I began a discussion about fundamental parameters in highway geometric design recently. The first parameter in my list is road type. A feature of “road types” is that they are associated with specific details of highway geometric parameters in all three dimensions of horizontal alignment, vertical aligment and cross-section. A particular road type is associated with particular values of (for example) number of carriageways, sight distance, and permitted gradient. Even the name of a road type from the UK, such as a 3-lane motorway dual carriageway (ref. 1038) itself tells us much about the road cross-section, design speed and so permitted gradient.
A search of just a few highway design guidelines will easily produce over a hundred different road types, so it can help to group them into a number of sub-sets. Table 1 gives an example of how this might be done.
The terms used in the “examples” column in table 1 come from various design guidelines (that is, they weren’t invented by me).
It does not help the highway designer if he is told to plan for a road type such as a recreational road. He or she will need to know specific details of the road type. The details can be presented in the form of a road type design brief. This should include
- Typical cross-section
- Design control values
- Values for parameters in horizontal design
- Values for paraameters in vertical design
Road type and road classification
Many guidelines talk about road classification. Classification however has less to do with highway geometrics than you might think. Knowing the classification of a road (e.g. “collector”) might tell you what the road is to be used for, but it does not tell you (for example) the width of a hard shoulder. How do we get from road classification to road type? What design guidelines should provide is a sort of conversion table,
Table 2 – road class / type conversion table
In fact one source does have such a table (ref. 1038), although arguably it would be better to replace the terms “road type” with “road class”, and “description of carriageway(s)” with “road type”.
Maybe highway engineers should concentrate more on road types rather than road classes. It might help if there were at least a common list of road types. Also I suggest that
- Any guideline which refers to road classification should include a conversion table of road class against road type
- Every road type should be accompanied by a table of parameters ( a road type design brief)
1038 – C.A. O’Flaherty and others, “Transport planning and traffic engineering”, Elsevier 2006