Shortly after last week's fiery rush-hour accident on the Cabin John Bridge, eastbound MacArthur Boulevard looked like a parking lot. Six hours later traffic had advanced only to a crawl.

Downstream on the Potomac, five cars overheated on Key Bridge and there was a bottleneck at the Wilson Bridge. Heeding radio bulletins, motorists abandoned their cars to seek shade in the 95-degree heat.

At the intersection of MacArthur Boulevard and Cathedral Avenue, one enterprising young man set up a stand to sell cold sodas to stranded motorists.

By 2 a.m. Thursday, nearly 11 hours after the crash that left two men dead and another injured, there was still a three-mile traffic backup at Cabin John Bridge.

It was a classic example of what in the traffic lexicon might be called "gridlock," a term coined for massive tie-ups that have plagued Manhattan commuters during subway strikes. Last week, all the frailties in the Washington system were on display.

Regional transportation authorities offered a variety of reasons. One conclusion was that area commuters suffer from a lack of imagination and rigid habits: Many know only one way home, leave work at the same hour every day, and are loath to try new routes. But blame is laid equally on geography, with the Potomac River creating a moat around the District that is bridged at only a few points.

Then there is the special problem of the Capital Beltway, which authorities say never was intended to handle the number of vehicles it now does. Even a minor accident compounds the already severe traffic problems. And in case of fire, the complications become nearly insurmountable, as the highway and the bridges were built without hydrants.

Ed Meehan, State Highway Administration (SHA) district engineer, blames the beltway's propensity for traffic jams on the area's tremendous commercial and residential growth. "When you try to place the beltway traffic on other roads, everything breaks down," Meehan says. "Most of the roads are at capacity. The network just won't handle it. It's a problem that's not going to ever be solved."

Suburb-to-suburb commuting also has increased beltway traffic over the years.

"The Washington beltway, like most beltways around the country, was envisioned as a highway to bypass traffic around the metro area," explains Francis B. Francois, executive director of the American Association of State Highway and Transportation Officials.

"It was to take New York-to-Florida traffic out of D.C. and put it in the countryside." Now, he says, motorists use the bypass instead as a main street.

And one of the "main street's" critical links is the Cabin John Bridge, where last week's gridlock resulted after a tractor-trailer collided with a passenger car and burst into flames at the beginning of rush hour.

Only a crisis on the Wilson Bridge could have snarled traffic the way the Cabin John accident did, Francois said. "Once you're onto a bridge or an approach, there's nowhere else to go," he said. Washington's Key, Memorial and Roosevelt bridges can be circumvented by using alternate routes.

The 20-year-old Cabin John Bridge handles an average of 120,000 vehicles a day, with 11,000 cars an hour at the peak of evening rush hour. It's due for a $20 million repair job starting in the summer of 1984.

"The problem is the alternates for getting across the river are limited out there," said Ron Welke, chief of the Montgomery County Department of Transportation's division of traffic engineering.

White's Ferry is about 20 miles north, past Poolesville. (The ferry, which takes six cars at a time on its 10-minute crossing, was overburdened last Wednesday. Police had to close off the road to the ferry after too many cars backed up.) And the next bridge, at Point of Rocks, is about 40 miles away in Frederick County.

"The only legitimate alternative was to go into the District, which on top of the usual congestion, was a mess," Welke said.

The Potomac may make Washington's traffic problems unique, but other cities also suffer beltway backups. Atlanta's beltway is being enlarged, which causes major delays as lanes are closed for construction.

Houston's, also intended as a bypass and now heavily used by local traffic, is seriously overcrowded. Planners there are looking at a double-deck beltway to ease the problem.

"Stacking them up on a double-deck is one possibility we may see in the '80s and '90s," Francois said. "But it's not justified in Washington at this point."

Experts offer little hope for avoiding future Washington gridlocks. Francois says there is only a remote possibility of building another bridge across the river. Environmental laws and lack of available land limit potential new crossing points, he says.

Plans for an outer beltway, widely discussed throughout the 1970s as an added way to bypass the metropolitan area, largely have been scrapped, partly for political reasons.

As Francois said, "In Montgomery County, it would mean going through Potomac a wealthy and politically influential community , and you can imagine what the reality of that is."

Ultimately, transportation officials are resigned to the traffic system's inability to cope in times of crisis. Says SHA engineer Meehan, "There's not enough land or money to build highways to have enough alternate routes when something like this happens."