[mschuetz]

Things like that may vary from country to country, though. In Austria, roundabouts do not have a special rule and therefore a car approaching the roundabout theoretically has the right of way. But pretty much any roundabout has a yield sign, I've never encountered one without a yield sign at the entrance. Doesn't mean that there can't be a roundabout that does not have one.

[provost]

As you described, the default is to yield, and so the vehicle can always yield and signal its intention to do so by breaking slowly. And I agree, there may not always be a sign (a storm or driver could knock it over), so the default should always be to yield.