turns are *totally* normal. Because of these central reservations, you pretty much *have* to do U-turns often. And the roads are wide enough to support it.
Many junctions have two lanes turning left. (We drive on the right in California...) You can usually do a U-turn in the leftmost one. But sometimes there's a sign saying you can't. That usually means if you try to, some other car will have right-of-way and you'll get creamed.
Anyway, U-turns are really rare in UK/IE. The roads aren't wide enough, so it wouldn't be practical, but in general it just doesn't happen. Instead, there are roundabouts, which are good enough. I quite like roundabouts - they're a great idea so long as everybody knows how they workd. (Hint: don't stop!)
A little difference!