Road capacity and design-standard approaches to road design.

The 'capacity' of a road, the extent to which it can provide for traffic movement under given circumstances, can be defined in three ways. The 'economic capacity' is the smallest of all traffic volumes that needs to be attained to enable cost-benefit analysis to justify a road project. The 'environmental capacity' is the upper limit of traffic volume, to enable the road not to exceed minimum standards for noise, air pollution, pedestrian and cyclist safety and amenity, and visual intrusion, for example. The 'traffic capacity' is the maximum traffic flow at which vehicles can reasonably be expected to traverse a point or uniform section of a road or lane during a given time period, under prevailing roadway, traffic, and control conditions. In practice, a distinction should be made between two types of traffic capacity, those determined by interrupted and uninterrupted flow conditions. The US Highway Capacity Manual provides a set of analysis techniques which traffic engineers can use to determine the capacity rate of traffic flow to use in road design, to ensure a specific quality of service to drivers. The UK is one of the few countries that has not adopted this US approach; instead, it uses empirically derived 'practical capacity' design standards for use in rural and urban road design. For the covering abstract, see IRRD 892228.