Ditto to the various folks who have encouraged riding in September and October. Even here in northern Minnesota it's usually nice until the end of October -- yes, there can be a few nasty days, but the weather usually is crisp and lovely -- actually our favorite time to bike.

I think MichaelW hit it on the head with his suggestions. Expanding on them, I'd say that in my view, the most dangerous places to ride are on the fringes of large metro areas -- lots of traffic, often fast, and a road net that is overwhelmed. Avoid those areas unless there's a clear safe way in and out (e.g. a rail trail), or take public transit through them. In contrast, the older parts of cities are usually fine because there are tons of side streets with little traffic.

For me, low traffic trumps everything else. If possible, therefore, bike in areas that have lots of secondary or tertiary paved roads -- I'm talking paved county or township roads. Iowa, Minnesota, Wisconsin, and Illinois are great for that. Leave the main highways for the cars unless there isn't an alternative.

Many states have bike maps, and you can get a lot of useful information that way.