Mikael Laakso and Bo-Christer Björk have self-archived "Delayed Open Access—An Overlooked High-Impact Category of Openly Available Scientific Literature."
Here's an excerpt:
Delayed open access (OA) refers to scholarly articles in subscription journals made available openly on the web directly through the publisher at the expiry of a set embargo period. Though a substantial number of journals have practiced delayed OA since they started publishing e-versions, empirical studies concerning open access have often overlooked this body of literature. This study provides comprehensive quantitative measurements by identifying delayed OA journals, collecting data concerning their publication volumes, embargo lengths, and citation rates. Altogether 492 journals were identified, publishing a combined total of 111 312 articles in 2011. 77,8 % of these articles were made open access within 12 months from publication, with 85,4 % becoming available within 24 months. A journal impact factor analysis revealed that delayed OA journals have on average twice as high average citation rates compared to closed subscription journals, and three times as high as immediate OA journals. Overall the results demonstrate that delayed OA journals constitute an important segment of the openly available scholarly journal literature, both by their sheer article volume as well as by including a substantial proportion of high impact journals.