Braess's paradox (http://en.wikipedia.org/wiki/Braess's_paradox) states that adding an additional road to a transportation network can actually cause an increase in traffic congestion.

This has actually been observed in multiple major cities, including New York [1], Seoul, Boston, and London [2].

[1] http://query.nytimes.com/gst/fullpage.html?res=9C0CE7D81530F936A15751C1A966958260&scp=8&sq=&st=nyt
[2] http://arxiv.org/abs/0712.1598