BRIDGES ARE closed as unsafe. Great highways are being ground up under volumes of traffic beyond the imagination of their designers. New York's subway has become a national symbol of municipal decrepitude. Douglas B. Feaver has described it in detail, in a series of articles in this newspaper, and readers can supplement his account with tales of their own adventures in trying to get around this city. What's going on here? Why this rapid collapse of quality and safety in a fine transportation system in which Americans once took great pride?

Don't bother with the narrow technical explanations. It's got a great deal more to do with, unfortunately, characteristic American habits of mind. Particularly over the past decade or so, Americans have slid into the habit of expecting a lot from their governments without wanting to pay for it--and, worse, without seeing any reason why they should. They generally prefer to elect political candidates who tell them that they don't have to pay any more --the current tenant at 1600 Pennsylvania Avenue is not the least conspicuous example--and then they are astounded to find that there's not enough money to buy what they expected. You can see it happening in Social Security and the defense budget as well as in the highway system.

Americans like large new structures of reinforced concrete. Don't snicker -- remember what life was like at rush hour before the Woodrow Wilson and Cabin John bridges and, especially, Metrorail. They have done great things for mobility and convenience of access in this area. But when funds are tight and it's a choice between building a new road or maintaining an old one, public enthusiasm favors construction. No governor is ever invited to cut the ribbon for a repaving project, and no highway commissioner ever sees his name on a plaque commemorating the repainting of a bridge.

Congress is about to raise the gasoline tax by five cents a gallon. The congressional nickel is barely half the nine-cent increase required merely to restore the purchasing power of the present four-cent tax at the time it was first imposed 23 years ago. As the passion for driving has waxed, the passion for paying for those magnificent roads has waned.

There was an enormous boom in both public and private construction throughout this country in the decades after World War II. As the subdivisions went up at high speed in the 1950s, people sometimes wondered if there wouldn't be unmanageable suburban blight in the 1980s as all those houses aged simultaneously. But it turns out that most people have been very careful about their private property. It is their public property on which they have chosen to save money by deferring the maintenance -- and deferring it, and deferring it. Now pieces of pavement are dropping out of the Wilson Bridge into the river, and the bills are coming due.