SSRN (Social Science Research Network) is an example of an "Open-Access Repository" or "Open Archive." Increasingly, these sites are being called "preprint servers." They are databases that contain freely accessible scholarly research output. They are easy to search and papers in them are available for anyone to find, use, download and distribute free of charge.
There are many types of Open Access Repositories (some are institutional, some are specific to a discipline) but one thing they all have in common is they provide free access to scholarship (in the form of indexed works and full-text access to those works). See our Guide To Understanding Open Access for more details about the concepts behind Open Access.
The world of academic publishing relies on "preprints" (sometimes digital preprints are called "eprints"), or early versions of scholarly or scientific papers that precede peer review and publication in a peer-reviewed scholarly or scientific journal. A "preprint" is typically an early version of a research paper that is often shared on an Open Access Repository prior to, or during, a formal peer-review process. In most cases these un-reviewed, un-edited and unformatted drafts can be made accessible while copyrighted final drafts are being processed by publishers prior to publication in fee-based journals. But lots of things can happen along the way, and in some cases the paper is never published through peer review, but does live on in the Open Access Repository. Some works may be contributed only to the OA Repository, but not submitted for formal publication--in essence they are "published" this way.
As of 2020 the OpenDOAR: Directory of Open Access Repositories (a UK-based website that lists academic Open Access Repositories) listed more than 2,300 such repositories, but it is hard to locate any relating specifically to "transportation." However, there are many well known OA Repositories that researchers might want to consider, depending on their topics. They often use a nomenclature that invokes the word "archive" (as in "a collection of historical documents or records") with a clever incorporate of the Greek letter chi (χ). They include the following:
arXiv.org (pronouced "archive"): Open access to over one million academic articles in Physics, Mathematics, Computer Science, Quantitative Biology, Quantitative Finance and Statistics. Created in 1991, this preprint sever set a standard by which similar servers in other disciplines have evolved.
engrXiv.org (pronouced "Engineering Archive"): A preprint server dedicated to engineering. It notes it "is a project of the nonprofit Open Engineering Inc." (a 501(c)(3) non-profit, that "provides a free and publicly accessible platform for engineers and engineering researchers to upload working papers, pre-prints, published papers, data, and code."
bioRxiv (pronounced "bio-archive"): A free online archive and distribution service for unpublished preprints in the life sciences.
RePEc (Research Papers in Economics): Index to publishing in economics and related sciences created by hundreds of volunteers worldwide.
PubMed Central: A free full-text archive of biomedical and life sciences journal literature at the U.S. National Institutes of Health's National Library of Medicine (NIH/NLM).
In theory, any of the sources above could contain research relevant to a transportation researcher (depending on the research topic), however, none of these repositories focus on transportation exclusively. The closest would be engrXiv.org
Note: See also the FAQ on Intuitional Repositories (IR) and this FAQ on Preprints
