New road classification / 4 : a two-dimensional geometrics classification

The previous post in this series gave an example of a two-dimensional classification for roads, and asked: What would be the features suitable for use in a 2-dimensional code for the geometry of roads? The same post argued that when considering the geometric design of roads, these basically have only two dimensions:

• characteristics along the line of the road and
• Characteristics transverse to the line of the road.

The longitudinal features (for example gradient, horizontal curve) are primarily dependent on design speed. In most cases, design speed is indicated in terms of increments of 10 km/hr. This does not apply to the USA, which still uses miles / hr, nor to the UK, where one main document (ref. 255) refers to design speeds of 85A, 70B etc.

A main descriptor for the features across the width of the road would be the number of lanes. A figure for right of way would give the full width of the road, but most of this width might be taken up by landscaping. It is not such a good indicator of capacity.

So a combination of design speed and number of lanes would give the matrix of geometric road types shown in table 1.

A particular road might then be coded as SL 80/08 (speed/lanes classification indicating a road with a design speed of 80 km/hr and 8 traffic lanes).

Adding terms to a 2D classification

If there is a desire to use words as well as codes to describe geometric road types, then terms might be added as shown in table 2.

Developing a pragmatic 2D classification

Not all these speed/ lane combinations are likely to occur in the real world, so table 2 might be modified as shown below (note that this is particular figure is only intended as an illustration)

Un-resolved problems

Arguably the concept of a 2-dimensional coding for the geometry of roads is interesting. However it would help if road engineers agreed on what they meant by design speed, lanes (and also perhaps carriageways).