James M. Donovan et al. have self-archived "The Open Access Advantage for American Law Reviews."
Here's an excerpt:
Articles available in open access formats enjoy an advantage in citation by subsequent law review works of 53%. For every two citations an article would otherwise receive, it can expect a third when made freely available on the Internet. This benefit is not uniformly spread through the law school tiers. Higher tier journals experience a lower OA advantage (11.4%) due to the attention such prestigious works routinely receive regardless of the format. When focusing on the availability of new scholarship, as compared to creating retrospective collections, the aggregated advantage rises to 60.2%. While the first tier advantage rises to 16.8%, the mid-tiers skyrocket to 89.7%. The fourth tier OA advantage comes in at 81.2%.