The government can either sell the cars it buys to licensed recyclers for scrap, or refit them with new emissions controls and resell them. But the government must not ship the cars to poor countries, where they would continue to belch pollutants.

Cash for Clunkers is not the pipe dream of some academic scribblers. Local variants are either now in operation or have been tested in California, Colorado, Delaware, Illinois, Texas, Virginia and several Canadian provinces. So there is no need for a “proof of concept.” Rather, a national Cash for Clunkers program could learn from all this experience in building a better system.

THE big need to date has been money, which is why the scope of Cash for Clunkers programs has been limited. And that, of course, is where the need for stimulus comes in. We now want intelligent ways for the federal government to spend money.

Here’s a high-end cost calculation for a national program. Suppose we took two million cars off the road a year, at an average purchase price of $3,500 (the top price in the Texas program today). Including all the administrative costs of running the program, that would probably cost about $8 billion. Compared with other nationwide income-transfer or environmental policies, that’s a pretty small bill. For stimulus purposes, it would, of course, be better to run the program on a larger scale, if possible. There are over 250 million cars and light trucks on American roads, and some 30 percent are 15 years old or older. That’s at least 75 million clunkers. At five million cars a year  an ambitious target, to be sure  the program would cost less than $20 billion, still cheap compared with the $168 billion stimulus enacted in February.

And what would all this money buy? First, less pollution. The Texas program estimated that clunkers spew 10 to 30 times as much pollution as newer cars. Second, the subsidy value (the 20 percent premium in my example) is a direct income transfer to the owners of clunkers, who are mostly low-income people. Third, these folks would almost certainly spend the cash they receive  not just the subsidy, but the entire payment, giving the economy a much-needed boost.

Oh, and I left out a fourth possible goal. By pulling millions of old cars off the road, Cash for Clunkers would stimulate the demand for new cars as people trade up. It need hardly be pointed out that our ailing auto industry, like our ailing economy, could use a shot in the arm right now. Scrapping two million or more clunkers a year should help.

With today’s concerns over stimulus, inequality and greenhouse gases, as well as an aging vehicle fleet, Cash for Clunkers is an idea whose time may finally have come. Write your congressman.