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Extending Debate on Contested Spaces: Urban Arterials

Extending Debate on Contested Spaces: Urban Arterials

L Kane. Urban Transport Research Group. University of Cape Town.


This Journal sets itself a worthy and laudable goal, to extend architectural debate well
beyond the lay-man’s definition of the domain of the Architect (the building) into the
broader urban environment and the city. Few practicing architects would argue against
the need to have a wide urban scope. In the main, South African cities are not human
spaces at all and can be sterile, hostile or demeaning. Adding some more design flair into
the urban design mix must surely be beneficial?
In practice, however, architects and urban designers are often frustrated by their
interactions with urban engineering professionals who seem to speak a different language
and who appear to share little common direction with them. These professional
differences are particularly acute in the design of ‘contested spaces’ (that is, those which
house competing needs and wishes), such as the urban arterial, which is the focus of this
article. Using some recent research of a Masters student on the MPhil Programme in
Transport Studies at UCT1

, aspects of the conventional traffic engineering approach to
urban arterial design is described, and selected international differences are highlighted.2
The urban arterial provides several key functions: a piece of public urban space; mobility
corridor to public and private vehicles; an interface between communities; part of a route
to schools, shopping centers and employment; access to properties. The various roles
which are required of the urban arterial inevitably lead to conflict situations, between
local and longer-distance traffic, between vehicle and pedestrian (leading to crashes, and
injury), between resident and transport needs. Planners and architects bemoan the poor
urban environment of many urban arterials, creating an unwelcoming urban space and
lack of opportunities for local development. Meanwhile the traffic engineers bemoan the
poor or worsening transport conditions.
Conventional traffic engineering wisdom
There are many different positions on how the issue can best be tackled, but conventional
traffic engineering wisdom3

places roads into two categories: mobility routes or access

1 Barros Feio, I (2004) An international review of urban arterial road space management and cross-section
design practices. Research project prepared in partial fulfillment of the requirements for the Master of
Philosophy in Transport Studies.
‘Urban arterial’ is a problematic term, and is not universal, but for the purposes of this work is describes a
route of metropolitan significance, what is internationally referred to as a ‘main street’. It would have
mixed functions and is described in the NDoH Guidelines for Human Settlement and Design as ‘mixed
higher order’.
3 As described in CUTA (Committee of Urban Authorities) (1986) Draft UTG1 – Guidelines for the
Geometric Design of Urban Arterial Roads. Urban Transport Guidelines, National Institute for Transport

Extending Debate on Contested Spaces: Urban Arterials


routes. Mobility routes are intended to give priority to longer distance traffic and to
maintain high standards of transport ‘service’ (that is, a comfortable driving experience
with few interruptions and the ability to maintain relatively high speeds). An access route
is for roads which mainly access property, and which do not perform a mobility function.
The problem with conventional wisdom is that it assumes a clear-cut, and rather idealistic
subdivision of function. In reality, the ‘ideal’ of roads falling clearly into ‘mobility’ or
‘access’ routes rarely occurs, and so the conundrum remains: how to deal with conflicts
along urban arterials which host many functions.
The differing philosophical positions taken by the professionals and public often
condense into a debate over two key issues: space and speed. Broadly speaking the urban
design professionals prefer an arterial which gives more or better space over to residents,
business and slower modes (walking and to a lesser extent cycling), and which reduces
ambient speeds. Meanwhile traffic engineers, wishing to safeguard the mobility
functions of most urban arterials, attempt to do so through the use of relatively wide road
lanes, and relatively high speed. To date, engineers have been rather successful in
enforcing their viewpoint. There are many reasons for this, but the ability of seemingly
‘scientific’ arguments about road hierarchies, and the links between design and safety to
convince those in power, has certainly been one reason. Another has been the vested
interests of the engineering profession and politicians in the provision of roads (for jobs
and their own mobility). Also helpful to South African engineers have been the
superficially convincing body of US design codes which South Africa engineers have
referred to. (Superficially since, while they appear rigorous in their own terms, the
underlying assumption is that there is a very high car ownership, and a very small
pedestrian body. Application of these ‘rigorous’ codes in a country where these
assumptions do not hold true should be questioned).
How common is the conventional traffic engineering wisdom? Considering not only the
US experience, but taking a broader international view, how is space on urban arterials
divided and how is speed managed? The ability to answer these questions, and to use
relevant overseas experience, has the potential to improve our own practice or at least to
raise the level of debate. Of course, such simple questions are complicated by the widely
varying international contexts, and the peculiar characteristics of South African cities.
Nevertheless, the remainder of this piece takes a look at the elements of a typical urban
arterial cross-section and considers guidance on the space allocation and speed for each
of them in a sample of five case studies: US; Australia; The Netherlands; South Africa
and Bogotá (Colombia).
The ‘Design Vehicle’
Most traffic engineering design uses as a starting point the ‘design’ vehicle – a notional
vehicle which represents the typical vehicle on the road. Not surprisingly it varies across
countries: from a width of 2.1m in the US; 1.8m in South Africa and 1.75m in The

3 Road Research of the Council for Scientific and Industrial Research, Pretoria, South Africa and NDoH
(National Department of Housing) (2000) Guidelines for Human Settlement Planning and Design.

Extending Debate on Contested Spaces: Urban Arterials


Netherlands. There is more international agreement on a typical bus, described as 2.4-
2.6m in the US, 2.5m in The Netherlands and 2.6m in South Africa.
Lane widths and speed for private motorized traffic
Differing philosophies about the preferred clearance between vehicles lead to
international differences in lane width standards. From a South Africa perspective, two
interesting points emerge from the comparison: the tendency for South African guidance
to always err towards greater rather than lesser space provision; and the dramatic
difference between all case studies and Dutch practice, which is not fully explained by
the smaller design vehicle assumed. For example, the generally recommended road
width in the Netherlands is 3.1m, versus 3.5m in Australia and 3.4-3.7m in South Africa
Hidden behind recommendations for lane width are a series of assumptions about
appropriate speeds for urban arterials; the link between speed limits and the speed the
road should be designed to cater for; and the link between road width and speed. Views
on appropriate speed limits for urban arterials vary widely. It is evident that Dutch limits
of 50 km/h are rather more cautious than South African ones of 60 km/h. Perhaps of
even more interest, however, is the relationship between legal speed limit and the speed
for which the road is designed. The philosophy in Bogotá and the Netherlands is that the
traffic engineering design should support the legal limit, and should make the driving
experience very uncomfortable for those who break the limit. In US and South Africa,
the recommended speed used in design is somewhat higher than the legal speed limit for
the road. The philosophy here is that there will be errant drivers, and it is important to
make the road safe for those who speed. This is a fundamental traffic engineering
concept, sometimes called designing the ‘forgiving road’. Unfortunately, the outcome of
a ‘forgiving road’ for the few who speed is to make the ambient, comfortable driving
speed, on average, higher for all. Few would argue that a typical ‘free-flow’ speed on
South African arterials tends more towards 80km/h than the legal limit of 60km/h.
In order to design for high speeds, traffic engineers need to provide relatively wide lanes
(which allow for the driver to wander in the lane, without encroaching oncoming traffic);
and a clear view of the road ahead. The clear view, in practice, translates into large
sweeping radii at corners or on bends, and roads relatively clear of street furniture and
landscaping. In addition, situations which could impede the traffic flow are removed,
therefore pedestrians are ideally confined to regulated crossing places, and turning traffic
is removed into turning lanes, preferably in a median.
It is difficult to understand where the ‘wider is best’ philosophy of many South African
engineers comes from, although it may be based on research regarding safety on rural
roads, which clearly indicates a safety benefit to wider roads (collisions at speed area are
avoided and wider verges also have benefits for vehicles running off the road). However,
the current urban evidence, although difficult to fully validate, indicates benefits to
narrower urban roads. This is probably due to the speed reductions, which can come with

Extending Debate on Contested Spaces: Urban Arterials


urban road narrowing, and the improved pedestrian safety offered by narrower crossings.
It is generally accepted that in urban areas lower speeds lead to a lower incidence of
crashes, and a reduced mortality from them. For vehicle/ pedestrian crashes a collision at
30 km/h is likely to lead to bone fractures and concussion, whilst a collision at 50 km/h
means a high probability of permanent disability or death. Certainly, empirical evidence
elsewhere from traffic calming and Dutch style ‘woonerf’ have shown reductions in both
speed and injuries in narrowed residential streets.
The traffic calming ‘toolkit’ includes: changing road width, adjusting horizontal and
vertical road re-alignment (for example, introducing or adjusting curves or bumps);
adjusting junction control (traffic light changes, or introducing circles); adjusting access
control (by closing off or increasing minor road access); landscaping. At present, due to
the high role of the motorist mobility function on arterial roads many of these tools are
not used, but there are certainly many examples internationally of ‘traffic calmed’ major
routes (see photographs).
Of course the debate around what constitutes a ‘good’ urban arterial goes well beyond
issues of width and speed. The starting point of road design (or road re-design, which is
far more common in urban areas) must be an open discussion of the roles of the road.
This discussion will often cut to a discussion of mobility versus access, safety, design
speed, and user needs. A full discussion will interpret the user needs broadly to include
those of the various types of resident; pedestrian (scholar, shopper, through traveler)
motorist (commuter, shopper, business-person) and so on. The changing nature of the
road through the day and night also need to be considered. Ultimately the success of a
road design will depend on its ability to broadly satisfy a number of, often competing,
wishes. The success of the design process will depend on the extent to which these needs
were highlighted, described and answered.

Extending Debate on Contested Spaces: Urban Arterials


Source: Kenworthy, J4

. A traffic calmed inner city street in Freiburg. The strong

landscape elements are clear.

4 Photographs are from Kenworthy, J. Techniques for urban sustainability. Traffic Calming. Institute for
Sustainability and Technology Policy, Murdoch University. Available online at

Extending Debate on Contested Spaces: Urban Arterials


Source: Kenworthy, J5

. A main road in the suburbs of Copenhagen redesigned from 4

lanes to 2.

Source: Kenworthy, J. Eppendorf , Hamburg. A main road reduced from 4 lanes to 2.

5 Photographs are from Kenworthy, J. Techniques for urban sustainability. Traffic Calming. Institute for
Sustainability and Technology Policy, Murdoch University. Available online at

Extending Debate on Contested Spaces: Urban Arterials


The challenge is to apply some of these lessons, about safe and creative road space design
to higher order routes in South Africa, in order to improve the living environment and
improve road safety. The biggest challenge, however, is probably not in the application
of new design, but in finding a language with which engineers, planners, architects and
users can debate the role and purpose of urban arterial roads: for access, mobility or…?

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