Complete Streets Act of 2021 This bill directs each state to establish a competitive program to provide technical assistance and grants for the design and construction of complete streets to make transit routes safer and more accessible. Specifically, the bill directs the Department of Transportation (DOT) to establish benchmarks and guidance by which states can implement complete streets programs and eligible entities can carry out complete streets policies and principles; eligible entities seeking technical assistance or funds for a complete streets program to adopt policies using complete streets principles that are approved by the state or metropolitan planning organizations (MPOs) with jurisdiction over such entities; states and MPOs to certify that their complete street policies meet the minimum requirements set out by DOT; states to set aside 5% of their federal highway money to implement a complete streets program; and states and MPOs to adopt design standards for federal surface transportation projects that provide for the safe and adequate accommodation of all users of the surface transportation network, including motorized and non-motorized users, in all phases of project planning, development, and operation.