Recent posts below (see “Taxing congestion: how I might justify it“) have sought reasons as to why toll-roads are so often touted as an economically efficient measure. For my part, I am quite sceptical that the are universally efficient, and struggle to find a compelling reason why they are even efficient most of the time. However there are some circumstances where it is quite conceivable that they can be efficient. Where there is a (slower) alternative to the road with a congestion charge, and different drivers place different values on congestion free travel, congestion charges/tolls can lead to an efficient sorting of road users between the (faster) toll road and the (slower) free road, resulting in socially optimal outcomes.
The intuition goes something like this: