Road design – the vehicle environment

veh 02This is the third of three posts which refer to the presentation by Dr. John Rolt of the UK’s TRL (ref. 2211). In the previous post, I said that

“It is easy to read through a document with road design standards, check out the values for a particular design parameter such as gradient, and then start to design the road. However the first steps should be to decide the technical environment in which the planned road finds itself”.

I then went on to list seven factors which define the technical environment. One of the factors was”vehicle type”. Here Dr. Rolt says:

“In countries of South East Asia, the traffic on rural and semiurban roads comprises many motor cycles, motor cycle taxis,  pedal bicycles and pedestrians. You may be surprised to know that a typical rural road in a country like Cambodia may carry 2000 motorcycles, 1000 pedal bicycles, 1000 pedestrians but only 5-10 cars or other 4-wheeled plus motorised vehicles”.


“It is fairly obvious that geometric standards should not be the same as in countries where motorcycle and pedal cycle traffic is almost non-existent”.


A few years ago I was able to work with other engineers to produce some documents on the same topic, that is of different vehicles in different countries. From this work it looks like the mix of vehicle types changes with country, and also probably with geographic region.

It may be possible that engineers from western countries often cannot produce a good design for roads in in other countries, because they do not understand the vehicle mix which will use the roads (and probably do not understand the social context of the roads either). So it may be possible to argue that:

  • You cannot directly use western road design standards in other countries, and
  • You cannot directly use western road design engineers in other countries

One comment made by Dr. Rolt is on PCUa, where he says:

“Consideration of other traffic  …. is taken into account by modifying the basic standards. This is done by combining the number of such road users using the PCU (passenger car unit) concept …. “

I disagree with this, because I suspect that the PCU concept doesn’t actually work. I discussed this in an earlier post.


2211 – UK, Dr. Rolt of the UK’s TRL, “Geometric design and safety for LVRR”, presented at the 6th Africa Transportation Technology Transfer (T2) conference in Botswana, 2013

The image comes from the document GTA 004 – Tanzania – Robert Bartlett, with UWABA, the Dar es Salaam cyclists organisation, “Tanzania, Gutas”  July 2012 (to be republished)


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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