Road classification is a means of recognising certain characteristics of the roads in a road network. Road hierarchy is a special type of road classification. It ranks the roads in a network in terms of some sort of graded importance. Very often the importance involves speed and journey distance – with the implication that these are related. Thus it could be argued that:
A motorway has a design speed of 120 km/hr, and is intended for long-distance journeys
An access road has a design speed of 20 km/hr and is intended for short-distance journeys
In fact, roads can be classified in many different ways – there are many different possible “classifiers” such as administrative ownership, journey distance etc. An earlier blog post suggested that there could be one “primary” classifier, and that this might be “speed” (see here). Speed as applied to highway design has less than 15 possible values.
Another idea with road networks is that they demonstrate fractal characteristics (discussed e.g. here) and that these characteristics are similar at different levels of scale. For example, at any scale a road network consists (only) of through links and access links. Again, we are implying some measure of scale when we talk about long-distance roads (motorways) and short distance roads (access roads).
Roads only make sense in terms of land use (you can’t have one without the other); and land use planners make frequent reference to scale. For example, Poland’s “National spatial development concept 2030” (ref. 381) refers to “the integration of the Polish space proceeds based on several parallel processes at various levels of management in diverse spatial dimensions” and at local, regional, national and international scales”.
The question then is, can we quantify scale when talking about roads and road hierarchy.
Straws in the wind
Some engineers and other specialists have already given numbers to road hierarchy and a measure of ‘scale’ in the sense of distance. For example:
- Eppell and others (ref. 249) discuss the relationship between land use and road hierarchy. The paper has a figure suggesting a square cell size for land use of between 250m and 500m for collector streets and 500m to 1000m for arterial roads.
- The paper by Stephen Marshall (ref. 388 page 62) says that Spain has a road classification based on trp length (a measure of distance), with for example:
- Taeke M. deJong in “An urban designer’s road hierarchy” (ref. 385) has a figure which suggests a relationship between road classification and a measure of spatial distance called “crossing distance”. (Mr. deJong’s figure also relates each road class to road corridor width and the number of inhabitants served). Crossing distance may be a measure of land use cell size.
Poland relates road classes and speed (see e.g. here). We can attempt to assign a value of speed to each of deJong’s road classes. For example, a metropolitan highway is likely to have a design speed of 120 km/hr whilst a neighbourhood road perhaps only 20 km/hr. If we produce such a relationship then we can also relate road classification and cell size (crossing distance) to time:
The details may certainly be disputable but it does seem possible to relate road hierarchy to scale measured in terms of speed, time and distance.
249 – Eppell and others, “A four level road hierarchy for network planning and management” (Australia 2001)
381 – Ministry of Regional Development, “NSDC national spatial development concept 2030” (Poland, 2012)
385 – Taeke M. de Jong, “An urban designer’s road hierarchy” published in the “Proceedings of the WSEAS International Conference on Urban Planning and Transportation” (Greece 2008)
388 – Stephen Marshall, “Artists – A First Theoretical Approach to Classification of Arterial Streets”, (England, 2002)