Congestion pricing (also known as Congestion charges) is a concept from market economy regaring the use of pricing mechanisms to charge the users of public goods for the negative externalities generated by the peak demand in excess of available supply. Its economic rationale is that, at a price of zero, demand exceeds supply, causing a shortage, and that the shortage should be corrected by charging the equilibrium price rather than shifing it down by increasing the supply. Usually this means increasing prices during certain periods of time or at the places where congestion occurs; or introducing a new usage tax or charge when peak demand exceeds available supply in the case of a tax-funded public good provided free at the point of usage.