Good OP, and good discussion.

Question: for those of us with access to hills, are hill repeats sufficient for interval training? Where I live, I can ride over to a 4% steady incline that takes me several minutes to climb. However, to do sprints in a protected environment (not worrying about cars as I come close to passing out) I have to ride on residential streets in a circle, going round and round and blowing through stop signs, which I would rather not do.