In the search for a suitable value for a geometric design parameter, it can often be a relief to find a simple table which seems to give the figure you are looking for. The trouble is that a simple table may omit the best information just as much as a complicated table can hide it.
To illustrate what I mean, take the example of maximum longitudinal gradient. The first figure below shows part of a table from the ASEAN highway standards (ref. 1884). For me the table is too complicated – the key figures do not stand out, and the table seems (for example) to suggest that gradient varies with shoulder width or minimum curve radius.
The recent Nigeria design standards (Ref. 1505) give a clearer presentation of values for gradient, as in the next table:
The table relates gradient to what I would call the primary factor influencing geometric parameters, namely design speed. If we try to show the ASEAN values in a similar table, we get:
This table shows in some cases a range of values for a particular combination of terrain type and speed, but there is nothing unusual in that. The figure below comes from the Austroads 2010 design standards (ref. 1887). Here the table shows a range of values for maximum gradient, on a similar basis to the layout above, although with a different orientation.
Clear, consistent layouts
If we now present the values for maximum gradient from the ASEAN, Nigeria and Austroads documents we can get a clearer picture of what each is saying, and it is also easier to compare the values with each other, as the following graphic shows:
- Simple tables with a common layout make it easier to compare recommendations from different sources.
- The three tables are not robustly comparable b ecause we dont know if the three sources use the same definitions for terrain and speed
I haven’t made it clear whether the three tables refer to the same road types – in fact, I also haven’t made it clear how gradient can be affected by other design considerations beside “road type”.
The next blog post will cover what I mean by design considerations, and how they might be covered.
1505 – Nigeria, “Highway manual part 1 Design / vol. I: geometric design”, Federal Ministry of Works, 2013
1884 – multi-country, “Ministerial understanding on the development of the ASEAN highway network project”, ASEAN, 2003
1887 – multi-country, “AGRD part 3: Geometric design”, Austroads; 2010