The term switchback comes from the word switch to turn a train from one railway track to another usign a switch and back so as to reverse the direction and return.
A switchback can refer to a mountain road, railway, or track which rises and falls sharply many times or a sharp rise and fall on such a road, railway, or track.
A switchback is a road which goes up a steep hill in a series of sharp bends, or a sharp bend in a road.
Hairpin turns are often built when a route climbs up or down a steep slope, so that it can travel mostly across the slope with only moderate steepness, and are often arrayed in a zigzag pattern.
A switchback is a type of path that follows a zig-zag pattern up a steep stretch of terrain such as a hill or mountainside.
Rather than climbing directly up the slope, switchbacks run from one side of the slope's face to the other before “switching back” and continuing in the opposite direction.
Switchback roads are better than straight roads over the mountains because going straight down a mountain would result in a VERY steep road - so steep that no wheeled vehicle could get up it - and you'd be unable to stop going down it.
The reason is a matter of grade - that is to say the amount of distance you go VERTICALLY for whatever distance you travel HORIZONTALLY.
Roads going up mountains are formed into hairpin turns, a sharp bend in a road on a steep incline.
These turns, also known as switchbacks, are named hairpin turn because its resemblance to a hairpin/bobby pin.
The USA is a vast country with the majority of roads being laid down after the advent of the motor car.
The roads are generally straight between urban areas with sufficient space to have 6 lanes each way.
That said when you get into the mountains the roads narrow down to 2 lane highways.