This isn't actually an essay that earns the conclusion you've put on it. What happened was that Strahilevitz wrote a too-clever-by-half piece designed to misuse a metaphor: he was going to prove the value of pollution emission permits by analogy with selling HOV lane access. You came along and took his HOV access material, his arguments, and his conclusion hook, line and sinker, including the social norms implications. But Strahilevitz is just a wise-ass law clerk, and he's writing a paper designed to get him a teaching job by sounding smart, so there's no reason to believe he's actually got a tight argument on his jumping-off point, which is just a metaphor anyway. When you come along and borrow this organ-grinder's monkey, you're now responsible for your data and your conclusions. So how about moving away from San Diego and DC, and looking at the San Jose metropolitan area, where the penalty for violating the HOV lane rules, like the penalty for littering on the coast highway, is heart-stoppingly high. If you charge people $1,000 for littering or $500 for driving under-loaded in HOV lanes, or for speeding in construction zones, you make even sporadic enforcement very effective, because the fines are larger than many drivers' cars are worth. You also create a good reason for the Highway Patrol to make enforcement of these rules a significant priority. That plus some points on the license, endangering the ability to make a living in a place that so utterly requires each worker to retain the privilege to drive, and you are likely to prevent behavior quite effectively. No significant littering happens on Highway 1, and at least in my experience with Silicon Valley traffic, HOV violation is very uncommon conduct.

So maybe you're right and maybe you aren't: perhaps there's data showing that San Diego does better than San Jose by some measurement we might be interested in (though it probably wouldn't do by some other measure of equal interest, because that's the problem with "policy science") but you're not even going to find that data as long as you're relying on Strahilevitz to do all your work for you. And whether the "social norm," which it might be simpler to call "other social control" is more powerful than law doesn't require extended analysis: law is always a fairly weak means of social control. If every church sermonized against HOV lane violation every Sunday, that would be more effective at deterring than high fines, though perhaps less effective than frequent public capital punishment. But other social control is not usually incompatible with legal social control, except in the thought experiments of smart-ass dudes trying to get a teaching gig at Chicago. There are other matters to consider: Whether enforcement is expensive, for example, depends on whether it pays for itself. And so on. Strahilevitz has a bunch of other problems, because he puts a ladder on top of this rickety stool and tries to climb many times higher than his head on it, but you've borrowed from him problems enough. Whether one wants to be responsible for this organ grinder's monkey isn't clear to me, but if you do, you need to ask some slightly broader questions and gather some slightly more comprehensive information.