The justification for “taxing congestion” appears to be Pigovian – someone clogging up the road has a negative externality on everyone else, and so we should tax that externality. However, I feel that this is just half the story.
In the case of congestion, everyone else on the road is also holding up that one person. In fact, on average, one person on the road is suffering the same negative externality as they are providing. As a result, doesn’t the existence of congestion effectively cancel itself out? Sure putting a toll on will reduce congestion – but if we already have the optimal solution why would we want to introduce a tax on top of it. Note: It may be efficient to actually have some congestion, as the goal of policy is to maximise welfare – not minimise congestion.
Now I have made the argument for an externality in the past (here and here) – my thinking was that the externality fell outside of the drivers and on other areas with which driving was a means to (eg work). Of course, I can’t think of a single situation where there isn’t a “price” mechanism to sort this out (eg with work people wages will adjust to sort out the optimal labour market solution in the case of the “externality”).
I would like to hear if anyone has an externality justification for toll roads – bonus points for using the term “non-linearity”.