I don't know where you're from, but around here highway dollars are spent expanding and rebuilding highways to increase capacity and throughput. Projects that focus on only safety -- removing left-lane exits and entrances, adding full-width paved shoulders, increasing grade, increasing curve radius, etc -- are rare-to-non-existant uses of scarce highway dollars.
States -- especially California -- have a serious interest in increased highway efficiency. The only thing about the original premise that I wonder is how much additional wear-and-tear the high speed lane would create, especially considering it would need to be kept in optimal condition to avoid deleterious safety affects at such high speeds.
Not much - wear and tear is strongly a function of vehicle weight, a lane with no big rigs doesn't suffer much.
And with enforced control over the vehicles you could make the lane narrower and even have a low kerb to stop people cutting in and out.
Popular in europe are guided bus ways. you put a low concrete kerb in a very narrow lane ie. 12 inches wider than the bus. There are small guide wheels that run against the kerb and servo the steering. So you can run a two way full size bus route in almost the same width as a single rail line - and at the end of the route it can pull out into normal traffic. This is just a software alternative.
States -- especially California -- have a serious interest in increased highway efficiency. The only thing about the original premise that I wonder is how much additional wear-and-tear the high speed lane would create, especially considering it would need to be kept in optimal condition to avoid deleterious safety affects at such high speeds.