According to US federal building codes, the maximum height for an access ramp is 30 inches. To reach entrances that exceed this limit, lifts or an elevator may be installed. Most cars cannot climb hills that have a slope of 30° (or rise about 1 foot for every 1.7 feet horizontally). Most local building codes describe a maximum stair rise of 8-1/4 inches and a minimum tread width of 9 inches. More than nine out of ten avalanches occur on slopes ranging from 25° to 45°. The ratio for slope is independent of units. Most black diamond ski slopes are around 30° to 35°, while a 40° slope would be considered the low end of extreme mountaineering. Parts of Tuckerman’s Ravine on Mt. Washington have slopes of about 50°. Slopes are rates of change, a fundamental idea of calculus. Another way to express a slope uses the trigonometry notion of tangent.
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