One possible perspective is that transport models try to predict changes
in flows, related to alternative policies, technologies, preferences, and,
obviously, locations (SOCIAL DYNAMICS), on a given spatial base, over a
relatively short time period. So this is a socio-economic model with a
spatial constraint. One would expect that morphological models similarly try
to predict changes in space, related to alternative SPATIAL DYNAMICS and
implicit social forces (which would come about only as triggers for a
essentially spatial process), over large time period (e.g. CA ones). As it
is well acknowledged in the literature, slow processes enslaves fast ones,
so that it would be expected that space constrains flows, not being its
generators in any way.
What space syntax does is neither one nor other thing, in fact SS doesn't
actually model anything, yet modeling is about change and, of course, time.
In this way, correlations between accessibility and flows are incidental by
definition, yet change and time are not considered, let alone trip purposes
which are at the heart of urban flows. This is made quite clear in urban
evolution studies that frequently display the disarray between ss measure
and distribution of flows in different moments in time of a same city.
In terms of spatial dynamics, SS has this suggestion that street
configuration, creating accessibility differentials would induce location of
attractors, and consequently street layout would survive in the resulting
complex system of activities and flows. Hansen had already explored that
idea, although he considered accessibility in a more sophisticated way. It
is a tempting idea, but perhaps too easy, too straight; what to do with
externalities, congestion, urban growth, sprawl, innovation, changes of
business scale, transport technologies, etc, all changing factors of urban
polarization?
Within transport models it is relatively easy to deal with them, as
spatial base, remember, is given; therefore every spatial new feature is a
new constraint, whereas every new social feature is potentially part of the
equation system already. Within space based models, every spatial feature
are derived from the spatial rules already inside the model, whereas every
new social feature is just one more external force exciting the system.
Regarding SS, every new spatial feature comes as a random change, which
requires a new measure, as the new and the old systems states have no
relation with each other. The new measure, even related to the old one,
doesn't tell about the transition from one state to the other.
Even adopting the suggestion that urban grid configuration is induction to
location, which is induction to flows, it is not a simple step to take grid
configuration as induction to flow directly; think of a perfect grid: before
its full occupation, every street has the same probability to attract
activities and to channel flow, but this probability will dramatically
change as soon as the first attractor is located. After that the grid itself
is not the same any longer, as the rich becomes richer (highly varied built
form topography of Manhattan is a good example). So, accessibility
differentials can be seen as probabilities of location; some configurations
have such a probabilities more distributed, others more concentrated, but
those probabilities change in time, both because space is finite, discreet
and costly, and accessibility does not depend solely on configuration.
Syntactic measures could come nearer to transport models through simple,
although not easily accepted improvements: a) let axial lines go and adopt a
new spatial unit representation which could better capture the distance
dimension of space. Links would do much better (bits of streets defined by
two consecutive corners) by reducing the length variation; b) keep the
topology but do introduce a distance decay function; c) consider built form
and activities as weights attributed to the links. Voilą, we“re back to the
Sixties' interaction models. The result would be a better assessed
distribution of probabilities of flows, still a bit faraway from real
transport models, but nearer to a morphological model of interaction
potential.
----- Original Message -----
From: "Michael Batty" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Saturday, May 08, 2004 6:54 AM
Subject: Re: What streets to include in axman
> Basically I think people should get this discussion in perspective and I
> agree with Andrew Smith.
>
> Space Syntax represents the network and that is all and even that is
> problematic as it is not the network that most transport planners use.
Most
> transport models begin with the network as a planar graph assumed,
> modelling flows through all kinds of human behavioral models from
transport
> activity to discrete choice to gravitational. There is 50 or more years
of
> work in this area with many many journals. and an extremely activity
> community world wide.
>
> Before we get too worked up by the claims about what space syntax can do
> when it comes to associating flows with accessibility indices because
that
> is what it is doing, we should realise that what is involved here is
simply
> saying that the density of streets is correlated with the density of
flows
> and vice versa. One would expect that. In fact probably better
correlations
> between flows and densities of streets over small areas could be
achieved
> than those paraded in space syntax if we simply counted streets by
number
> and distance in small areas.
>
> But that is not the point. We would expect high correlations anyway.
Saying
> that "streets cause flows or flows cause streets" is not what urban
theory
> is about. And in any case there is a massive debate about the
> appropriateness of using parametric statistics like correlation to
measure
> the coincidence of street density or accessibility or integration -
> whatever one wants to call it - with flow volumes. That is yet another
debate.
>
> Finally I cant see how flow volumes which are measures over small areas
and
> can therefore be converted into densities, can be tagged to axial lines
> anyway as axial lines are not normalised with respect to length, and are
> topological construction without density. They have no dimension because
> the Euclidean geometry is not included (see my paper on "Distance in
Space
> Syntax" again - http://www.casa.ucl.ac.uk/publications/full_list.htm ).
How
> do you associate a flow with Oxford street when the flow is over small
> segments which do not coincide with Oxford Street as an axial line? I
> suppose one could interpolate but most flows are computed for networks
> which are not those used in space syntax
>
> All this is why we need new theories of urban morphology based on
streets
> which link to what we know about geometry as well as toplogy and all
else
> in urban geography and transportation besides
>
> Mike
>
>
>
>
> At 19:55 07/05/2004, Andrew Smith wrote:
> >If you'll permit me stepping into the discussion of space syntax and
> >transport modelling, as a transport modeller / planner with a mere
> >inkling of space syntax.
> >
> >In essence, transport models are *pivot* models. A transport model's
> >utility is in reliably forecasting the *change* from a current
> >situation that a policy or scheme would have, rather than in
> >describing the status quo.
> >
> >So a model that explains 80% of the traffic flows in Central London
> >(before congestion charging), based on an axial map, will predict no
> >changes after such road pricing is introduced, as the axial map does
> >not change. Similarly with bus lanes, traffic light phasing, traffic
> >calming: all the standard tools of urban traffic management. But
> >flows have changed significantly ... and a useful transport model
> >would predict those changes reliably.
> >
> > > If we combine the Nantes results ...
> > > we won't be has puzzled than the
> > > transport engineer/planner seems to be in
> > > their conclusions.
> >
> >There's little new in the Boston conclusions in the document that was
> >linked to, earlier. I'd say that it seems to be somewhat behind the
> >times IMHO. The transport planning profession has known about the
> >linkages between land use mixture, density and travel demand for over
> >40 years (not that it's been reflected in policy and action, but
> >nevertheless the knowledge has been there), and there's been lots of
> >very good work done in the last 4 decades on describing those
> >linkages.
> >
> >Total mobility is driven by relative and absolute accessibility of
> >individual modes, of course, and so a useful network model needs to
> >have coded within it which modes can use which links.
> >
> >As traffic (or more usefully, general transport) models use
> >origin/destination matrices, I can't see how one would implement Alan
> >Penn's suggestion of: "using space syntax measures ... in the matrix
> >estimation phase of the construction of traffic models", but am
> >willing to be enlightened. I'm often agnostic as to the usefulness of
> >models based on such O/D matrices, but they do have lots of
> >advantages in terms of being able to inform policy decisions on land
> >use and transport.
> >
> >Andrew Smith
>
> ___________________________________________________________
> Michael Batty, Director, Centre for Advanced Spatial Analysis
> University College London, 1-19 Torrington Place London WC1E 6BT
>
> [t] 44 (0) 207 679 1781 [f] 44 (0) 207 813 2843 [m] 44 (0) 7890 071 838
> ___________________________________________________________
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