What is Geometric Design of Highways and Factors Affecting it?

Geometric design of highway is the determination of layout and features visible on highway. The emphasis is more on satisfying the need of the driver as well as to ensure the safety of the vehicle, the comfort while driving and efficiency. Other related factors are also considered based on the project.

The main features considered 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:

  • The characteristics of the vehicle
  • The behavior of the driver
  • The psychology of the driver
    Traffic characteristics
  • Traffic Volume
  • Traffic Speed

Severity of movement and accidents can be reduced largely by implementing a proper design. The main objective of geometric design is to get optimum efficiency in the traffic operation period and maximum safety.

All these features must be attained with maximum economy in the cost and construction. Unlike the construction of pavement, the planning process is carried out in advance.

Geometric Design of Highways

Factors Affecting the Geometric Design of Highways

The various factors that govern the geometric design of highways are:

  • The Design Speed
  • The topography factors
  • Other Factors

Design Speed of Highways

The most important factor affecting the geometric design of the highway is the design speed. The design speed parameter affects the:

  • Length of the vertical curves
  • The horizontal curves
  • The sight distances

Speed is a parameter that have huge variation will the person driving, the vehicle type, the topography etc. This is the reason why the design speed is taken as a primary factor in the geometric design of highways.

The highest continuous speed, given that the weather conditions are conducive, to facilitate safety travelling of the vehicles is called as the design speed. Legal speed is different from the design speed. The legal speed is the speed at which the drivers tend to travel beyond a safe speed. Desired speed is the maximum speed at which the driver can travel when they are constrained by means of a local geometry or traffic.

As mentioned before, speed is a factor that bring variations of different types. This requires having different vehicle design speeds to satisfy the requirement of all vehicle drivers. So, by standard: 85th percentile design speed is the normally adopted design speed.

85th percentile design speed can be defined as the speed which is higher than speed taken by 85% of the drivers on that road. For some countries this value will be in the range of 95 to 98 percentile speed.

Topography Factors

Second important factor that affects the geometric design is the topography. For a plain terrain, it is very easy to construct the highway as per the standards. As the terrain and the gradient increases the construction cost will increase for a specific design speed.

So, to keep the construction cost and time under control, the geometric standards vary with different terrain or topography. This is hence classified as steeper gradients and sharper curves.

Other Factors Affecting Geometric Design

Other miscellaneous factors that will affect the geometric design of highways are:

  • Vehicle
  • Human
  • Environment and Economy
  • Traffic
  • Other Factors

Large variety of vehicles are now made which range from tiny to massive units. The weight of the axle, the dimensions of the car and the characteristics of the vehicle influence greatly the design aspects. The design aspects involve the pavement width, the clearances, the radii of curve and the parking geometrics.

To facilitate this requirement, a design vehicle is set which own a standard weight, operating characteristics, and dimension. This helps to establish a design controls so that vehicle of designated type is accommodated.

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

Always a reasonable value of traffic is considered for the geometric design. The design for a higher traffic value result in a design that is uneconomical. This value is collected from various and previous traffic data collected and recorded.

While developing a geometrical design, it is very essential to give importance to the environmental concerns like noise and air pollution. The design developed considering all the above factors have to be economical in nature. It should come within the capital and the maintenance cost that is allotted.

The highway geometric design has to be such a way that the overall aesthetics of the environment is not affected.

Also Read: Classification of Roads and Their Details

Neenu Arjun
Neenu is a Civil and Structural Engineer and has experience in Design of Concrete and Steel Structures. She is an Author, Editor and Partner at theconstructor.org.