Geometric Design Standards

Geometric Design Standards

  • The various Design Standards being followed in the India for the design of Hill Road are

  • IRC:SP:48-1998 Hill Road Manual. 

  • IRC:52-2001 Recommendations About the Alignment Survey and Geometric Design of Hill Roads. 

  • IRC:SP:91-2010 Guidelines for Road Tunnels. 

  • IRC:SP:73-2015 Manual of Specifications and Standards for Two Laning of Highways with Paved Shoulder. 

  • IRC:SP:84-2014 Manual of Specifications and Standards for Four Laning of Highways through Public Private Partnership. 




  • Visibility is an important requirement for safety on roads.  It is necessary that sight distance of sufficient length is available to permit drivers enough time and distance to stop their vehicles to avoid accidents.

QUESTION- What is the type of sight distance? 

SOLUTION-Sight distance available from a point is the actual distance along the road surface, over which a driver from a specified height above the carriage way has visibility of stationary or moving objects. Three sight distance situations are considered for design: 

• Stopping sight distance (SSD) or the absolute minimum sight distance 

• Intermediate sight distance (ISD) is defined as twice SSD 

• Overtaking sight distance (OSD) for safe overtaking operation 

• Headlight sight distance is the distance visible to a driver during night driving under the illumination of headlights 

• Safe sight distance to enter into an intersection.

QUESTION-What is meant by widening of pavement on horizontal curves?

SOLUTION-Extra widening refers to the additional width of carriageway that is required on a curved section of a road over and above that required on a straight alignment. This widening is done due to two reasons: the first and most important is the additional width required for a vehicle taking a horizontal curve and the second is due to the tendency of the drivers to ply away from the edge of the carriageway as they drive on a curve.

QUESTION-What is the maximum and Minimum super -elevation?

SOLUTION-Depends on 

(a) slow moving vehicle

(b) heavy loaded trucks with high CG.

IRC specifies a maximum super-elevation of 7 percent for plain and rolling terrain, while that of hilly terrain is 10 percent and urban road is 4 percent.

The minimum super elevation is 2-4 percent for drainage purposes, especially for the large radius of the horizontal curve.

QUESTION-Define camber.

SOLUTION-Camber, also called cross fall, is the convexity provided to the cross section of the surface of carriageway. It is the difference in level between the highest point, known as the crown usually located at the center of the carriage, and the edge.                                                        

Camber is provided so as 

To drain surface water 

To separate the traffic in two opposite directions 

To improve the appearance of the road.

QUESTION-Define sag curves?

SOLUTION-Sag vertical curves are curves that connect descending grades, forming a bowl or a sag. Designing them is very similar to the design of crest vertical curves.

QUESTION-What are overtaking zones? 

SOLUTION-The intervals or zones on the highways provided for the purpose of safety overtaking operation of fast moving vehicles with slow moving vehicles, without any collision of vehicles from opposite side are called overtaking zones.

QUESTION-What are the types of curves in highway geometric design and write any two salient features of any one curve?

SOLUTION- I. Horizontal curve 

       a) Simple curve 

       b) Compound curve 

       c) Reverse curve 

       d) Transition curve

II. Vertical curve 

       a) Sag curve 

       b) Summit curve

    Salient Features of summit sag curve

       a)Safety and b) Comfort traveling between gradients

As per IRC:SP:48-1998 and IRC:52- 2001


It should be ensured that there is sufficient length between the two curves for introduction of requisite transition curves.

  • Curves in same direction separated by short tangents, known as broken – back curves.

  • Should be avoided, as far as possible, in the interest of aesthetics and safety and replaced by a single curve.

  • If this is not feasible, a tangent length corresponding to 10 seconds travel time must at least be ensured between the two curves

Vertical Alignment

Vertical curves are introduced for smooth transition at grade change. 

  • Both Summit curves and Valley curves should be designed as Square parabola.  

  • The Length of vertical curves is controlled by sight distance requirements.  

  • Curves with greater length are aesthetically better.

Recommended gradients for different terrain conditions, except at hair pin bends, are given below.

At unavoidable circumstances Hair-pin Bends may be designed as Circular Curve with Transitions or as Compound Circular curves. 

Design Criteria for Hair-pin Bends as per IRC: SP: 48-1998 and IRC: 52- 2001