For 48 years, two infamous right-angle turns on high-speed Lake Shore Drive slowed commuter traffic to a wriggle and caused perhaps the greatest number of fender-benders, bumps, sideswipes, scrapes and rear-enders at any point in the city's road system.

Built by planners attuned to horse-and-buggy speeds and still referred to as the "S-curve," the legendary bottleneck became history earlier this month when officials opened the first half of its replacement, projected to cost $101 million.

The southbound lanes were closed in April 1984, detouring traffic through city streets until at least 1987. Northbound traffic negotiated the old turns until one side of an elevated, gently curving highway was opened several weeks ago.

The S-curve, a ghost road awaiting only demolition, lies within view of its replacement. The new road, built on landfill along the shoreline, sweeps past its predecessor like an interstate bypassing a dirt path.

The two turns offered an incomparable challenge to the nerves and skill of 100,000 drivers each day. Now, commuters converging on downtown Chicago must find a new source of early-morning excitement.

"Some things are good to lose," traffic Patrolman Ed Altman said before the change, as he watched drivers experience the gravitational thrill of steering through a 90-degree right turn, slinging down the straightaway into a 90-degree left turn and coming out the other side.

Chicago traffic officials have consistently described the hard S-turn as "the most dangerous roadway segment in the city."

Motorists cruising Lake Shore Drive at the 40-mph limit and higher encountered a 20-mph limit while trying to snake through the right angles. Police reported an average of an accident a day.

The new configuration, which allows drivers to maintain 40 mph is no doubt safer than its predecessor, police say.

"Everybody's got something to say about how they spun out on the old S-curve," a city traffic engineer said at dedication ceremonies for the new road. Lore accumulated quickly.

John LaPlante, assistant commissioner of the Department of Public Works, read from a newspaper clipping dated Oct. 8, 1937, that said: "Auto Crashes at New Bridge Turn; Two Hurt." The report said two drag-racing drivers skipped over the center divider "less than 72 hours after the dedication."

In years since, traffic police have filed endless paperwork for minor and major accidents on the curve. "Now we have a road that's a little more forgiving," LaPlante said of the design, which planners describe as a much gentler, "smooth, sweeping curve."

The old turns inspired the Law of the Worst Car, according to a city planner. In the struggle to maneuver through the turns, he noted, "the worst car always wins . . . . If you're in your shiny BMW, and I'm in my old beater, you're going to pull over."

The origins of the bending roadway remain shrouded. In a 1938 clipping provided by LaPlante, a Park District employe asks, "Why are there right-angle turns?" and answers, "Very often the employe is unable to answer this question intelligently."

Explanations for the design range from the pragmatic "it was the cheapest way to join the north and south ends of the existing roads" to the romantic "back then, planners still thought in terms of how fast horses traveled, and right angle turns weren't a problem."

At the dedication ceremony Oct. 7, a brilliant autumn sky, mirrored in Lake Michigan, gave the new road a postcard backdrop. Mayor Harold Washington recalled the 1937 dedication as proof of "something momentous going on." He called the straightening of the road "part of the renaissance of Chicago."

After freeing a flock of red, white and blue balloons and cutting a purple ribbon stretched across the new piece of road, the mayor took a northbound spin in a gleaming black 1937 Cadillac Phaeton. The Great Lakes Navy Band struck up marching music and guests helped themselves to coffee and doughnuts.

The north-south arc of Lake Shore Drive traverses Chicago's eastern flank as drivers pass 12 1/2 miles of Lake Michigan shoreline. Along the way are harbors full of sailboats rocking at their moorings, wooded parks full of color in autumn, ice formations on rock jetties in winter, cyclists and runners in spring and acres of volleyball players and sunbathers on the beaches in summer.

All four replacement lanes are open for northbound traffic, but southbound motorists must detour through city streets. Demolition of the abandoned S-turn is scheduled for the coming months. "We can't build in the winter, but we can tear down," LaPlante said. City officials hope to finish the project by late 1987.

Ed Wileyto, project manager for the Lake Shore Drive relocation, said that while this month's dedication ceremony marked "a big moment," he suspects that his new road also could become obsolete. "Maybe in 50 or 70 years, this will be outdated," he said.

The scrapes and sideswipes Chicagoans endured on the S-curve were part of daily life here. But while traffic on Lake Shore Drive is being straightened out, nobody has figured out a civilized way to unravel Chicago's other famous bottleneck: the City Council.