How are roads designed?

When a road needs to be designed, the manuals for these are produced by Highways England. There are 16 documents which comprise the Design Manual for Roads and Bridges (DMRB); they can be found here.

The manuals contain all process from getting a road approved through to design, construction and maintenance. When planning a road, there are many considerations. The road must fit with relevant government policies and plans. These include the mix between private and public transport, goals for economic growth, political promises to reduce casualties on the road, etc. Governments also define funding allocations for different modes of transport. One party may prioritise roads while another prioritises rail.

Roads are built in response to existing traffic demand (for example, a bypass to a town or village where safety and efficiency benefits are desired, or adding lanes to improve capacity where there’s severe congestion), projected traffic demand (for example, where a new industrial park is planned) or as a replacement to an existing road (for example, if a road is destroyed due to a landslip or erosion, or deemed unable to be reopened or maintained due to other concerns). The road’s position must fit in with the current network and there will be a projection for the useful life of the road as it stands, for example, 20-40 years.

The various communities on and around the road will be affected and may want input as they will be understandably self-interested. Roads can sometimes bisect communities and may require houses to be requisitioned for the land. They can cause unwanted noise and, if poorly planned, can actually increase congestion in other areas if feeder roads aren’t upgraded. Residents may be worried about construction noise and disruption, visual pollution and air pollution. Local communities will have diverse factions that will disagree on speed limits. For example, a local freight company may want a higher speed limit to improve delivery times while concerned parents may want lower limits if they fear for their children near the road.

Road construction is costly therefore a cost-benefit analysis is always applied. This will take into consideration any reduction in travel times, increases in throughput which may have a positive impact on freight costs,  improvements in the public transport network, improvements to safety for cyclists, pedestrians and motorists and whether the road will bring a business benefit to the area (for example, it creates a new opportunity for a shopping centre or business park).

Public transport frequency and placement are often contentious. People are reluctant to have bus stops right outside their homes, for example. The budget might not extend to implementing bus or cycle lanes immediately but the road could be constructed bearing those in mind. Cyclists and pedestrians must be able to safely use the road. Positions and types of crossings need to be considered.

Volume
Volume 0	Introduction and General Requirements
Volume 1	Highway Structures: Approval Procedures and General Design
Volume 2	Highway Structures: Design (Substructures & Special Substructures), Materials
Volume 3	Highway Structures: Inspection & Maintenance
Volume 4	Geotechnics & Drainage
Volume 5	Assessment & Preparation of Road Schemes
Volume 6	Road Geometry
Volume 7	Pavement Design & Maintenance
Volume 8	Traffic Signs & Lighting
Volume 9	Traffic Control & Communications
Volume 10	Environmental Design
Volume 11	Environmental Assessment
Volume 12	Traffic Appraisal of Road Schemes
Volume 13	Economic Assessment of Road Schemes
Volume 14	Economic Assessment of Road Maintenance
Volume 15	Economic Assessment of Road Schemes in Scotland