Maybe toll gates could be set on roads through the map editor? Then a road with the toll gate would HAVE to remain in place (demolishment disabled), and would affect player's decisions - for example, if toll roads happen to be all the bridges over a river, connecting two parts of a city, then making a metro line between two good points on both sides of the river, or a boat line with car parks on both sides, could easily make a nice profit. But then again, making a bus route through such toll gate would make more operation costs.