Hunter College student and photographer Andrew Lynch recently posted Google Map mashups of the unbuilt freeways  that made up many of the master plans in cities around the country in the 1950s and 1960s. San Francisco, New York City, and Boston avoided the worst of automobility, while the map of Los Angeles freeways was pretty well paved.
The maps are a stark reminder of how devastating the plans would have been to San Francisco's livability had the public not revolted against the master planners . I'd currently be huffing fumes from the Mission/Bernal Heights freeway, there'd be no Panhandle, and North Beach and the Marina would likely have much lower property values. But you can imagine the pleasure of motoring past Ocean Beach at 70 mph without forced stops at traffic signals!
Alternately, can anyone tell me why the upper portion of Market Street where it becomes Portola Drive still has elevated segments? I would think the value of new development with those views would have trumped the convenience of speeding through that neighborhood with limited stop lights.