Why are mountain roads generally made winding upwards rather than going straight up ?

Explanation

While going up a mountain , the force of friction acting on a vehicle of mass m is t = `mu R = mu mg cos theta` , where `theta` is the angle of slope of the road with the horizontal . To avoid skidding force of friction (f) should be large and therefore , cos `theta` should be large and hence `theta` should be small.
That's why mountain roads are generally made winding upwards rather than going straight upto and avoid skidding.