Importance of Geometric Design

Importance of Geometric Design The geometric design of a highway deals with the dimensions and layout of visible features of the highway such as alignment, sight distance and intersection.

Optimum efficiency- Maximum safety- Reasonable cost The geometrics of highway should be designed to provide optimum efficiency in traffic operations with maximum safety at reasonable cost. The designer may be exposed to either planning of new highway network or improvement of existing highways to meet the requirements of the existing and anticipated traffic. 2500 2015 12500 2035 PCU/day Year

Improving Geometric Elements in Stages Improving in stages is very difficult and more expensive Design geometric features at the initial stage itself considering future traffic scenario For example, in 2015 the road may be designed for 80 kmph. But the designer has to consider for 120 kmph as the road may be upgraded in future. Accordingly Sight distance, super-elevation etc are to be designed.

Elements of Geometric Design of Highways Width of pavement Cross section elements Formation and land Surface characteristics Cross slope of pavement SD available in horizontal curve Sight distance consideration SD available in Vertical curve SD available at intersections Horizontal alignment details Super-elevation Transition curves Vertical alignment details Intersection elements

Design control criteria

Design Factors The geometric design of highways depends on several design factors which control the geometric elements are: Design Speed Topography Traffic Factors Design hourly volume and capacity Environmental and other factors

Design Speed Most important factor Decided based on overall requirement of highway In India depends on category of road design speed assigned (NH/SH/MDR/ODR/VR) Design speed varies with topography

Design Speed Every geometric elements depends on speed For example, Pavement surface characteristics Width & side clearance Sight distance requirements Radius of curve Super-elevation Summit or valley curve

Topography Design Speed of NH/SH 100 Kmph Plain Terrain 80 Kmph Rolling Terrain Mountainous Terrain 100 Kmph 80 Kmph 50 Kmph Design Speed of NH/SH

Traffic Factors Different vehicles have different characteristics like height, weight, braking nature etc.

Design Hourly Volume Flow fluctuate with time Off-peak versus Peak Uneconomical to design for PH traffic Reasonable traffic value called design hourly volume DHV for urban 8-10% ; for rural 12-18%

Environmental & Others Factors such as aesthetics, landscaping, noise pollution and other local conditions should be given due consideration in the design of road geometrics Grade-separated intersection for expressways to achieve higher speed standards