The European Commission recently released the most comprehensive study of open access (OA) publishing in peer reviewed journals ever done, an analysis of over 1 million articles indexed in Scopus from 1996-2013. Open access articles are free for anyone to read. This massive study is filled with data that includes the growth of OA, the proportion of various types of OA articles, the availability of OA articles by discipline, and the citation advantage OA papers have over articles requiring a subscription to read. It is the only study to my knowledge that includes a breakdown of OA articles by country, region, discipline, publication year, and type of OA (Gold: published directly in an OA journal; Green: made freely available by the author via an official institutional or disciplinary repository; Other: Freely available on the Internet via some other means).
A few of the main findings are:
1) More than 50% of articles published between 2007-2012 are freely available as of April 2014.
2) OA articles overall are cited 40.3% more than non-OA articles, ranging from 26% to 64% based on discipline.
3) The proportion of Gold OA articles (published directly in OA journals) is currently doubling every 4.1 years.
At a minimum, the 7-page executive summary is worth a careful read. See: Archambault, E., Amyot, D., Deschamps, P., Nicol, A., Provencher, F., Rebout, L., & Roberge, G. (2014). Proportion of Open Access Papers Published in Peer-Reviewed Journals at the European and World Levels: 1996-2013 (D 1.8 version 11p ed., pp. ix, 41): Science-Metrix.