Is it ethical for me to take a shortcut that involves leaving an expressway a few miles before an overcrowded bridge and taking local roads only to re-enter the expressway just before the bridge. I have observed that much of the slowdown at this bridge is caused by merging traffic coming from this shortcut.

I'm not sure that ethics has much to do with it: whether to take theshortcut doesn't seem like a moral question. But your situation doeshave a paradoxical flavor: the very fact that you and others take theshortcut to avoid the slowdown at the bridge is what causes theslowdown at the bridge. If everyone could just agree not to take theshortcut, then there'd be no slowdown, and no need to take theshortcut. Of course, it's difficult to get everyone to cooperate. Andif somehow you could, it wouldn't last long: someone would realize thats/he could take advantage of that cooperation by taking the shortcutand not encountering any traffic at the bridge. And then another person wouldrealize that; and then another. Until enough people began taking theshortcut to cause a significant slowdown at the bridge. Philosophershave been very interested in such unstable attempts at cooperation whicheventually break down and leave all participants worse off than theymight otherwise be. (They've been baptized Prisoner's...