Establishes the "Motor Fuel Tax Fund of 2021"

HB 3538 establishes a "Motor Fuel Tax Fund of 2021" by setting tiered taxes on various fuels used in vehicles. It imposes rates like 17 cents per gallon for regular gasoline, 5-17 cents per gallon equivalent for natural gas/propane (increasing over time), and a supplemental tax rising from 2.5 cents to 12.5 cents per gallon starting in 2021. The revenue from these taxes flows into the fund, which must be used for state road and bridge projects. Businesses that qualify (e.g., commercial fleets using fuel for non-highway purposes) can claim refunds by submitting documentation annually, with refunds paid from the fund.