Lombard Street in San Francisco is widely regarded as the curviest street in the United States (or in the world, depending on who you ask. :) In a single block, it curves nearly 180 degrees eight times. Here's a view from the top:
And here's a view from the bottom:
I've driven down Lombard in a car and I was terrified. I can't imagine doing it in a big van! :)
The switchback design, first suggested by property owner Carl Henry and instituted in 1922, was born out of necessity in order to reduce the hill's natural 27% grade, which was too steep for most vehicles to climb and a serious hazard to pedestrians used to a more reasonable sixteen-degree incline. The speed limit is a mere 5 mph (10 km/h) on the crooked section, which is about 1/4 mile (400 m) long.
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