Most people who think they're doing HIIT are not doing HIIT. HIIT is not climbing a hill, it is max effort sprinting every 1-2 minutes until your legs are jelly. If you can sustain what you think is HIIT for more than 10-15 minutes, it ain't HIIT. Your HR will be maxed and you will be gassed, not really safe to do on a bike IMHO.