When driving through active school zones in my previous car, I'd hit 15mph and set Cruise Control on. After picking up my Air, I found out cruise control won't engage under 20mph. Ugh.
You can set the speed to 20mph, but it will adapt and drive slower than that to keep distance. So if everyone is going 15mph, it will also go 15mph.
You just can't set the minimum cruising speed lower than 20mph. If you set it to 20mph though, it will gladly cruise slower than that if everyone else is too.