The Interstate Highway Act was proclaimed the "largest public works program since the Pyramids" when it was passed in 1956. Now it is being hailed as the single most important influence on the American metropolis in the last 50 years.

That distinction was bestowed on the 41,000-mile highway system and the dominance of the automobile by 149 members of the American City and Regional Planning History, an interdisciplinary professional organization composed of urban historians, social scientists, planning faculty and working planners and architects.

The Interstate highway system "transformed the American metropolis in ways its planners never anticipated," says Robert Fishman, a history professor at Rutgers University who led a panel of urban specialists in developing a list of 25 possible choices for the past as well as the future. "More than any other measure, the 1956 highway act created the decentralized, automobile-dependent metropolis we know today."

Tomorrow, we'll cover the ten most likely major influences over the next half-century. For now, lets look at the highway act and some of the other ten most important influences of the last 50 years, including the Federal Housing Administration, urban renewal, Levittown, enclosed shopping malls and air conditioning.

The highway system was supposed to save central cities by rescuing them from automobile congestion, and to provide high-speed, cost-to-coast travel "without a traffic light." But instead, they themselves became clogged with cars moving in and out of downtown, and their construction destroyed many viable urban neighborhoods.

At the same time, the new peripheral by-passes known as beltways that were intended to allow traveler to go around crowded cities turned into main streets themselves, linking housing- hungry families to cheap rural land. In turn, the houses, shopping centers and office parks they bred drew people out of the city core and into suburbia.

The experts also recognized that the suburbs were built on the FHA's low-downpayment, long-term fixed rate mortgage, which was developed to replace the 50 percent down, five-year loans that were normal prior to the post-war era.

"By the seemingly simple expedient of insuring loans issued by federally-chartered thrift institutions against default, the FHA created the financial instrument that would raise American home ownership from 44 percent in 1940 to the record 67 percent of today," Fishman points out.

But the FHA also was responsible for a couple of other things it rarely gets credit for. For one thing, it imposed standards for both home and subdivision design that quickly became the norms for the home building business. But for another, it limited loans to what amounted to race-restricted new housing in the suburbs, in effect excluding minorities from access to better lives.

Here's a rundown on the other eight most important influences on the American landscape: