I think SS people should send more private messages to this list...
After all jiscmail is owned by space syntax...
Or am I just getting too cynical?
Rui
On Tue, 23 Jan 2007 11:42:10 -0000, Alan Penn <[log in to unmask]> wrote:
>Mike,
>
>
>
>That's a very good hypothesis - it makes complete sense of what we found in
>the London study where the pedestrian model (no traffic management)
>correlated better with flows than the model with oxford street cut to
>vehicular movement. My analysis of this was that what we were in fact
>observing was a long term historical evolution of land uses and densities
>according to the un-traffic managed space structure. Now, in the case of
>Kowloon, I am not so sure that it is as evenly distributed in terms of
>density and use as you suggest, certainly at the large scale, and I suspect
>that there may be issues to do with data consistency, mapping and size of
>the map all of which have been raised, and should be checked for. In
>particular, at about 5pm every evening a whole series of streets get cut to
>traffic by the setting up of street markets. These produce crowds of
>pedestrians and severely disrupt neighbouring streets. One could see these
>as both attractor destinations - I am sure that some people drive to get to
>them, and as blockages in the space structure to vehicular movement. Given
>this kind of complexity I am not surprised that a first cut analysis doesn't
>show a correlation, however you have provided a very nice working hypothesis
>for Chengke to test. I wonder whether data on land use and development
>density are available?
>
>
>
>Alan
>
>
>
>
>
>You know, I think the problem with this is that there is very little spatial
>autocorrelation in HK. This is because of the density of the place and
>because the traffic pattern is not radial like organically growing western
>cities that grow from a core, usually.
>
>In western cities there is a lot of spatial autocorrelation which means that
>more central points in the city are naturally the most accessible like the
>centre of a circle and therefore there is more traffic travelling there due
>to more people there. Some would argue that this shows that any such
>correlation with traffic going to the centre and accessibility is spurious
>and it is true that it doesn't explain much at all, forthe centre of any
>bounded space is the most accessible by definition.
>
>In fact in a lot of spatial analysis, accessibility or integration is
>factored out before analysis begins - this is like getting rid of spatial
>autocorrelation before the modelling begins. We wrote a couple of papers on
>this in the 1970s and here they are:
>
> 1976 (with L. March) The Method of Residues in Urban Modelling,
>Environment and Planning A, 8, 189-214.
>
> 1978 (with L. March) Urban Models based on Information-Minimising,
>in R. L. Martin, R. J. Bennett and N. J. Thrift (Editors) Towards Dynamic
>Analysis of Spatial Systems (Eds., Pion Press, London, 127-155.
>
>There is a good book on this - in fact the original book by Andy Cliff and
>Keith Ord (1981) Spatial Processes: Models and Applications, Pion which
>tells you why this effect is important but there are a lot of more
>intelligible primers such as Spatial Regression Analysis on the PC: Spatial
>Statistics Using Sas (Resource Publications in Geography) by Daniel A.
>Griffith, Assn of Amer Geographers (Nov 1993).
>
>So all this means that what one might do is once one has computed the
>integration accessibility values then use these to explain how traffic
>patterns differ from these values and hence all the explanation is heaped on
>the residuals. So in a sense in HK this is less of a problem and it means
>that to explain traffic we need new independent variables such as density,
>land use type, trip demand and so on.
>
>If you have a density of population that is not declining inversely with
>distance from the centre which is probably the case in places like HK and
>also the place is hemmed in by mountains - Lion Rock to the north of
>Kwowloon and the Peak to the south of Victoria, then it must be almost
>impossible to satisfy the conditions for high accessibility at the centre in
>terms of just the street geometry.
>
>Mike
>
> At 12:03 22/01/2007 +0000, Alain Chiaradia wrote:
>
>
>
>Dear Chengke,
>
>
>To add to Alan comments:
>
>a/ What kind of road counts do you use? Are they mainly expressways, trunks
>road, main roads? If so try global measure. Traffic flow level is also
>dependant on capacity (useable width or lane number) so this may impact on
>your analysis.
>
>b/ if there are global movement roads it might be that your model coverage
>is too small. Large scale movement require a regional model size.
>
>________________________________________
>
>Alain Chiaradia
>________________________________________
>
>
>-----Original Message-----
>From: Chengke LIU
>Sent: 21 January 2007 10:18
>To:
>Subject: Poor correlation between local integration and traffic flow in
>Kowloon, Hong Kong
>
>Deal all,
>
>I have finished drawing axial lines of one district of Hong Kong - Kowloon,
>totally 3004 axial lines. Also I pin-pointed all the counting stations on
>those axial lines. I calculated local integration of axial lines and
>compared it with the natural logarithm of the average traffic flow of that
>corresponding axial line. To my disappointment, the correlation is quite
>poor, r square value is only 0.10. I am not sure why the correlation is so
>poor, the possible reasons are listed as follows:
>
>1.The street network in Hong Kong is very complex, I digitized some road
>lines with only one axial lines, since they represents different lanes for
>different traffic direction. And even, somewhere a cluster of roads are
>represented for only one axial line for simplification.
>2.Due to lacking of time and manpower, I did not take those overpass and
>underpass roads into consideration when I calculated the connectivity value
>of axial lines. When I digitized the axial map, I only referred to the
>digital street and building maps of Hong Kong at hand. Some lines are
>apparently intersected on digital map, but actually they are not, since one
>of them is overpass or underpass road.
>3.When I assigned the traffic flow the axial lines, I first pin-pointed
>counting stations on roads to corresponding axial line, and then averaged
>the traffic flow.
>4.The spatial configuration (space syntax) is not suitable in Hong Kong to
>predict traffic flow.
>
>The above factors are responsible for the poor correlation from my point of
>view. What do you think about this problem? Since Kowloon is only one
>district of Hong Kong, I need to continue with the whole Hong Kong
>territory. If something wrong in above 4 factors, please point it out, so I
>can correct it early. Your comments are highly appreciated!
>
>
>Michael Batty Director Centre for Advanced Spatial Analysis (CASA)
>University College London - 1-19 Torrington Place - London - WC1E 6BT UK
>________________________________________________________________
>
>tel 44 (0) 207 679 1781 New CASA Web Site at http://www.casa.ucl.ac.uk/
>Personal Web Page: http://www.casa.ucl.ac.uk/people/MikesPage.htm
>
>
>
>
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