You might try mapquest.com to get an idea, then like you said go to the LBS when you get there. Try to find the "B" roads, like "B1112". "A" roads are busier, and may be off limits. A good way to guess, I think, is if there are only two numbers, it is more like an interstate highway "A10", whereas "A1012" is more like a B road (but still a chore to ride on).