The design principle Functionality of roads implies that road sections and intersections have only one function for all modes of transport (mono-functionality) – a traffic flow function or an exchange function – and that these functions are built into the road network in a hierarchical and efficient way.
Public space consists of dwelling areas and traffic areas. The traffic area consists of road sections and intersections to which the functionality principle applies.
We distinguish between two road functions: a traffic flow function and an exchange function. The flow function means that traffic participation does not involve interaction with the environment, whereas during exchange there is interaction with the environment and abrupt manoeuvres are also possible. These functions do not combine safely [1] [2] [3]. Road function is therefore the basis for a safe design and use of roads. A direct result of this is the division into road categories and the ideal structure of the road network.
In a hierarchical and efficient structure of the road network three road categories may be distinguished according to their functions (see also Figure 1 [4]):
- Through-roads (flow function on road sections and across intersections);
- distributor roads (flow functions on road sections and exchange function at intersections);
- access roads (exchange function on road sections and at intersections).
Figure 1. Functional classification of roads.
This functional classification of roads relates to the ‘traffic space’. ‘Dwelling’ functions such as playing and shopping do not combine safely with traffic, least of all with through-traffic (for example see [2]). Access roads, and in particular home zones, are the only type of traffic space that, where necessary, combines with an area’s dwelling function (for example see [5]). In cases where function, design and use do not correspond we talk about ‘grey roads’ (for example in a 50km/h shopping street; see also the design principle ‘Biomechanics’
See Aarts & Dijkstra [6], Chapter 3 for a more detailed discussion of this principle.
