I do HIIT on treadmill all the time; machines go FASTER than 15 mph !! I've gone only up to 14.8 mph.

You can learn to control the adjustments at fast speeds. However, it is okay to momentarily step on the sides of the machine, straddle it, to bring the speed back down to recovery pace. I do this all the time. At end of sprint, step feet off to side and change speed, then get back on.

OR, when I'm done with a sprint, I just get off the machine and walk around for part of the recovery, then get back on and slow it down for the remainder of the recovery.

DO NOT HOLD ONTO THE MACHINE WHILE SPRINTING. THIS IS CHEATING BIG-TIME.

you don't always have to go faster, if you are uncomfortable with that raise the incline as it is your heart rate that is the bases for HIIT its like running hills while trying to maintain the same pace-just food for thought

I've never felt good on a treadmill, even going slow boat. Doing the hiit on a stationary bike can be kill though. Right now I'm doing 50 seconds at about 22 mph, then all out (I've had myself going 40mph according to the computer) for ten seconds. Legs get a really good burn.

try the rower for a change of pace it is a killer too

For cardio I typically jog or use the elliptical. So if I use the elliptical, what's a good pattern for HIIT? I usually just do like 25mins on level 7. I never realized you need to do like a 30seconds of really hard stuff. What levels would be good to alternate between? I'm not sure what the highest setting is, but its probably around 20.