Geometric Road Design Considerations

Association of Construction and Development
March 12, 2013 — 4,263 views  

Highway geometric design entails the analysis and calculations that are made by engineers regarding highways. This is done to ensure that the highway fits the topography of a particular site while ensuring service, performance, and safety at the same time.

There are several considerations that need to be made that will determine the highway design, these include:

  • Design traffic volume
  • Level of service
  • Super-elevation, alignment and grades
  • Lane width
  • Cross section
  • Slight distance
  • Number of lanes
  • Design speed
  • Vertical and horizontal clearance

Design Speed

This refers to the highest speed at which vehicles can travel on the road safely, when the weather is favorable and the density of traffic is low, while ensuring that the safe speed is only determined by the road’s geometric features.

Minimum design elements are established with the design speed. These elements include super-elevation, the alignment and the grades. Other elements include width, clearance and side slope ratio.

Level of Service

Level of service refers to the qualitative measure of functional conditions within traffic stream. This is commonly described as freedom to maneuver, travel time, speed, comfort, safety, convenience and interruptions. This criterion differs for multi-lane highways, and it is defined in density.

Design Traffic Volume

The traffic volume is the average density of traffic that will run on a road in an hour of every day. With the help of these volumes, service flow rate and other calculations that will help determine how the road should be constructed.

Roadway Context

The context of the road or highway is critical when developing a road construction project. This factor will determine fundamental design decisions like the cross-section determination, detailed design elements and street fixtures to name a few.

In order for a project to be successful, it is important for the highway design to consider its surroundings and respect it. A design that is context sensitive will start with analyzing the contextual elements like the community and environment that surrounds the area. Once the designer clearly understands the surroundings of a potential highway and its users, the rest is much easier to design and plan.

Sight Distance

This refers to the length of the road ahead that can been seen by the user of the roadway. Most of the time, specific distances applies to cyclists and motor vehicles, stopping sight distance, decision sight distance, and passing sight distance are usually taken into consideration.

Safety

The safety that all transportation facilities provide is the primary consideration when it comes to geometric road design. While constructing roads, certain projects are specifically made to tackle existing safety hazards. It is important that all projects do eventually result in facilities that are extremely safe for its users.

Conducting corridor safety audits and crash record analysis are a great way to discover any hidden safety hazards. It is important for designers to pick project design elements according to their expected characteristics while operating and according to historic performance when it comes to safety. All highways are tested before they become functional.

Association of Construction and Development