Not just between car manufacturers, but also between the cars and the area conrol system. Basically, all car manufacturers whould have to agree on a common API that allows the control system to take over, with full access to sensors and drive controls. A manufacturer could not simply refuse to implement the API, because this would effectively make their cars unable to use certain parts of the road network.

Even if manufacturers would somehow manage to agree on an API, it could then be "abused" by competitors or accessory vendors to sell their own customized car assistants, which would instantly work with any car brand - without them having to negotiate with the manufacturers.

I fear we will sooner have a usable open IoT standard than manufacturers giving up that level of control.