I can't stand riding indoors so I ride on the road as much as possible. I also do intervals on the road because my heart rate is much different and the sensations don't feel the same. Like how much effort I am putting out compared to my heart rate indoors. I try to do all my intervals outdoors where time and weather permits. I like to do them on a hill or on a uninterrupted road. Luckily there is a few good places around. Also if your interval is 10 minutes, but you can only find 5 minutes of road then make a U-Turn and just turn around and head back. Or if you have a 10 minute climb, you'll need less road. And you can use the descent as your recovery time.