An autumn chill has arrived, and the beach crowds have vanished, yet the lines of cars on the Chesapeake Bay Bridge say it's still summer.

The immediate problem is that a botched paving project forced Maryland authorities to close two of the five lanes on the Route 50 bridge. But the slow traffic that beachgoers have long endured on summer weekends is becoming more common, and drivers can find themselves stuck in jams at any time.

A growing segment of the Eastern Shore population uses the bridge to get to jobs, school, family, entertainment and most anything else that doesn't include hunting, fishing or farming. Likewise, booming populations in the Washington and Baltimore areas count on the bridge as their gateway to beach destinations, vacation spots and other getaways along the Delmarva Peninsula.

The result is that commuters are causing backups heading west on weekday mornings and east in the afternoon. Weekends that used to be predictable -- regular bridge users knew not to go east on Friday night and west on Sunday afternoon -- have become anything but. Now drivers are just as likely to be riding the brakes going toward the Eastern Shore on Sundays or heading to Washington on Friday afternoons.

"More people moved over to the Eastern Shore than we ever thought would when we built the bridge," said Catherine Leahan, director of public safety and information for the Maryland Transportation Authority, which operates the Bay Bridge. "That means more traffic on the bridge on a daily basis. I'm sure if the engineers had a crystal ball when they built that bridge, there'd be a lot more lanes on it."

The first of the bridge's two 4.3-mile spans opened in 1952. The structure was hailed at the time as an engineering marvel that would make a splintered state whole.

It did. By 1973, so many people were crossing it that a second span, this one three lanes, was opened.

The improvements to Route 50 on both sides of the bay have made travel easier and attracted more traffic. In the early 1990s, some stop signs and stoplights were removed from the road, and other lights were timed so drivers could buzz through towns at reasonable speeds. A bypass around the city of Salisbury cut 10 to 15 minutes off trips to the shore.

Traffic volume increased from about 19 million vehicles a decade ago to more than 25 million last year. The backups can be staggering, even by Washington standards: Bridge officials said the worst they've recorded was 15 miles, a notch longer than a 14-mile jam that was caused in September by the repaving project.

What makes the bridge so popular -- and important -- is that there is no real way around it. The closest alternative is about 65 miles north, near the Delaware line. To the south, drivers would have to travel nearly four hours to cross the Chesapeake Bay Bridge-Tunnel at the far southern tip of Virginia's portion of the Eastern Shore to bypass the Bay Bridge.

"I think there's no doubt that the improvements that have been made in the early '90s to Route 50 . . . has allowed people to get to the beach a lot quicker," said Russ Brinsfield, mayor of the tiny shore town of Vienna, Md. "But the unintended consequences were that people started to see they could live in Queenstown or Easton or Cambridge" and work on the western shore.

"The first bridge and second bridge made it really easy for people to get back and forth," said Brinsfield, who remembers the days when residents took a ferry across the bay. "But we've reached critical mass now, where the number of people commuting is exceeding the capacity of the bridge."

In 1990, the population of the five upper-shore counties was 182,000. Over the next decade nearly 30,000 people moved there, increasing the population to 210,000 in 2000. Nearly 13,000 more moved between 2000 and 2003, and there are plans for thousands more houses in towns up and down the shore.

Maryland Transportation Secretary Robert L. Flanagan said that traffic problems have reenergized talks about adding bridge capacity, although he said additional lanes could be as much as 15 years away. As for the ferry service that many would like to see in the meantime, Flanagan said, the state looked into that option and concluded that "a ferry trying to have an impact on Bay Bridge traffic is like a gnat trying to stop a Mack truck."

At the Club One health club in Chester, Md., where most exercisers work on the Washington side of the bridge, a steady stream of sweaty bodies peers out the front window each morning to see if traffic has slowed on the handful of miles between the club and bridge, said Melissa Hartman, a club employee. If it has, that means no shower for many members who know they're already going to be late for work.

Hartman said she used to go over the bridge all the time, but traffic has confined her to the two times a week she drives to classes at University of Baltimore.

Traffic on the bridge "disrupts everybody's life," she said, bemoaning the loss of regular trips to Trader Joe's and movie theaters.

Jim Cotter moved to the Eastern Shore town of Centreville, Md., this year after 34 years in Anne Arundel County and said that while he's happy he did, bridge traffic is a "crapshoot."

"Sometimes it's not too bad during the week," said Cotter, who commutes to Greenbelt. "Other days, you have tremendous problems." Such as the night it took him three hours to get across. "The occasions when you get stuck are pretty miserable," he said. "But it's not like you can go another way."