It says on the US embassy website that traffic convictions which did not result in arrest do not count.
I have no personal experience of applying for a visa but from what I have heard from other people the most likely reason they will refuse you a visa is if they don't think you are just going there on holiday. So if you have just changed your job for instance, or have no permanent place to live - that kind of thing. The fact you have previously entered under the visa waiver probably won't count against you as the rules weren't so strictly enforced before and I think they take this into account.
From reading on the various boards it would seem most people get their visa so your husband should apply in plenty of time. And make sure he takes all the documents they ask for with him.