Who should pay to repair and improve the nation’s transportation system? Someone has to. About 10% of the nation’s approximately 600,000 bridges are structurally unsound. Another 14% are antiquated. Tunnels, highways and mass-transit systems also cry out for improvements. Yet the federal Highway Trust Fund, which is supposed to pay for most of this, is shaky. It has needed infusions from general revenues a few times in the past six years, and will run out of money this year unless Congress gives it a new allocation. So who will be stuck with the bill? • Taxpayers at large? This would come through the federal income tax. • Trucking companies that put extra wear and tear on roads? That might be done by raising tolls on federal highways. • People who drive a lot? This would involve raising the gasoline tax, as proposed this week by a couple of Senators. Sen. Bob Corker (R-Tenn.) and Sen. Chris Murphy (D-Conn.) want to raise the federal gasoline tax to 30.4 cents a gallon over two years, from 18.4 cents a gallon at present. Perhaps mindful of the Tea Party’s growing power, the senators say their plan won’t necessarily raise the total tax burden on Americans. To offset the gas-tax increase, they say, Congress could extend certain tax breaks that have expired or are scheduled to expire. As one example, they mentioned the deduction for teachers who spend money on classroom supplies. Another example was the federal deduction for taxes paid to states. These deductions may be good or bad, but none of them offer much comfort to people who use a lot of gasoline. The federal gasoline tax is only part of the picture. When I fill the tank of my wife’s Subaru in the Boston suburbs, I pay about $55. The gasoline itself costs me roughly $48, Massachusetts takes $4 and Uncle Sam takes $3. If I lived in California I would be paying about $11 in combined state and federal tax for the same tank of gas. It would be about $10 in Connecticut or Hawaii. Taxes on specific goods and services, such as gasoline or cigarettes, have two main purposes — to raise revenue and to discourage the use of the item in question. Proponents of higher gas taxes often point to the health effects of air pollution and a desire to encourage mass transit. As a clean-air proponent, I’m tempted to endorse a higher gasoline tax. But it’s a regressive tax, hitting hardest those people who – often by necessity, not choice – have to commute a long distance to work. It is more logical to have funds for transportation infrastructure taken from general revenue. That way the benefits of each item — defense, Social Security, Medicare, bridge repair, highway construction, and so on — can be weighed each another, and we can decide how much of each we can afford. John Dorfman is chairman of Thunderstorm Capital LLC, an investment management firm in Boston.