Fractal road hierarchy (and other ideas)

Road hierarchy has been a topic of interest to engineers and planners since at least the early 1960’s, when the Buchanan  Report “Traffic in towns” was published (link).  Wikipedia says

“the hierarchy of roads “categorizes roads according to their functions and capacities. While sources differ on the exact nomenclature, the basic hierarchy comprises freeways, arterials, collectors, and local roads” (link)

Whilst Transit New Zealand (ref. 138) says road hierarchy is:

“the grading of roads according to increasing or decreasing importance of their traffic carrying or other function”

And a paper by Andrew Macbeth in 2007 (ref. 250) says:

“A  functional  road  hierarchy  is  a  system  of  classifying  roads  for  different  functions  and  for managing roads and traffic according to this classification system.  The roads carrying (or intended to carry) most traffic are at the top of the hierarchy, while those with least traffic (and which serve primarily to provide property access) are at the bottom”

 However there can be problems with terminology and definitions. For example,  when most people talk about road hierarchy what they really mean is “a system of roads designed to benefit motorised traffic, with more important roads being those that either carry most traffic (and/or) roads that carry longer-distance journeys”. There is also the  implication that the higher level roads are the most important, and that therefore they should have more money spent on them.

Anyone who is a pedestrian or a cyclist would be unlikely to accept this particular idea of road hierarchy – although perhaps both would argue that for most of their journey they would like to travel on comfortable, high-quality footways / cycle lanes. Further, an environmentally conscious society would surely argue for reducing the number of longer-distance journeys. Certainly every journey made by a person starts on a local road (the lower level of a road hierarchy) whilst fewer and fewer use the roads in the upper levels of a hierarchy. In this sense it is the lower levels which should have more money spent on them.

Arguments

Anyone talking about road hierarchy should really begin by talking about road networks. And these are not as simple as you might suppose. Here are some arguments about road networks (not in any particular order):

1. A road network can have many road hierarchies and / or functional road classifications (not just one)

2. There are different road networks for different users (car drivers, pedestrians, emergency service vehicles)

3. The road networks are only valid if they are contiguous / complete (an incomplete road network for is pointless)

4. Links in one network can also be used for links (serving a different function level) in other road networks

5. Road networks are fractal, that is they look and work very similarly whatever level of scale the viewer is using. Here for example India has a network of NHAI and international roads; if you delete the background mapping and zoom down to part of India’s rural road network (of through and link roads, see ref. 282), you would be looking at very much the same thing.

6. The fractal number for one road network may be different for different road networks (car-based trips, cycling network, goods vehicle network)

7. Mathematical measures of connectivity could be developed for networks. These could then be used to indicate where a network is deficient, and how connected (complete) a particular network is.

8. Shallow road hierarchies – the number of levels in a road hierarchy could be reduced to (say) three, based on long-distance, medium-distance and short-distance trips; terminology for these three levels could be applied at each increasing implement of scale.The same ideas should be applied to each “road network” (e.g. HGV network, cycling network etc)

9. The terminology of road networks should break away from national or other locally-based terms. To describe a functional road network the same terminology should be used in every country.

10. Road classification refers to “classes”. There can be many classes – for example a road may be

  • Owned by the county council (administration class)
  • Have a concrete road surface (pavement type class)
  • Be single or dual carriageway (road width class)
  • Be suitable for HGV traffic (vehicle type / weight limit class)
  • And so on. Here a class is a set of information.

11. Roads are spatial objects, so they are suitable for modelling by geographic information (GIS) systems. This is already being done for example in India and Pakistan. GIS systems include relationall databases which provide “analysis engines”. They are not merely intended simply to create attractive-looking maps.

12. Road classes are elements of information which relate to specific spatial objects, so any road classification should preferably be entered into a geographic information system which covers the network to which it belongs.

 

References

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

250 – “A national road hierarchy – are we ready?”, Andrew Macbeth, IPENT Transportation Conference 2007

282 -PRADHAN  MANTRI  GRAM  SADAK  YOJANA , programme guidelines, 2012

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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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