It's standard here, when shopping for policies, they always ask your miles per annum. The more miles you do, the more likely you are to be involved in an accident, the more your insurance will be. I don't think they check, but if they can, they will.
I've never heard of a mileage cap before. Do you get a discount for less miles traveled? I should check into my policy to make sure I don't have anything like that in it!
Since the wife and I have 4 cars between the two of us, and my daily work commute is only about 5 miles one-way, the cars don't see as much road use as the average American vehicle (which is about 15,000 miles). State Farm offers a limited-use provision in our auto insurance policy that reduces the premium significantly for vehicles driven less than 7,500 miles per year (ours are mostly around 5000 per year). We keep unrestricted coverage on the SVT Focus, as it's the most practical of the bunch, and gets driven a lot more than the others - especially on long trips.
A girl at work recently got caught out. As a recently passed learner she had a tracker fitted to lower her premium. When asked her annual mileage, her "friend" who drove the same route each day told her to put 5,000. Her real mileage was 5,000 by 6 months, and she ended up paying out a nearly £1,000 fine and losing the policy....