Optimizing the Social Cost of Congestion Games by Imposing Variable   Delays
  We describe a new coordination mechanism for non-atomic congestion games that leads to a (selfish) social cost which is arbitrarily close to the non-selfish optimal. This mechanism does not incur any additional extra cost, like tolls, which are usually differentiated from the social cost as expressed in terms of delays only. 
Josep D\'iaz, Ioannis Giotis, Lefteris Kirousis, Yiannis Mourtos,   Maria J. Serna
cs.GT