You're making a number of assumptions, most of which I believe are wrong.

a) No, it's not the first thing that springs to everybody's mind. Experience says it's the first thing that springs to a relatively small percentage of minds, actually.

b) No, someone who didn't ask about the kind of bus isn't necessarily under the impression that it's a toy problem. Again, experience says that for a large percentage of people the problem, or more specifically any problem of the ilk of this problem, is completely overwhelming.