Curves - Types of Curves - Horizontal and Vertical

Definition:

Curves are provided whenever a road changes its direction from right to S (vice versa) or changes its alignment from up to down (vice versa). Curves are a critical! element in the pavement design. They are provided with a maximum speed limit that should lie followed very strictly. Following the speed limit becomes essential as the exceed in speed may lead to the chances of the vehicle becoming out of control while negotiating a turn and thus increase the odds of fatal accidents. Also, it is very necessary that appropriate safety measures be adopted at all horizontal and vertical curves to make the infrastructure road user friendly and decrease the risks of hazardous circumstances.

The low cost safety measures that can be adopted at curves included chevron signs, delineators, pavement markings, flexible posts, fluorescent strips, road safety barriers, rumble strips etc.

Types of Curves

There are two types of curves provided primarily for the comfort and ease of the motorists in the road namely:

1. Horizontal Curve
2. Vertical Curve

Horizontal Curves

Horizontal curves are provided to change the direction or alignment of a road. Horizontal Curve are circular curves or circular arcs. The sharpness of a curve increases as the radius is decrease which makes it risky and dangerous. The main design criterion of a horizontal curve is the provision of an adequate safe stopping sight distance.

Types of Horizontal Curve:

Simple Curve:

A simple arc provided in the road to impose a curve between the two straight lines.

Compound Curve:

Combination of two simple curves combined together to curve in the same direction.

Reverse Curve:

Combination of two simple curves combined together to curve in the same direction.

Transition or Spiral Curve:

A curve that has a varying radius. Its provided with a simple curve and between the simple curves in a compound curve.

While turning a vehicle is exposed to two forces. The first force which attracts the vehicle towards the ground is gravity. The second is centripetal force, which is an external force required to keep the vehicle on a curved path. At any velocity, the centripetal force would be greater for a tighter turn (smaller radius) than a broader one (larger radius). Thus, the vehicle would have to make a very wide circle in order to negotiate a turn

This issue is encountered when providing horizontal curves by designing roads that are tilted at a slight angle thus providing ease and comfort to the driver while turning. This phenomenon is defined as super elevation, which is the amount of rise seen on a given cross-section of a turning road, it is otherwise known as slope.

Vertical Curves

Vertical curves are provided to change the slope in the road and may or may not. be symmetrical. They are parabolic and not circular like horizontal curves. Identifying the proper grade and the safe passing sight distance is the main design criterion of the vertical curve, iln crest vertical curve the length should be enough to provide safe stopping sight distance and in sag vertical curve the length is important as it influences the factors such as headlight sight distance, rider comfort and drainage requirements.

Types of Vertical Curve:

Sag Curve

Sag Curves are those which change the alignment of the road from uphill to downhill,

Crest Curve/Summit Curve

Crest Curves are those which change the alignment of the road from downhill to uphill. In designing crest vertical curves it is important that the grades be not] too high which makes it difficult for the motorists to travel upon it.