Roads aren’t actually that expensive. Resurfacing a highway costs about a million dollars a mile. Rebuilding the track on the CTA Redline recently cost $40 million per mile, and that was a miraculously cheap and fast rail project. A new freeway costs 5-10 million per mile. A new subway tunnel can cost a billion dollars per mile. Building the DC Silver Line through existing freeway medians cost $240 million per mile.
Maybe in the boonies. Adding a carpool lane on 10 miles of the 405 cost 1.1 billion and construction was a shit show. Then we need to add sensors and whatever else into the road surface so private companies can get their profit margins? We can't even maintain our existing road surfaces. We have the lanes already, remove a car lane on every road over 1 lane wide and make it bus only; same grade separating benefits of a subway but requiring zero investment beyond retiming lights and buying plastic bollards and maybe a few more busses to pick up demand. The insurmountable difficulty is in not letting emotional NIMBYs kill the bill.
