Are roads built to the wrong design?

One of the key features of the geometric design of roads is horizontal curvature. Most countries base their guideline values on the formula which relates horizontal curve radius to speed, side friction and superelevation. So, for a speed of 100 km/hr and a superelevation of 6%, all you need to know is the side friction value and you can calculate the radius – problem solved.

Well, perhaps not. There are other ways of determining minimum radius. Australia n (or rather, Austroads) practice says that (ref. 1968): “Trucks have a higher centre of gravity than cars. Consequently, the limiting condition for trucks negotiating a circular curve tends to be rollover rather than skidding“. Austroads then developed a method of calculating horizontal radius for trucks which is based on a measure of truck rollover which they call “static roll threshold”.

The problem is that, first, most highway design guides only quote minimum radius values for cars (and usually don’t make it clear that this is what they are doing). As (ref. 1968) also says, “most road geometric design standards,particularly alignment design standards, are based on the performance characteristics of passenger cars and car drivers“.

Secondly, appropriate values for horizontal radius for trucks can be larger than those for cars. The table and chart are based on figures taken from (ref. 1968).

design 1

and

design 2

Comment

Roads should be designed for different vehicle types, and the appropriate design based on whichever vehicle type gives the limiting value for a particular parameter. There may be more than two vehicle types (just cars and trucks), as for example buses and bicycles have their own performance characteristics.

The question also arises whether some road accidents which involve trucks may be the result of incorrect road design (if the road design was based on parameters for cars) and not on any incorrect behaviour on the part of the truck driver.

References

1968 – Australia / New Zealand,”Geometric Design for Trucks — When, Where and How?” Austroads, 2002

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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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