Chevy cars have this in most of their stereos and it seems to work nicely, anyone know how they do it?
Ford has this as well, at least in some models since 1999, they call it AutomaticVolumeControl and worked very well. I'm not sure how it works most likely a mic input that measures ambient noise, as there is a small grill on the dash of the vehicle about the size of a mic either that or it's the temp sensor.
You actively set the AVC amount thru the stereo from values of 1-9? this setting determines how aggressive, it seems, the volume is raised compared to speed or whatever determines the volume increase.
The system works very well, better at low AVC settings as you don't notice the increase in volume as much as higher settings.

I would rather have it inactive below 40mph or so and then kick in with adjustments every 10-15mph after that. This would avoid the "city driving" effect.

When set right via the stereo control you don't notice any pumping or the increase/decrease just that it's NOT blaring at you when you slow to stop or it's NOT too weak to be heard at highway speeds.
I like the it alot and it would be a great feature on the Empeg if someone who is profecient at such things since I'm not.