I'm gonna go with "don't be stupid" again. Putting stuff down on your floorboards that interfere with the full operation of the controls is asking for trouble. Your device could fall into the brake pedal, preventing operation. It could catch your foot at a critical moment. It could just surprise you after a long period of inattentiveness by acting "different" than what you're used to.

You want it to be a 'safe' device that simply prevents over acceleration. It comes with a royal boatload of possible unintended and unimagined consequences.

Don't be stupid.

Edit: (On reread). If "don't be stupid" makes you feel angry or defiant, please read it as "Please don't do this. Its more dangerous than you think.", with my apologies for sounding parental.

Where's the feedback? A constant throttle might get you a stable speed on flat ground (after adjustment), but when its time to climb a hill, you're going to have to press down further to even make it up at all.

I do wish cruise controls were designed such that you'd still push down on the gas, and it acted as an upper limit. I avoid cruise control entirely if I'm tired, as I like the feedback from unconsciously slowing down as a sign that I'm too tired to be driving.

That's one drawback. If you're going up a hill, you'll slow down. It's not as bad as you'd think though, your inertia is usually enough to only cause a difference in 5mph spread over ~5-10 seconds.

I think we've got different magnitudes of hills in mind. Perhaps using memory foam for a spring would allow you to deliberately change position while still providing decent support.

Sorry, but this seems monumentally stupid, and likely illegal.

Personally I never use cruise control, even when I do 400 mile trips. I just don't find it safe, since you can only steer in case of an emergency + my legs become restless. At least when you gas/brake your legs have something to do