I agree, but as
Winot pointed earlier the biggest challenge at the moment is not technological but political (and financial).
The cost of installing GPS- controlled interactive speed limiters (a big proportion of zipcars don’t have cruise at all) or the required software in their fleet would be very substantial, much more so if it was a compulsory feature as it would have to be absolutely fault-proof to avoid any potential legal issues (you can bet that any collision or accident that happened at a time when the limiter had allowed the car to travel at, say, 30 mph in a 20 street would be blamed on zipcar by the driver regardless of the circumstances of the accident).
AFAIK no car hire/ car fleet in the world has yet implemented a functioning interactive limiter that reacts and changes according to location. I’m sure once they become commonplace and proven-technology enough all car hire/ car club companies will adopt it. But I can’t see zipcar being the first to take the plunge. And there is absolutely no way imo the potential savings on insurance costs to them would be greater than the costs of introducing such technology in their fleet. As it is, car hire companies tend to generate juicy profits from insurance fees and claims anyway.
I think autonomous cars will be provide the first realistic opportunity of working interactive speed limiters. But retrofitting them in ordinary non-autonomous cars is a financial nonstarter.