Definition of gradient
In terms of the vertical alignment of a road, one of the primary design parameters is gradient. A good definition of gradient is given in (ref. 148), which says:
“The slope of the grade between two adjacent Vertical Points of Intersection (VPI), typically expressed in percentage form as the vertical rise or fall in metres/100 metres. In the direction of increasing stake value, upgrades are taken as positive and downgrades as negative.”
This definition reminds people that gradient is dependent on direction. What is uphill in one direction of travel is downhill on the other – and of course, how you deal with gradient depends on whether you are travelling uphill or downhill.
Definition of maximum gradient
The above definition says what gradient is. Maximum gradient may be related to the ability of vehicles to maintain a constant speed. For example, a definition from an Austroads document on the geometric design for trucks (ref.1887) says:
“General maximum grade: this is the sustained grade at which the truck can maintain a speed corresponding to the design criterion”.
Gradient and vehicle performance
For engineers who are designing a road, the ability of vehicles to travel the road must be a primary input. Of course, subsequent analysis may look at cost/benefit figures, at environmental impact and so on. The following table, taken from (ref. 857), relates gradient with the performance of vehicles. Note that it deals with uphill and downhill gradients separately:
Some values for maximum gradient
The following table gives one example of suggested values for maximum gradient. It is taken from the Austroads design guide (ref.1887), and “shows maximum grades over long lengths of road in various terrain types”.
I believe there should be a health warning to tables of values such as the above, along the lines of:
“These values only apply to motorised traffic travelling along a straight alignment of a long length of a surfaced road in a temperate climate, and where the road is not in tunnel. They do not necessarily represent values at which trucks will be able to maintain a constant speed. They do not necessarily relate to the design speed of the road, or to the operating speed of trucks, or to gradients suitable for non-motorised traffic, or to special roads such as scenic routes (where truck volumes may not exist). The values do not apply to definitions of terrain which are different from those used to develop the table. Further, these values suggested may have been affected by allowance for factors such as cost/benefit and factors of safety and so may not be solely related to engineering considerations”.
Engineers should bear in mind that summary tables of values might not include the values which they really need in their design.
148 – South Africa, Geometric design guidelines, 2002
857 – Austroads: Rural road design (8th ed.) 2003
1887 – Multi-country, Austroads: “Austroads Guide to Road Design, Part 3: Geometric design” 2010