When planners mix cars and trucks on a crowded interstate highway, accidents happen. That is one of many reasons why the off-again, now on-again Barney Circle interstate should not be built. It is bad news for almost everyone.

The Barney Circle project is described by planners as a substitute for the unbuilt "East Leg" of the 1960s "inner beltway." Like the "East Leg," it would connect the Southeast Freeway (I-295) to the Anacostia Freeway, I-95 and I-495 (via the newly widened Rte. 50 and the Baltimore-Washington Parkway). Community opposition stopped most of the inner beltway, including the "East Leg."

East Leg redivivus, or Barney Circle, would carry at a minimum 77,000 vehicles per day to and from the Southeast Freeway. With almost 20,000 vehicles per lane daily, it would instantly become the District's most congested freeway.

It means, too, at least 150 more trucks each hour on the Southeast Freeway -- in each direction -- and a minimum of 340 trucks hourly at midday. It would more than double existing truck volume on an already congested road.

These estimates represent regional traffic only -- not interstate or Beltway traffic. Planners claim, despite their stated purpose to "link" interstates, that interstate or Beltway traffic would not be "diverted" to this new cross-Washington Beltway bypass. Their argument -- that interstate drivers will prefer the increasingly congested Woodrow Wilson Bridge crossing -- ignores that one-half of Beltway traffic now is regional traffic.

These traffic volumes underscore how potentially destructive the inner beltway concept has become. Conceived in the 1960s as a means of routing traffic around the urban core, it acts in the late 1980s -- as the Anacostia Freeway and the Beltway capacity near exhaustion -- to bring circumferential traffic into the urban core.

Apparently embarrassed by candor (the environmental impact statement never mentions the figure "77,000 vehicles per day"), planners have sought, for four years, to market the project as a boon to commuters and adjacent communities alike. The argument won't wash. Adding yet more vehicles, and weighting the traffic mix with more trucks, can only exacerbate the dangers of existing Southeast Freeway congestion. This is so even if scheduled -- and needed -- safety improvements are made.

Northeast corridor commuters -- who might well "divert" to the Green Line if the District could find money to finish it -- won't "divert" to a congested freeway. The inescapable fact is that there's no more room during either rush hour. (The project's main impact may, in fact, be to relocate evening congestion to the vicinity of Anacostia Park and River Terrace.) Planners recognize this: they want to annex M and 7th streets SW to service freeway traffic connecting with the 11th Street (Anacostia River) bridge!

The project won't help adjacent communities either. Long-cherished hopes of reducing through traffic will inexorably be set back as an increased total traffic volume, encountering an already-congested rush-hour freeway, "spills over" into neighborhoods. (One example, among many, is 8th Street SE, which is projected to see 1,000 freeway cars each weekday morning.)

District planners know this too -- hence their inability to claim that the link, by itself, will reduce traffic on local streets. Hence, too, their twin refusals to study the effectiveness of controls alone, without freeway link, to reduce such traffic or to implement any controls until "after Barney Circle" is "constructed" and "evaluated." These conditions are a prescription, not for realizing those goals, but for placing them permanently beyond reach.

Additional reasons not to build this $140 million highway are its effects on Anacostia Park, the communities surrounding the Southeast Freeway and the historic Congressional Cemetery. Anacostia Park, which lost 110 acres to freeway construction in the 1960s, would lose another 17 acres, mostly on the razor-thin west bank. The remaining park would be cut in half and engulfed in the interstate din of trucks and cars. Yet Frederick Law Olmsted Jr. inspired the McMillan Commission in 1901 to propose this park to create "an aspect of spaciousness and serenity."

The noise and fumes of 77,000 vehicles would not be confined to the park but would diminish the amenity of all neighborhoods along the Southeast Freeway corridor.

Last, it would permanently degrade the still-wooded vista from, and tranquillity of, the nation's first national cemetery. One hundred and forty million federal dollars for an eyesore and decibels well in excess of federal limits for historic sites are unfitting and perhaps ungrateful memorials to the distinguished federal officers, congressmen, mayors, soldiers, diplomats and artists buried there.

The Barney Circle project is opposed by 2,500 residents of Capitol Hill and a variety of community and political groups. It remains unbuilt, years after the last interstate segment was started, for a very good reason. Whether embodied as the East Leg, requiring the bulldozing of homes, or as an interstate bridge engorging scarce river-front parkland in the nation's capital, it helps neither commuter nor community resident and could hurt most everyone except those seeking to bypass the Beltway. There are better ways to help commuters and communities deal with their problems than by repackaging the failed panaceas of the past. -- Christopher C. Herman is vice chairman of the Congressional Cemetery Association. -- Janet Wintermute is chairman of Citizens Committee to Stop It Again. BY LARRY FOGEL -- THE WASHINGTON POST FIRST LEG 2 LINES LONGER