Primary road classification

Terminology

There are various definitions of road hierarchy and road classification. The basic difference (should) be that a hierarchy groups roads into different classes and then gives the classes some sense of ranking –  for example, in order of decreasing importance (although “importance” is not always well explained). A typical example of a road hierarchy might be as quoted for the USA and Canada in a wiki page (link):

  • Freeways or motorways
  • Arterials
  • Collectors
  • Local roads

 This sort of road hierarchy brings with it a number of implications. It seems to suggest that

  • The road hierarchy is only relevant to motor vehicles
  • Faster roads are more important than slower roads
  • Roads intended for longer distance traffic are more important than those for local traffic

 With a hierarchy there isalso the implication that roads which are at a higher level in a hierarchy must have more money spent on them. Any use of a road hierarchy should really come with a health warning which explains what traffic it is aimed at and what ranking it is based on.

Road classification is a means of grouping a set of roads into a series of classes. The classes can be based on any one of dozens of different parameters, for example: administrative responsibility, type of access, road surface etc.

Functional road classification is a particular method of grouping roads into classes. The USA’s Federal Highway Administration (link) says

“Functional classification is the process by which streets and highways are grouped into classes, or systems, according to the character of traffic service that they are intended to provide. There are three highway functional classifications: arterial, collector, and local roads”

A New Zealand definition of functional classification (ref. 138) is the

“classification of roads into groups according to their function, ranging from, for example, principal routes for communication between major regions and capital cities, to those roads which provide almost exclusively for local residential traffic. Also known as road amenity classification”

 There is often a lack of consistent use of terminology, even between countries which share a common language, and even between organisations in the same country. The problem can be even worse where engineers working on road classification do not share the same language. The correct sequence of the terms should be (after assessing the road network and listing road user classes):

  • Road classification
    • Functional road classification
      • Road hierarchy (and there are arguments for not using the term”road hierarchy” at all)

A primary road classification

Whatever scale of road network the observer is looking at, and regardless of the country he is working in, there is arguably one primary classifier for roads: speed. Speed determines many features of the road, for example:

  • the road’s geometrics ( horizontal  radius, visibility distance, junction spacing)
  • the road’s relationship to a sense of “place” (zero at 120 km/hr speed, 100% at 0 km/hr speed)

George Gummarra in (ref. 355, Australia) relates his recommended classes for low volume roads to geometric design guidelines, and points out that design speed is one of the most important features in arriving at these. We can use this to suggest a primary road classification as:

classification 01

References

138 – Transit New Zealand, “State highway geometric design manual, glossary of terms” (2005)

355 – ARRB, AR354 “Road classifications, geometric designs and maintenance standards for low volume roads” (2001)

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About roadnotes

Robert Bartlett is an international consultant with over 30 years of professional experience as a highway and traffic engineer with leading companies and organisations in several countries, including Germany, China (Hong Kong), Qatar and the UK. Specialised in urban studies, transport and the use of GIS, research has included new ideas on subjects such as the study of social justice using GIS, the dimensions of vehicles, and comparative geometrics (highways and transport).
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2 Responses to Primary road classification

  1. Pingback: Primary road classification (2) | Comparative Geometrics

  2. Pingback: Classification – two main road types | Comparative Geometrics

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