Geometric Design of Highway | Highway Geometric Design Factors

What is a Geometric Design for Highway?

There are several noticeable layouts and features on the highway. The determination of such visible features on highways is known as the Geometric Design of Highways. 

Its aim is to sustain the need of a driver, to guarantee the safety of the vehicle, ensure comfort while driving, and also the efficiency while driving.

 Features taken under consideration for geometric design of highway are:

• Cross section elements
• The gradient
• The intersection
• The consideration of sight distance

The geometric design of highway is influenced by:

• Characteristics of the vehicle
• The behavior of the driver
• Psychology of the driver  
• Traffic characteristics
• Traffic Volume
• Traffic Speed

Implementation of a proper design can largely reduce the severity of movement and accidents. 

The main objective of geometric design is:

• Optimum effectiveness in the traffic operation period 
• Maximum safety. 

These features need to be accomplished with the maximum economy in the cost and construction. The planning process is carried out in advance, unlike the construction of pavement.

Factors Affecting the Geometric Design of Highways:

Various factors affect the geometric highway design. The factors governing the design are categorised into following:

1. Design Speed of Highways:

The major factor responsible for affecting the geometric design of the highway is the design speed. The design speed parameter affects the Length of the vertical curves, horizontal curves and sight distances.

Speed is a parameter that hugely varies with the driver, the type of vehicle, the topography etc. This is the reason why the design speed is considered as a primary factor in the geometric design of highways. The highest continuous speed along with favourable weather conditions are, to facilitate safety travelling of the vehicles is called as the design speed. Legal speed is slightly differs from the design speed. 

The legal speed is the speed at which the driver travels beyond a safe speed. 

Desired speed is the maximum speed at which the driver can travel, being controlled by means of a local geometry or traffic. 

As stated earlier, that speed is a factor bringing different types of variations which requires having various vehicle design speeds to please the requirement of all vehicle drivers. 

By standard normally adopted design speed is 85^th percentile design speed.  

85^th percentile design speed is the speed higher than the speed taken by 85% of the drivers on that road. For some countries this value ranges from 95 to 98 percentile speed.

2. Topographic Factors:

Next important factor affecting the geometric design is the topography. 

For a plain terrain, construction of highway is easier to as per the standards. 

With increase of the terrain and the gradient, the cost of construction also increases for a specific design speed. So, to keep the cost of construction as well as time under control, the geometric standards vary with terrain or topography. Therefore it is classified as steeper gradients and sharper curves.

3. Miscellaneous factors:

Other factors besides the above mentioned ones, affecting the geometric design are:

• Vehicle

Large range of vehicles is now made stretching from tiny to massive units. The weight of the axle, the dimensions of the car and the characteristics of the vehicle has a great influence on the design aspects which involve the width of pavement, the clearances, the curve radii and the geometrics of parking. A design vehicle is set to facilitate this requirement, which own a standard weight, operating characteristics and dimension that helps to establish design controls to accommodate that vehicle of designated type. 

• Human

The physical, mental, and psychological characteristics of the human seriously affect the geometric design of the highway.

• Environment and Economy

While developing a geometrical design, it is vital to give importance to environmental concerns like noise and air pollution. The design developed considering all the above factors also needs to be economical in nature which should come within the capital with the allotment of maintenance cost. The highway geometric design has to be in such a way that the overall aesthetics of the environment remains unaffected. 

• Traffic

A reasonable value of traffic for the geometric design always needs consideration. Design for a higher traffic value affects an uneconomical design. The value is further collected and recorded from various and previous collected traffic data.