Road User Charge Advancement Act of 2021 This bill revises the Surface Transportation Systems Funding Alternatives program. The program provides grants to states to demonstrate user-based alternative revenue mechanisms that utilize a user fee structure to maintain the long-term solvency of the Highway Trust Fund. Specifically, the bill revises the program to set forth certain eligibility requirements, ensure that activities carried out using grant funds include test solutions to ensure the privacy and security of data collected for the purpose of implementing a user-based alternative revenue mechanism, increase the maximum amount of federal share that may payable for the costs of an activity from 50% to 80% of the total costs, and modify reporting requirements for the Department of Transportation to include annually publishing a report that contains a determination of the characteristics of the most successful mechanisms with the highest potential for future widespread deployment. The bill also provides funding for FY2022-FY2025 to carry out the program.