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
- 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).
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?