Road hierarchy and bendy roads

The normal concept of a road hierarchy is that of sorting roads into groups or levels of increasing importance, often with each level related to increasing maximum permitted speed(s). In terms of the horizontal curvature of a road, this means that with increasing levels in the hierarchy the road curves become flatter and flatter. The roads become increasingly straight.

 If engineering principles result in roads becoming flatter as speed and level in the hierarchy increase, then conversely roads could become more curved as speed and level in the hierarchy decrease. Indeed, if a road is meant to be self-explaining (see e.g. here), then a straight road in a residential area is the equivalent of bad engineering.

A basic  design principle could therefore be: as a road’s place in a hierarchy decreases, it should become more curved, and visibility distances and other geometric parameters should be deliberately reduced.

To take the idea a step further, for any design speed there should be a maximum value for design parameters as well as the existing, minimum values. For example, for a road with a design speed of 50 km/hr, the minimum radius is the minimum safe radius for 50 km/hr, the maximum could be the minimum safe radius for 60 km/hr.

Nikos Salingaros criticises straight roads in (urban) architectural terms. For example (from ref. 389)

“One of the stated aims of modernism was to eliminate any architectural interface with fractal dimension. These were replaced by long, straight roads, and reinforced with the strict alignment of buildings. The reason given was to clean up the perceived messiness of older cities; yet that messiness was really the organized complexity that made them alive” (own emphasis)

 Safer roads, designed for lower speeds, and reintroducing a human scale environment. Bendy roads could be good for urban architectural as well as engineering reasons.


Thanks to Marija Jevric of the University of Montenegro for the introduction to Nikos Saingaros’s writings.


389 – Victor Padron, “Conversation with Nikos Salingaros” (2000)


About roadnotes

Robert Bartlett is an international consultant with over 30 years of professional experience as a highway and traffic engineer with leading companies and organisations in several countries, including Germany, China (Hong Kong), Qatar and the UK. Specialised in urban studies, transport and the use of GIS, research has included new ideas on subjects such as the study of social justice using GIS, the dimensions of vehicles, and comparative geometrics (highways and transport).
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