Unenforceable Laws--Speeding, Immigration, Prostitution

Shankar Vedantam, in his Monday Post column, cites the opinions of Douglas Husak and Lawrence Solum, who probably unknowingly follow in the footsteps of Robin Williams, my sociology professor 45 years ago:

Husak and Solum, legal theorists and philosophers, argue that laws on immigration are part of a broad pattern. In recent decades, they say, Congress has passed innumerable laws that no one seriously expects will be enforced. Such laws largely seem to serve symbolic purposes and are often designed to placate some powerful constituency -- conservatives in the case of immigration, or the entertainment industry in the case of laws that seek to deter people from swapping copyrighted music and movies.

Williams cited the case of prostitution, illegal in most states. Laws aren't effective in such cases--you get pro forma prosecutions, "show cases" for effect, not something that works. It's easy to take this line of thought too far. For example, I disagree with Husak and Solum about speed limits. They believe that, if cars on the Dulles toll road go 65 to 80 in a 55 mph zone, the limit ought to be raised. That's carrying the power of the majority too far--let me creep along at 60-65 in the right lane without feeling guilty about not going the speed limit.

It is a problem for bureaucrats whenever the gap between law and behavior becomes great. Do you enforce the rules or exercise the discretion and then become arbitrary?