RALEIGH – When government works at cross-purposes, it’s unlikely to achieve its purposes – and likely to make its citizens cross.

There are many cases of state policies with multiple objectives that conflict, or are at least in tension. For example, North Carolina derives hundreds of millions of dollars a year from excise taxes on cigarettes and alcohol. One goal of this tax policy is to raise revenue for expenditures on public education, mental health, and other government programs. Another goal, though, is to discourage cigarette and alcohol consumption. If the state hopes to make any significant progress towards the second goal, that will undermine its ability to accomplish the former one.

This is one of the problems with using the tax code to dictate economic decisions or police personal behavior, rather than simply to raise revenue for government programs. I fear that we are about to see the same dynamic play out in the area of transportation funding.

For decades, the state has relied on two main sources of revenue for transportation: taxes on each gallon of motor fuels plus taxes and fees on the purchase and operation of automobiles. As households and businesses placed more vehicles into service and consumed more gas driving them around, the state gained more revenue with which to invest in new capacity and maintenance.

Over the years, however, the relationship between these two revenue sources and transportation usage began to change. On the plus side, increasing household income, the popularity of leasing, and other factors increased the turnover of the auto fleet, thus creating more taxable “churn.” On the minus side, and with greater import, increases in fuel efficiency meant that drivers consumed less taxable fuel per mile traversed, thus reducing the real tax collection per use.

At the same time, political pressures intensified to reduce the consumption of gasoline and the purchase of certain kinds of automobiles. Some of these pressures were based on legitimate public-policy concerns. Others, such as a widespread misinterpretation of air-quality trends and a misguided fixation with “Smart Growth,” were wasteful detours. Still, whether justified or not, some politicians and activists began to see gas and car taxes not simply as means, however imperfect, to raise transportation revenue in proportion to consumer use, but also as policy levers to engineer social, environmental, or even foreign-policy outcomes.

With this in mind, consider the proposed replacement over time of gas taxes with mileage-based fees to finance road construction and maintenance. In theory, there is much to be said for the idea. In practice, there are important challenges such as guaranteeing that mileage charges would truly replace, not just supplement, gas taxes.

Political opposition may well come from those who wish to use the gas tax to force people into compliance with carbon-emissions caps and out of their sport-utility vehicles. SUVs will be more expensive to own and operate in gas-tax regimes than in mileage-tax regimes. Furthermore, advocates of alternatives such as electric cars and hydrogen fuels have been counting on a continued tax penalty on gasoline to make their pet ideas feasible. If drivers are charged per mile, it won’t matter to the state which fuels you use to travel those miles.

Because I dislike using the tax code for something other than apportioning the cost of necessary government programs fairly and efficiently, I like the idea of keeping the state neutral in the market for fuel. Somehow, though, I doubt rent-seeking politicians and interest groups will agree.

Hood is president of the John Locke Foundation.