I grew up in southern California which means that everything I ever needed to know I learned on the 405. Driving in traffic serves as a useful metaphor for a lot of life and it wasn’t until this morning that I made the connection and started to understand what Simon Johnson has been talking about all this time in blog posts like What Is Finance Really?
The parallels are clear between financial markets and driving in traffic. Arbitrage is the controlling force. For example, on the freeways arbitrage equalizes the traveling time across lanes, the commuters version of the efficient markets hypothesis.
You don’t have to have spent much time on the freeways to understand why arbitrage is not always efficient. An individual driver can get where he is going faster by changing lanes, but since there is a fixed capacity on the road this is always at the expense of somebody else. In equilibrium the total distance traveled by all is the same as if everybody were required to stay in their lanes. The arbitrage turns out to be a pure social loss due to the increased frequency of accidents.
Addendum: Calculated Exuberance has a nice take.