[Hatman]

^^^^
It's all about the phases of adoption. Obviously at first autonomous cars will need to be able to share the road with everybody. But later, when a majority of vehicles in a given area are driving autonomously, then the urge to require autonomy in vehicles may be too much for governments to resist.

Like you say, motorways are a great example. They are limited access and grade separated, meaning you would have easily delineated autonomous zones. Motorways are also much more expensive to build and maintain because of the higher speeds and use. A motorway adapted for autonomous vehicles will be much less expensive to maintain and operate (no lanes, signs, lighting, shoulders, barriers, policing ect). The cost savings will become so great that making them legally autonomous-only zones is already inevitable. 15 years is ambitious, but very achievable.

Urban cores in cities will also be ideal candidates for autonomous-only zones. I imagine a city where autonomous cars take up less than half the space human-driven cars used to, most likely in the center of the street, while the remainder of the space will be designated for bicycles and other pedestrian uses. I think that the appeal of a city so transformed from traffic-jams and surface parking to bike lanes and currently-unbelievable density will be so strong that governments will create autonomous-only zones within the same time frame as motorways.

As for everywhere else, such as the suburbs and the countryside, it will probably be a very long time before any of those places becomes mandatory autonomous zones - if they ever do.