> It's clear that if everyone only went to work half as much we'd only need to build half the roads and trains and airports.

Not only is this not "clear", it's also false. To connect two towns, you need 1 road. If everyone only went to work half as often, you still need 1 road – you can't have 0.5 roads. And if people work half as much, but only 4 hours per day, you wouldn't save anything.

Sure, with less utilization comes less need for maintainance, but not all wear and tear results from direct use.