My understanding of what's called HIIT is that the second letter is the key to it. High intensity is not 2 X 20 or hill repeats or whatever we commonly do out on the road. There are many different workouts, Tabata intervals probably being the most well-known. On the road the simplest is probably 1 minute on, 1 minute off, maybe 6 times. Best on the flat. Go very hard, but not as hard as you can for the "on" minute, then spin easy for the easy minute. It will take a few times to get the hang of it, that is, to see how hard "HARD" can be and still allow a set of 6 intervals of about the same RPI or power. HR will rise the whole time. You don't want to blow up, at least not until the 6th. Anyway, that's one way to do it. It takes maybe a mile of straight flat w/o stops, or a shorter chunk where you can U-turn during the easy bit.

I've done these in the context of a much longer road ride. You want to do them maybe 1/2 hour - 1 hour into the ride. Then ride the rest of it at whatever pace feels OK. You should be able to recover just fine if you're in good enough shape to do the HIIT in the first place. Road riding is all about being able to recover during the ride.