I think just adding it to new cars would be enough. Eventually all unregulated cars would age out.
For the upgrade path, it could be treated like building codes, which change every so often without requiring immediate retrofit of all homes/buildings: require it as a part of resale.
When new cars are forced to go 55 on that one long curvy downhill where the big trucks are all going 80, you just increased fatalities on that corridor by a LOT. And probably for like 15+ years without forcing regulation that requires updates. The body count will be very substantial when you multiply it across all of the roads, or even just California roads.
Not trying to be contrary or throw up imaginary hypotheticals, I drive a car that sometimes has trouble keeping up with traffic on such downhills because I’m very top heavy.
People that talk about stuff like this seriously have no appreciation for the variety of car and road types that are already present in the real world. Robot assistance being offered is fine. But robot enforced compliance killing people in the name of safety is absurd.
I think realistically the limit would not be as low as 55, and also commercial trucks would be a likely target for actual immediate retrofit. I was talking about non-commercial privately owned cars.
For the upgrade path, it could be treated like building codes, which change every so often without requiring immediate retrofit of all homes/buildings: require it as a part of resale.