I'd prefer a system that was dependant on vehicle speed

Again, I get more difference in noise due to changes in the road surface than I do changes in speed.

There's a stretch of Interstate 80, the one that goes over the mountain pass between California and Nevada, that's particularly bad. I travel this road quite frequently. It's some of the roughest road surface I've ever seen (for a heavily-traveled interstate, that is). It's not necessarily bumpy or full of potholes, it's just got a very rough, pebbly texture. It's in dire need of resurfacing. Certain sections are being resurfaced right now, but the project is taking a long time and it will be years before the whole road is smooth again.

The funny thing is that the state of Nevada seems to have a better budget for maintaining its highways. As you pass the sign saying "Welcome to Nevada", it's like someone hit a switch. Suddenly, all the noise is just GONE and it's whisper-quiet in the car again. This is with no change in speed, just crossing onto a better-maintained stretch of road.

My primary reason for wanting noise-sensitive volume control is to deal with this exact stretch of road, and with the various increases and decreases in road noise that go with its changing texture.