Horizontal curve

When we say it's a straight road that usually means that there is no curve and by curve it usually means horizontal curve where you need to steer the wheel to the left or right. If you turn to the left, the curve is also called a left-turning curve and similarly it is defined for right-turning curve. The horizontal alignment of a road is how the road is stretched on the surface of the earth. That is, when you look at a road from above how it is laid on the ground. Well, sometimes it is straight and sometimes it turns in different directions. If we slice the road along its length, we see a profile of the road that is called vertical alignment, which includes all the longitudinal slopes of the road along its length.

Now Think of driving on a local road with a high speed and you come across a curve that you feel like you must reduce your speed to be able to safely pass the curve, or otherwise, you may roll over because of the centrifugal force that is applied to you driving through the curve. That is because you have a higher speed than the design speed of that curve. There is a standard for designing the horizontal curves based on the safe passing speed, road friction, sight distance, etc. According to the dynamics laws, the centrifugal force which is a function of the speed has to be counteracted so that vehicle can stay on the road, and not roll over. This problem is taken care by adding a little transverse slope (called superelevation) to the road so the road's friction and the transverse component of the vehicle's weight together counteract the curve's force and we stay on the road. There are tables to relate design speed, superelevation, and curve radius for low speed and high speed roads. 

When there is an obstacle on the side of the road, there is minimum sight distance that has to be determined and from that, a minimum curve radius is determined. related details about this matter can be found in this page (accessed 9/24/2015). Here are all the equations for horizontal curve.