Many modern cars have traffic sign recognition systems that extract the speed limit. The BMW system is fairly amazing to watch work as the speed limit display changes juuuust as you pass the sign and are surprisingly good at reading signs that are even partially obscured. Although I can't remember exactly which cars feed the detected speed limit into their adaptive cruise control systems, I think some of the Range Rover models do. Some even allow you to set a percentage or absolute speed value above or below the speed limit that you want the car to travel.

Others use a speed limit database and GPS/accelerometer system to establish the speed limit.

I haven't used (non-adaptive) cruise control in years. I find that a lot of the time in the Northeast US roads are busy enough that you have to be constantly dropping out of cruise control or end up driving in ways that are suboptimal other than for keeping the cruise control active. To be honest, I just stopped using it at some point and just don't think to use it even when it's a match for conditions.

When I get adaptive cruise control--presumably when I eventually get a new car--I may well change my tune.

Teslas recognize speed limit signs. Driving on Autosteer my Model S respects speed limits unless it recognizes that it is on a dual carriageway (divided highway for US readers) in which case it allows you to speed if you want to but warns you that you are over the limit.

Mercedes have had this feature as an option for many years.