In my opinion, to prevent the line passing from the center of gravity of a body (e.g bike or car) trying to climb the mountain or hill to fall outside the base of support so to maintain stable equilibrium and balance. This prevents them from falling down the hill. Is my answer right?
closed as unclear what you're asking by ACuriousMind♦, John Rennie, Kyle Kanos, Brandon Enright, BMS Jul 28 '14 at 15:56
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I think I know what you're getting at here. Mountain roads are windy for a different reason: Power.
Yes, if the hill is steep enough, some vehicles would have to worry about tipping over backwards. However, if you take a look at off-roading vehicles, you'll find they can go up some pretty steep slopes just fine. So why not go straight up a mountain?
The real issue here is power. Most vehicles don't have the ability to exert enough force fast enough to climb up a steep slope like that. Or, if they do have enough power, they don't have enough gasoline to do that again and again. Either way, by making vehicles go up a longer, but less steep slope, the vehicle has to exert less power to do so. The idea is that it's easier to exert less power over a longer period of time.
Also, going down those steep slopes may cause more accidents than the winding mountain road. It would be too steep to rely on engine brakes, so you'd have to use your disk brakes more, which could result in more burnt-out brakes.
To reduce risk of slipping by increasing friction between tyres and road. while going up a mountain the opposing frictional force $F = μN =mg\cos\theta$ where $\theta$ is the angle of slope with horizontal .To avoid skidding $F$ should be large,so $\cos\theta$ should be large and hence $\theta$ should be small.so roads are made winding upwards.the road straight up would have larger slope.