The Future of OA: A large-scale analysis projecting Open Access publication and readership by Heather Piwowar, Jason Priem and Richard Orr is an important study of the availability and use of Open Access papers:

This study analyses the number of papers available as OA over time. The models includes both OA embargo data and the relative growth rates of different OA types over time, based on the OA status of 70 million journal articles published between 1950 and 2019.

The study also looks at article usage data, analyzing the proportion of views to OA articles vs views to articles which are closed access. Signal processing techniques are used to model how these viewership patterns change over time. Viewership data is based on 2.8 million uses of the Unpaywall browser extension in July 2019.

They conclude:

One interesting realization from the modeling we’ve done is that when the proportion of papers that are OA increases, or when the OA lag decreases, the total number of views increase – the scholarly literature becomes more heavily viewed and thus more valuable to society.

Thus clearly demonstrating one part of the value that open access adds. Below the fold, some details and commentary.

First, they show that a tipping point has been reached, because the majority of views are now to open access papers:

We found that Green, Gold, and Hybrid papers receive more views than their Closed or Bronze counterparts, particularly Green papers made available within a year of publication. ... In 2019 ... 52% of article views are to OA articles

Because they are more likely to be read, open access papers are more likely to be cited. This is despite the fact that only about 1 in 3 papers are open access:

In 2019 ...31% of all journal articles are available as OA

Second, and more interestingly, they show how open access got to this market share, and project the historic trends forward to 2025 to conclude:

By 2025 ... 44% of all journal articles will be available as OA

So the second tipping point, at which the majority of articles are open access, is probably late in the next decade unless some dramatic change occurs.

But the minority of articles will be garnering a substantial majority of the views:

By 2025 ... 70% of article views will be to OA articles

Note in both graphs that by 2025:

Gold: published in a fully-OA journal

forms the majority of both the open access articles and views. This is because it has been the most rapidly growing type of open access, outstripping in particular:

Green: published in a toll-access journal and the only fulltext copy available is in an OA repository

This is good news. While it is true that Green open access is better than being firmly paywalled, the complexity of Green rather than Gold makes all use of the content, from authorship through human reading through text mining to archiving, harder and more expensive. Green adds much less value than Gold.

Green also subtracts value by adding delay, since the open access typically happens after an embargo period. The authors have a blog post, Green OA Lag, detailing this:

less than half of papers papers published in, say, 2015, were made available the same year — most of the papers have been made available in subsequent years. ... a very few articles that were published in 2015 were actually posted in a repository in 2014. Those are preprints. A lot of articles published in 2015 appeared in a repository in 2015, but even more had a delay and didn’t appear in a repository until 2016. A full 40% of articles had an OA lag of more than a year, including some with an OA lag of four years!

Both the paper and their series of blog posts are well worth reading. And you should definitely use the Unpaywall browser extension.