I got a TomTom navigator as a present and set it to narrate in English (Irish voice). Every time I'm about to go left or right in a roundabout it says: ”Go right on the roundabout, first exit”
The choice of preposition (on) strikes me as odd. I would've expected an in but perhaps that's just because how we say in my country (Sweden; where on would be plain wrong).
After some searching on the internet I realize that both in the roundabout and on the roundabout are commonly used. However, I can't really tell if they're completely interchangeable or if one of them should be picked in favour of the other depending on the context.
Best Answer
When you are driving, you are driving on the road, not "in" the road. However you could be playing in the road, meaning between the curbs is a close vicinity.
A possible usage for in though it may be a stretch:
meaning the sign is on land inside the roundabout.