Mike,
That’s a very good hypothesis –
it makes complete sense of what we found in the
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:
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 -
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
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
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
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
district of Hong Kong, I need to continue with the whole
territory. If something wrong in above 4 factors, please point it out, so I
can correct it early. Your comments are highly appreciated!
________________________________________________________________
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