In a noncongested transportation network where each user chooses his quickest route, the
creation of an additional route between some origin-destination pair clearly cannot result in
an increase in travel time to users traveling between that od pair. It seems reasonable to
assume the same can be said of congested networks. In 1968, D. Braess presented a
remarkable example demonstrating this is not the case: a new route can increase travel time
for all. The present paper gives, under reasonable assumptions, necessary and sufficient …