The Metropolitan Planning Council estimates Illinois must pour an extra $43 billion into transportation during the next 10 years to do away with a maintenance backlog and bring roads, bridges and highways into good condition. Illinois imposes a base tax rate of 19 cents per gallon for gasoline and 21.5 cents a gallon for diesel, though other fractions of a penny are added on to pay for costs associated with environmental cleanup and underground storage of fuel. The gas tax has not increased since 1991, and money raised has remained relatively flat as vehicles become increasingly fuel-efficient but construction costs go up.