Sure, I did it all summer and fall but it really has to be on roads that make sense for what you are doing. If you are doing 2 or 3x20s, you need a long stretch without interruptions so you can go 20 minutes non stop a the power you are targeting for instance. Shorter intervals are easier to find places that work. I'm doing work on the trainer now due to the weather, but will go back outside when a break comes.