I came across with Matt Hickman's 8 of the World's Steepest Streets and this picture...
I feel dizzy and vertigo is already trying to imbalance me. :)
"Full of twists and turns and lined with beautiful trees and scenery, Hawaii's Waipi'o Valley Road has an average gradient of about 25%, with some stretches reaching gradients of up to 40%.7
Waipi'o Valley Road on the northeast coast of Hawaii's Big Island is one of the only steep streets on this list that isn't accessible to public transit. In fact, only four-wheel-drive vehicles may drive on this paved, one-lane road through the lush Hawaiian rain forest. Many local car rental companies do not permit customers to drive down this street in rented vehicles due to the high frequency of accidents and breakdowns."
Check the other 7 of the 8 of the World's Steepest Streets described by Matt Hickman.