Basics – Traffic composition

motocarros excerptPeru, motocarros (photo: Gregorio Villacorte Alegria)

One fundamental component of road design is an understanding of the types of vehicles which will use the roads concerned. Initially I thought that – give or take a few percentage points – the composition of traffic on the roads in any country would be pretty much similar. Of course, this is not the case. Traffic composition varies from location to location (and from society to society), and also on a macro-scale, a micro-scale and a time-scale.

Macro-scale variation in traffic composition

For example,

  • Peru’s roads can be used by unusually large numbers of motocarros
  • Nigeria’s roads can be used by unusually large numbers of motorbikes
  • Germany’s roads can be used by unusually large numbers of bicycles

Here I use the term “unusually” in the sense of comparing local values with a theoretical global average vehicle composition.

Micro-scale variation in traffic composition

  • Traffic on some roads in a city can include large numbers of bicycles, whilst other roads can include large numbers of cars.
  • The composition of the sub-type “cars” can vary from place to place – see for example the article on “Rural commuters claim railway station should have bigger parking spaces for their 4x4s (source: Daily Mail, 2011 )

Time-based variation in traffic composition

  • In the early 1950’s, Portsmouth dockard was a big local employer. At the end of the working day there used to be a huge stream of workers cycling their way home. Today the number of dockyard workers (and the number who bike to work) is much smaller.
  • In a recent pro bike day in Berlin saw 120,000 cyclists taking part (link) whilst according to an article in “Der Spiegel of 2013 (link), the German city of Groningen hoped to see cyclists increase to 65% of traffic (the article doesnt define this modal split any more precisely).

Comment

What the above suggests is that perhaps

  • the life cycle of a road design does not primarily depend on its structural resistance to use, or on the volume of traffic which will use it, but on the changes in the composition of traffic which use it
  • The design of a road should be such that its cross-section can be varied over time, as the composition of traffic using it changes

I wonder if it is possible to come up with a few standard road cross-sections which allow for changes in traffic composition over time.

There is a hidden question in this discussion. Engineeers know that traffic composition varies with time, location, economic growth, population distribution and so on. They have to make pragmatic decisions in order to reduce these variables to a manageable level, and so that they can develop a few, robust estimates of traffic composition which they can use in their design studies. (one explanation of the difference between engineers and scientists is the difference between pragmatism and precision). Perhaps the simplifcations of traffic composition as used over the last years are no longer the best?

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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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6 Responses to Basics – Traffic composition

  1. Pedro says:

    Hello, that´s good considerations about traffic composition, it´s true that we have to be concerned that the cross section must change over time. but it´s difficult to take into account because it increases the budget. In a developing country the budgets are resticted.
    thanks for the post

  2. Pedro says:

    Hi its missing in the blog a way of contacting you and a short self presentation

  3. pawber says:

    Very interesting point Robert, but wouldn’t it lead to traffic oriented development rather than people oriented development? Urban mobility suggests that more public transport should be utilised (of course increasing the frequency); however, more and more cars are running on the roads with only one person per car (I am not anti-car). Shouldn’t the cross section focus on PCUs? But if this was to be implemented how would cross section work in dense landuse and populous regions?

    • roadnotes says:

      Hallo Pawber. What I had in mind is that the design – and particularly the cross-section – of a road needs to be more adaptable to changing circumstances. For example, with increasing bicycle traffic in cities, more road space is needed for bikes. But policy makers sometimes come up with good ideas (more bicycles) but don’t plan through to specifics, such as changing or “flexible” cross-sections, re-routing lorries away from some roads, re-thinking the local road hierarchy. These changes are taking place on quite short time-scales now. (by the way, on PCU’s I am a bit doubtful about there usefulness, see my post at – https://comparativegeometrics.wordpress.com/2015/03/16/pcu-factors-are-they-just-pseudoscience/ )

      • pawber says:

        Hi Robert, sorry for the delay in replying.

        You are absolutely right about the fact that sometimes policy makers do generate better ideas, sometimes best, but doesn’t work at the implementation level. Along side “flexible” cross-section they need to monitor/understand driver’s behaviour and modify the routings.

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