The Directory of Open Access Journals (DOAJ) has a subject list for transportation. Open access generally means the reader is not paying to read the articles in the journal. Libraries do not have to subscribe for readers to use the articles, though some are asked to "contribute" in order to sustain open access. Some open access journals are supported by taxes and/or research projects. Policies for access vary by journal and are often subject to change. Publication fees may be required to publish and, for some journals, only some articles are open access. Open access may also occur after a certain time period has passed. This embargo period may be of considerable length.