Tangents, circular and transition curves

The horizontal alignment of a road is made of three elements: tangents (straights), circular curves and transition curves. Each has its own use.

1. Tangents (straight sections)

• Shortest and cheapest route between two points; easy to survey and construct.
• Good sight distance and overtaking opportunity; drivers can maintain the highest speeds.
• Needed to join curves; a minimum tangent length between two reverse curves is provided to introduce superelevation.
• Drawbacks: long tangents bring monotony, sleepiness, headlight glare and high speeds, so very long straights are avoided in the design. Straights also have to be broken by flat curves in hills to follow the terrain.

2. Circular curves

• Change the direction of the alignment at the points of intersection of the tangents.
• Simple to design and set out in the field (deflection angle, tangent length, chord).
• Radius is chosen for design speed: $R = V^2/127(e+f)$. Minimum radius is an important design control.
• Types: simple, compound, reverse, and broken-back; compound curves for hill roads and turning movements at junctions.
• Require superelevation and extra widening.

3. Transition curves

• Provided between a straight and a circular curve (or between circular curves of different radii).
• Gradual introduction of centrifugal force, superelevation and extra widening; avoid the sudden jerk and lateral shift.
• Improve appearance and safety, and drivers follow a natural path.
• Used for curves of higher speed and small radius; IRC suggests a spiral and its length from criteria of comfort and superelevation rate.

A good alignment uses long, smooth curves and moderate tangents, balanced with the terrain.