For nearly four decades, Connecticut has designated a portion of its annual budget specifically to finance upgrades to its aging transportation infrastructure. And while fuel taxes were the principal source of revenue for the STF throughout most of its history, sales tax receipts recently have supplanted them — albeit just barely. The Special Transportation Fund, which is projected to spend $2.3 billion this fiscal year, represents nearly 9% of the state’s $26 billion budget. Initially, the fund was used to pay debt service — principal and interest — on the bonds Connecticut sells on Wall Street to finance repairs, improvements and upgrades to highways, bridges and rail lines. Currently, nearly $1 billion — over 40% — of the STF is used for that purpose. The fund is now used to finance the operations of Connecticut’s Department of Transportation and Department of Motor Vehicles as well. Critics fear the STF surpluses will keep growing and have renewed their call for gasoline and other tax cuts. But the Lamont administration insists big unused resources won’t be a problem again because a long-anticipated surge in the rebuilding of Connecticut’s aging infrastructure will take off this fiscal year.