Surface Transportation Reauthorization Act of 2021 This bill reauthorizes through FY2026, revises, and establishes multiple transportation programs and activities, including those supported by the Highway Trust Fund. This includes incorporating data-driven practices and other changes to transportation planning activities; modifying requirements concerning contracting methods, compliance with federal environmental laws, and additional aspects of delivering and improving processes related to transportation projects; mitigating climate change through, among other efforts, programs to reduce emissions and supporting alternative fueling infrastructure; and addressing transportation safety, new transportation technologies and systems, storm water management, cybersecurity, active transportation, and other miscellaneous matters. In addition, the bill modifies eligibility criteria and other elements of a program that provides loans and other types of credit for surface transportation projects of regional and national significance. Further, the bill makes changes to research and related activities carried out by the Department of Transportation (DOT). For example, DOT must carry out and support research on (1) user-based alternative revenue mechanisms to support the Highway Trust Fund, (2) automated vehicles and other new technologies, and (3) development of data sets to measure performance and other aspects of transportation programs. The bill also authorizes through FY2026 funding for the Department of the Interior for road maintenance and otherwise addresses transportation programs for Indian tribes.