This is the second of three posts which refer to the presentation by Dr. John Rolt of the UK’s TRL (ref. 2211).
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.
In earlier posts (see for example here) I suggested design is affected by a primary influencer, and by secondary influencers such as design speed and road type. Dr. Rolt’s presentation includes a list of 12 items which geometric standards depend on. These are the sort of items which define the technical environment.
Six of Dr. Rolt’s 12 items more or less match my own list:
- design speed
- the design vehicle;
- pavement type
- soil type and climate
Another of Dr. Rolt’s items – construction technology – should really be added to my own list. Of the remaining 6 items I am hesitant to add them to my list, as I think:
- traffic volume expected to use the road – probably affects road cross-section rather than alignment (which is the area I am concentrating on at the moment)
- composition of the traffic – probably covered by my “vehicle type”, with the note that there may be several vehicle types, just as there may be several design vehicles
- roadside population and land use – to an extent, already defined by road type
- economic and environmental considerations – these are non-technical and also sometimes hidden in the values which standards suggest for design parameters
- safety – I am reserving my opinion on this
- functional classification of the road – a term (such as level of service and passenger car unit) which has probably reached its sell-by date
I’ll modify my own list to include construction technology, which gives these main technical factors which affect the technical environment for road geometric design:
- design speed (primary influencer)
- road type
- vehicle type
- road surface
- construction technology
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