Except when the posted speed is 20 mph, 25 is more common.if you're driving at 20mph you have to keep your wits about you, and also chances are that you're only going to be driving at 20mph for thirty seconds at most in general.
from a quick google, some car models allow low speed cruise control, though most of them are from places that measure in km/h and probably adaptive
https://www.quora.com/What-is-the-minimum-speed-required-for-a-cars-cruise-control-to-operate
and why the specification for non autonomous cruise control?
another thread about low speed cruise control and how there were possibly mechanical limitations in the past as well as potential safety risks to implementing it now
https://community.cartalk.com/t/cruise-control-limit/74651/8
I'm not saying I don't want auto cruise control, but I don't want to rely on a vehicle in front of me slowing down to 20mph to reach that speed myself.
Yes, Jag and RR have a low speed cruise control that is intended for off road use but can obviously be used on a road. I think it clicks off at 18mph or so.