## Understanding road geometrics (1)

Selecting a suitable value for a geometric road design element – perhaps horizontal radius – isn’t always easy. The most common way design standards use to present their data is through many pages of text and tables. These often present an almost impenetrable jungle to the exploring engineer.

In a recent blog post “Geometric parameters (1) a question of layout”   (see here) I argued that simplifying the layout of these pages and tables would make it much easier to understand and compare recommendations from different sources. However, pages and tables are most useful e.g. in providing information on the longitudinal design of a road.

Putting together the pieces which make up the cross-section of the road is a separate design exercise, which can perhaps be best understood by the use of pictures. The example abovecomes from the Los Angeles 2010 bicycle plan, technical design handbook (Ref. 917).

It shows a possible cross-section for a “major highway – class I”. My own first impression was that the bike lane seems to be narrow (at 5 feet / 1.5 metres), given the presence of a drainage channel – but this point may be discussed in the text part of the reference.

The point is that cross-sections are a great help in understanding road geometrics, and their design for this purpose is different than the design of cross-sections which aim to visualise the pavement’s vertical structure. In fact, standards from countries as diverse as Bangladesh (ref. 687) and Germany (for example ref. 1615) present a series of road cross-sections for a geometric design purpose. The pictures they show would be more useful still if each were accompanied by a brief description and a data table.

References

687 – Bangladesh, Standard cross-sections for RHD roads, RHD 2014

917 – US, “Los Angeles 2010 bicycle plan, technical design handbook”; 2010

1615 – Germany, “RAA Richtlinien für die Anlage von Autobahnen”, fgsv, 2010