President George W. Bush and his conservative allies like to portray themselves as reformers, intent on modernizing government programs like Social Security that were built for a bygone era and no longer fit the demands of the current age.

There’s something to the claim. If you look hard enough, you can see Bush’s impulse for reform showing itself in the unlikeliest places – even, oddly enough, on the nation’s highways.

Bush has threatened to veto the highway bill now slithering its way through Congress. Bush’s budget allowed $284 billion to be spent on highways through fiscal 2009. The highly disciplined House of Representatives deferred to that figure, but the unruly Senate brought in a fatter version of the bill, worth at least $295 billion.

Conferees from both bodies will now slip into their butcher’s smocks and dismember the thing, hoping to put it together again to everyone’s satisfaction before sending it off to the president by the end of summer – though whether they can do so at $284 billion is an open question.

Bush has yet to veto any legislation presented to him by Congress, and his threat to veto the highway bill is usually described as the act of an exasperated executive who’s finally cracking down on the spendthrifts.

Well, maybe. But there may be something more at work here, too.

Needless to say, Bush would be fully justified in vetoing the highway bill on grounds of fiscal extravagance alone. The bill, which is funded largely through gas taxes, is a sausage-maker’s fantasy, stuffed with weird cuts of pork that might even astonish a French gourmand.

The lawmakers did try to restrain themselves. They haven’t thrown just any old pork into the highway bill. In deference to its origins as a bill for funding roads and bridges, they larded it mostly with projects that can somehow, by some generous stretch of the imagination, be linked to the general subject of human mobility.

Thus it provides a freshet of funding to a private program called “Safe Routes to Schools.” The program has nothing to do with building roads, of course. Instead, “it promotes walking and biking to school through education and incentives that show how much fun it can be.” Another grant totaling $3 million will rain down on Warren, Ohio, famous around the world as “The Birthplace of the Packard.” Bucolic Warren will spend the money on its National Packard Museum.

According to an analysis by Paul Gessing of the National Taxpayers Union, a Washington based group that advocates less federal spending, the House bill alone contains more than 4,000 such “earmarks,” as the pork projects are called. There’s money for ferryboats, graffiti-removal programs, and new lighting at bus stops.

Bush’s veto would be an admirable act of fiscal discipline, forcing Congress toward a new standard of truth in labeling: A bill that’s meant to fund road-building should pay for roads, period. If a congressmen or senator wants a program to remove graffiti from subway trains, he should fund it through a graffiti-removal bill.

A Bush veto could do more than this. As Gessing points out, the federal highway program is a relic from another age – a classic case of a government project that can’t be killed, even though it long ago fulfilled its original purpose. Begun in the 1950s, it was originally set to expire in 1972.

“It was set up to fund the interstate highway system,” Gessing says, “but we’re no longer funding a nationwide system. The federal government is just collecting money from the states” – in the form of gas tax revenue – “and then returning it to the states to fund state road projects.”

A far more efficient system would simply leave that revenue under state control, allowing states themselves to determine which highway construction projects would best suit the requirements of an increasingly congested road system.

And that’s the reform a Bush veto would advance. Restricting runaway spending at the federal level could begin to transfer responsibility for road-building from the feds to the states – where spending and financing decisions will be more rationally made.

Or so we can hope. Already states are showing a willingness to end-run Congress, experimenting with alternative ways of funding road construction, from electronic tolls to privately financed highways.

Such experiments in economic efficiency are unlikely to be advanced seriously at the federal level, where congressmen and senators continue to see highway spending as a slush fund for satisfying favorite constituencies, like the bicycle enthusiasts at “Safe Routes to Schools.”

Yet that wasteful course is probably unsustainable. From 1990 to 2003, according to the U.S. Department of Transportation, vehicle travel on U.S. highways jumped 35% while the population grew 17%.

“As gridlock gets worse, and people get more and more frustrated, states are going to see their real self-interest lies in saying, ‘Gee, we can do this better than you can in Washington,’ ” says Gessing.

Is an economically rational, politically workable system for funding road construction really possible? It’s an audacious goal – and a legacy that any would-be reformer, like George Bush, could point to with a happy heart.

Mr. Ferguson is a columnist for Bloomberg News.