CONTENTS DIARY OF EVENTS - The Urban Design Group

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Reinventing Road Hierarchy
Stephen Marshall suggests the possibility of reinventing ‘road hierarchy’ as
part of a street-based urban code
The historical transformation from traditional street-grids to the
Modernist system of open plan, hierarchical road layouts and back
to street-grids again must be one of the most significant reversals in
urban design history. As part of the urban counter-revolution, mixeduse
streets are now back in vogue, while old style roads-dominated
approaches to urban layout are on the back foot. Yet, while a variety of
professions and design guidance documents may now be sympathetic
to the street-oriented urban design agenda, many of the principles
which underpin urban layout are still to a significant extent based on
Modernist ideas of road hierarchies and separate land use zones.
Therefore, although it is possible to design neat urbanistic enclaves
such as Poundbury with official approval – as illustrated in the UK
design guide Places, Streets and Movement – it is not so clear what
happens when we come up against the main network of ‘distributor’
roads. In other words, while neo-traditional style developments such
as Poundbury may internally have an urbanistic ‘hierarchy’ of streets,
squares and mews, this hierarchy effectively goes no further than the
nearest distributor road, where it comes up against the constraints of
conventional road hierarchy.
‘GOOD’ AND ‘BAD’ HIERARCHY
Although modern roads layouts are sometimes criticised for being
hierarchical, urban designers and planners do sometimes appreciate
some kind of hierarchy, whether as part of traditional unplanned or
planned settlements. Design guides sometimes call for a recognisable
hierarchy of streets and spaces; this may promote diversity of street
type and legibility of the layout as a whole.
The Craig Plan for Edinburgh’s Georgian New Town, despite being
an ‘urbanistic grid’, employs a hierarchical structure similar to modern
road layouts. The difference is that in the case of the Craig Plan, all the
types are streets of one sort or another (streets, lanes, mews), and they
connect ‘upwards’ to the main streets – that is, to focal ‘people places’,
that form the main backbone or armature of urban structure.
In contrast, in modern road layouts, streets only feature at the
subordinate level of the access road. There is no strategic focus for these
14 | Urban Design | Spring 2005 | Issue 94
streets; they often lead or connect ‘upwards’ only to the non-place
disurban realm of the distributor road. In effect, the structural logic of
conventional road hierarchy leads to a discontinuous scatter of street
enclaves forming oases of urbanity in a desert of distributor roads.
So the problem with modern urban layouts is not that they are
hierarchical as such; but that they may use the ‘wrong’ combination of
hierarchical elements for today’s taste – ‘wrong’, that is, for promoting
mixed-use, walkable (and bus-catchable) urban neighbourhoods, rather
than mono-use, car-oriented suburbs.
Accordingly, to achieve more urbanistic layouts does not necessarily
mean abandoning road hierarchy, but could mean reinventing it as a
component of a wider urban code.
HIERARCHY AS CODE
One of the benefits of conventional road hierarchy is that it actually
allows a decent degree of flexibility when it comes to road pattern
or configuration. An observer from space looking down on typical
suburban layouts from the past four decades would note the ‘organic’
feel of the curvilinear layouts bristling with culs-de-sac, noting some
degree of common familial resemblance between configurations,
yet with no two being the same. (This could be contrasted with the
configurational regularity of traditional rectangular grids.)
This combination of consistency and diversity arises because road
hierarchy, while systematic, is not a fixed blueprint or template. It only
governs relationships between components – for example, immediate
adjacency and strategic connectivity. It is effectively a ‘code’, woven
as it were into the urban fabric. The pervasive embedded nature of this
code gives road hierarchy a potency that accounts in part for the impact
that highways and traffic have had on urban form and structure for
decades.
While in practice road hierarchy might have turned out as an antiurban
or at most sub-urban code, it is nevertheless capable of being
reinvented towards more urbanistic ends. That is, a street-based (rather
than road-based) code would have the possibility of generating more
urbanistic layouts, including traditional street grids of various sorts.