Highway Cost Allocation Study Act of 2021 This bill directs the Department of Transportation (DOT) to carry out a highway cost allocation study to determine the direct costs of highway use by various types of users. In carrying out the study, DOT must (1) ensure that the study examines only direct federal costs of highway use, (2) capture the various driving conditions in different geographic areas of the United States, and (3) distinguish between costs directly occasioned by a single highway user class and costs occasioned by all highway user classes. Once the study is completed, DOT must develop recommendations for a set of revenue options to fully cover the costs occasioned by highway users, including recommendations for changes to existing revenue streams and new revenue streams based on user fees.