General Road Design Principles:
Roads should be designed to:
- provide safe, short and fast thoroughfare and access to all road users, being motor vehicles, cyclists and pedestrians
- clearly convey the primary function to road users and encourage appropriate driver behaviour
- deliver traffic volumes at speeds compatible with function
- provide convenient location for services
- provide an opportunity for landscaping
- allow for parking, where appropriate
- have due regard to topography, geology, climate, environment and heritage of the site
- provide low cost of ownership
- comply with relevant standards and road authorities’ guidelines
Traffic speed: In general, design speed has been a basic parameter in determining road standards and is a function of the road classification. However, new guidelines have introduced an operating speed concept. Operating speed in these guidelines is defined as 85th percentile speed of cars when traffic volumes are low.
Sight distance: A minimum stopping sight distance should be provided at all points along the road. Stopping sight distance is a function of vehicle speed, reaction time, eye height and object height. A car driver eye height of 1.05m is used for the geometric design of both urban and rural roads. For commercial vehicles, a driver eye height of 1.8m is used for general geometric design.
Superelevation on Road:
Superelevation is the rotation of the pavement on the approach to and through a horizontal curve. Superelevation is intended to assist the driver by counteracting the lateral acceleration produced by tracking the curve. Superelevation is expressed as a decimal, representing the ratio of the pavement slope to width.
Selection of a maximum superelevation rate is based on several variables, such as climate, terrain, highway location (urban vs. rural), and frequency of very slow-moving vehicles.