Road hierarchies

This is one of a series of posts on highway engineering. It discusses roads in terms of hierarchies. The text comes from a google knol published in January 2009. The other material from this google knol will be added as separate posts.

Concept

This idea was proposed by Buchanan in his “Traffic in Towns” publication. The argument is that roads should not be considered simply as individual, unrelated transport links. Rather they should form a network. In this way the duplication of routes between individual origin and destination pairs would be avoided.

The next step was to argue that a road network should be seen as a sort of “3-dimensional object” with some roads being of a higher level of importance than others (higher up in the hierarchy).The importance of the roads can be based on their transport function – for example, a road which is meant to take traffic over longer distances could be considered to be more important and so placed at a higher level in the hierarchy. A short access road would be less ‘significant’ and at a lower level in the hierarchy.

To put this argument in a different way, a road hierarchy simply structures the individual links of a road network in terms of functions. Thus in a 4-level hierarchy level 1 could represent long distance travel link (e.g. motorway) whilst level 4 could represent a short distance travel link (such as an access road). Here the function is one of “journey length”. Alternatively, level 1 could represent high-speed roads and level 4 the lowest speed roads. Here the function is “speed”.

Table 1. Simplified road hierarchy based on longer distance = more significant

 Function >
 1) Journey distance  2) Speed
 Level in hierarchy:
 1 (top)  long distance motorway  120 km/hr
 2  urban distributor  100 km/hr
 3  local distributor  70 km/hr
 4 (bottom)  access road  30 km/hr

 

 

Inverted hierarchies

In the notes above there is no suggestion that a level 1 road is more important than a level 4 road.  However decision-makers could easily misunderstand that a road hierarchy does imply importance, with the result (for example) that more money is spent on motorways than on access roads. Conversely, a society may consider accessibility to be important. After all, most journeys are short journeys. Generally speaking the longer the journey length the fewer trips are made. For example a person may visit the local shops almost once a day but visit another continent less than once a year.

This reasoning suggests that the above table could be inverted to give the order of ranking shown below:

Table 2. Simplified road hierarchy based on short distance = more significant

 Function >
 1) Journey distance  2) Speed
 Level in hierarchy:
 1 (top)   access road  30 km/hr
 2  local distributor  70 km/hr
 3  urban distributor  100 km/hr
 4 (bottom) long distance motorway  120 km/hr

 

Multiple hierarchies

In the real world many different organisations use a region’s road network. These organisations have their own tasks and priorities, and so can each give a different level of ranking to one and the same road. It is even possible that one road may be very important for one organisation but completely un-important for another.

This argument is not merely a play with words. Today’s road network managers have to provide a safe, functioning road network which matches the needs and priorities of the road users – which means that half their task is understanding what the route needs and priorities of these users are.

Road networks are a form of spatial infrastructure. To study this complex aspect of their nature, road network managers should use a spatial analysis system or GIS. Indeed a GIS allows them to display and study layers representing e.g. different (function/level) combinations on the same screen.

Table 3. Road network, multiple hierarchies

 Function >
 1) Journey distance and 2) Speed  and 3) Vehicle  type, e.g. goods vehicle routes  and 4) Use type, e.g. emergency service routes
 Level in hierarchy:
 1 (top) long distance motorway  120 km/hr  weight > 40 tonnes  regional hospitals connector network
 2  urban distributor  100 km/hr  weight 7.5 – 40 tonnes  roads to district centres
 3  local distributor  70 km/hr  weight 3.5 – 7.5 tonnes  roads to polyclinics
 4 (bottom)   access road  30 km/hr  micro goods vehicles only  access to medical practice / surgery

 

Proposition

  • There is a risk that decision-makers will make the simple assumption  that the top levels in a road hierarchy are the more important, and that therefore more money should be spent on them
  • A road hierarchy is a 3-dimensional structuring of a 2-dimensional road network.
  • Different measures can be used to structure a road network into a road hierarchy
  • Shorter roads are more important than longer roads
  • In the real world, a road system is a multiple of several different road hierarchies (and several different road networks)
Advertisements

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).
This entry was posted in basics, roads and tagged . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s