There's an alternative to only on motorways and never on motorways: sometimes on motorways.
90% of the time you can use ordinary roads, with autonomous driving acting like a normal truck, giving the meat bags plenty of space. The most congested 10% you can build restricted access paths only for your smart trucks where they can act more like a train, with minimal spacing, increasing the traffic capacity of the road/tunnel/bridge/whatever so the extra cost of constructing it pays off sooner and/or it's cheaper in the first place because it's not built for drivers so doesn't need to be as safe.
90% of the time you can use ordinary roads, with autonomous driving acting like a normal truck, giving the meat bags plenty of space. The most congested 10% you can build restricted access paths only for your smart trucks where they can act more like a train, with minimal spacing, increasing the traffic capacity of the road/tunnel/bridge/whatever so the extra cost of constructing it pays off sooner and/or it's cheaper in the first place because it's not built for drivers so doesn't need to be as safe.