The graceful loops and simple design were meant to let motorists maneuver in any direction on a freeway interchange without braking.
So ubiquitous is the roadway design with its four circular ramps -- resembling a four-leaf clover -- that cultural critic Lewis Mumford once joked that the freeway cloverleaf had become the national flower.
But the bloom is off.
The cloverleaf interchange has outlived its usefulness, creating traffic jams instead of free-flow movement, transportation experts now say.
Across the country, transportation agencies have launched expensive reconstruction projects to replace obsolete cloverleaf interchanges -- in New Jersey, where the cloverleaf first grew; in Ohio; in Texas; and in California, where Caltrans has budgeted millions of dollars to replace cloverleaf intersections from Orange County to San Jose.
The cloverleaf's biggest flaw is that motorists merging onto a freeway and drivers trying to get off converge on the same short length of road, creating a gantlet of speeding vehicles battling for a few precious gaps in traffic.
The design works fine when traffic is light, but when traffic gets heavy -- as it does regularly in Southern California -- motorists slow in every direction, transportation experts say.
A 1999 study by the Virginia Department of Transportation concluded that cloverleaf interchanges begin to bog down when traffic surpasses 1,000 vehicles per hour. During peak rush hours, the cloverleaf intersection of the Riverside (91) Freeway and the Pomona (60) Freeway -- now being rebuilt at a cost of $317 million for four years of construction -- carries about 14,600 vehicles per hour in each direction.
The Virginia study also concluded that cloverleaf interchanges have the most "fixed object accidents" of any interchange design. Such accidents usually happen when motorists are forced off the road and into a wall or tree in the battle to merge with other drivers.
Studies of freeway accidents have yet to determine which freeway design is the most accident-prone overall.
An engineer from Maryland named Arthur Hale patented the cloverleaf design in the United States in 1916 for use primarily on surface streets. The advantage of the design was that a motorist could turn from one road to another without stopping and without having to make a left turn across the path of oncoming traffic. On a cloverleaf, motorists make the transition left by swinging right along a circular loop under an overpass, turning 270 degrees.
The first cloverleaf was built in 1929 at the intersection of two major roads in Woodbridge, N.J., the state's oldest township.
In the 1920s, New Jersey roadways had a traffic density seven times the national average, or nearly 60,000 vehicles per day, largely because of motorists traveling between New York and Philadelphia.
To accommodate demand at the intersection of the two heavily used routes in Woodbridge, the state was willing to test the innovative cloverleaf design.
The designer of the intersection, Edward Delano of the Pennsylvania-based contracting firm of Rudolph & Delano, got the idea after seeing an illustration of a cloverleaf interchange in Argentina on the cover of an engineering journal, according to the New Jersey Department of Transportation.
The design caught on and was used on roads throughout New Jersey and, eventually, throughout the nation. The disadvantages did not emerge until much later.
"In 1929, we didn't have that many people in the U.S., and not many were driving cars," said Jim McDonnell, an associate program director for the American Assn. of State and Highway and Transportation Officials. The association has been meeting and distributing road design specifications since 1914 and played a key role in spreading word of the cloverleaf design throughout the country.
The design gained popularity because it was usually cheaper than alternatives that included long, elevated ramps to carry traffic over two or three roadway bridges. Although the cloverleaf design needs extra real estate for the loop ramps, cheap land was plentiful when the nation's freeway system began to expand in the 1940s and '50s.
Beginning in the years after World War II, California added more cloverleafs to the state's freeways than did any other part of the country. At that time -- before "rush hour" and "SigAlerts" became part of the region's lexicon -- the looping ramps moved motorists fairly efficiently from one road to another.
But "traffic levels grew faster than anyone predicted," said Robert Poole, director of transportation studies for the Reason Foundation, a Los Angeles-based think tank.
In retrospect, installing cloverleaf interchanges in fast-growing Southern California "wasn't the smartest move," said former Caltrans Deputy Director Chuck O'Connell, who helped design several Los Angeles County freeways in the 1950s and '60s.