 |
 |
Jake Desyllas of Intelligentspace.com posted the following on 10/06/2003: > For those interested in pedestrian modelling, this paper can be > downloaded at http://www.casa.ucl.ac.uk/working_papers/paper62.pdf > Summary details below: > > "Pedestrian Demand Modelling of Large Cities: An Applied Example > from London" > by Jake Desyllas, Elspeth Duxbury, John Ward, Andrew Smith At the heart of the paper are what look like some good, if familiar, ideas, which are unfortunately marred by what appears to be a misguided attempt to cut out any trace of the origins of the research methods used, and some rather suspect statistical analysis. In this note we will try and set the record straight and go on to point out where these methods seem to us to need to go from here. A multivariate regression model to explain and predict pedestrian movement using a set of independent variables including measures of the configuration of space is an excellent idea. Many space syntax researchers have been doing this for some years, and this presumably is why the authors thought that these mail lists would be interested in their results. The paper points out how important it is to avoid what Harvey (1973) would call confounding spatial and social measures, and although it misinterprets the ecological fallacy, the need to analyse space at a fine resolution in order to avoid the problems posed by aggregation are well made. The response to Lee's (1973) criticisms of large scale urban models through the use of statistical and data based modelling are spot on. All this is excellent, so it is a pity that the paper does not properly review the background to the methods it has used nor the extensive literature on related multivariate regression models of pedestrian movement. A number of other extremely similar models have been developed in recent years, most notably Penn et al's analyses of vehicular and pedestrian movement in central London (1998). These regression models used a wide range of variables, including land use, development density measured by building height, street and pavement width, parking availability and of course the full range of accessibility measures provided by space syntax analysis using axial analysis. The observation data are consistent, thorough and gathered by the same team working to common observation methodologies and standards, for the sole purpose of developing a high resolution data set of pedestrian movement. The studies observed all street segments in seven areas of London (rather than sampling a subset of segments) giving over 460 observation locations. Along quite different lines Dongkuk Chang's 1998 PhD developed an integrated regression based model for pedestrian movement in multi-level circulation systems (see Chang & Penn, 1998). These models recognized that spatial configuration alone would be hard put to it to explain movement patterns in highly complex and unintelligible environments such as London's Barbican and South Bank areas where invisible staircases and major attractors or generators of pedestrian traffic play a powerful role. Chang's methods quantified visibility, grade separation, and, along with axial analysis, incorporated them in a multi-criteria model, which effectively 'calibrated' the axial model for the effects of these other factors. Both sets of studies have been peer reviewed and published. Both are clear precursors to the work on walkability in London that Andrew Smith initiated at Transport for London along with Space Syntax Limited (Stonor, Campos & Smith, 2002), however, all these are unmentioned, even though Alan Penn was Jake Desyllas' PhD supervisor, Chang a contemporary doctoral student and Smith a co-author. One wonders why? Perhaps it is merely an oversight in an unreviewed working paper on the web. But the oversights go further. One of the most serious omissions is the origin of a key component of the model, Visibility Graph Analysis (VGA). VGA is a technique developed at UCL by Turner and Penn, through a joint project led by the Bartlett and CASA, on which the first author was employed as a research assistant. It seems at least odd and at most obloquious that in the paper he should attribute the development of the technique to Braaksma (1980). Braaksma does use visibility graphs of service networks (although he uses the term 'adjaceny matrix'), but this is more related to the networks of radio transmitters proposed De Floriani (1994) and many other more traditional visibility graph applications. The key to Visibility Graph Analysis of urban areas is the dense grid form of the visibility graph, developed by Turner et al. (2001), and implemented in Depthmap and subsequently in Intelligent Space's 'Fathom' program. The term itself, 'Visibility Graph Analysis', and the application of these techniques to analysis of pedestrian movement in building interiors (and the possibility to analyse urban areas) originates in the Turner et al. (2001) paper. VGA was built on and resulted from research into space syntax. It extends local and global measures from graphs of axial lines and convex spaces to a complete visual domain. The CASA working paper also builds itself into this heritage, following the introduction of VGA with a discussion of how it may be used to examine features such as 'two step' relationships to an accessible 'neighbourhood'. Global analysis of spatial networks is described, but the provenance of the ideas is misattributed to Wasserman and Faust (1994) who applied two step measures in social network analysis. The idea of examining global network relationships in spatial systems is introduced in Hillier & Hanson (1984) and developed into radius measures during the 1990's. Indeed, one of the best correlates discovered for pedestrian movement in space syntax research, radius-3 axial integration, is the forrunner of radius based measures in VGA. That the authors appear not to understand the methodological basis of these techiques beggars belief. The proper referencing of the origins of the research is certainly our major concern, but there is also a problem with data analysis using linear regression that needs to be commented upon. Regression models are notorious for giving false correlation coefficients with skewed data sets. At the very least, some form of data normalization or test for normality should be performed before claiming a linear regression result. As can be seen from the published scatterplots in the paper, data on both axes are highly skewed to the bottom end of the scale. This needs to be corrected using a transformation of the data sets to produced normally distributed data sets if any reliance is to be placed on linear regression. After these negative paragraphs, it may seem odd for us to say that this paper could be a good one. Much of the literature review is enlightening, pointing towards the often overlooked origins of research into pedestrian movement. Fruin's (1971) levels of service are nicely woven into the idea of a complete pedestrian model. Using VGA to develop a regression model of a large area of central London is also a huge improvement on the state of the art in urban modelling in general. The parameters are well chosen: the literature has shown that spatial variables of street width and land use and transport interchanges are important factors in pedestrian movement. The importance of incorporating tube stations and rail terminii is clear given that these account for the major (and most concentrated) change of mode sources in London. Shop frontages are also a sensible choice, at least for the West End of London, where shopping streets see significantly more usage than, as pointed out in the paper, nearby side streets. This leads us to ask where this stream of research needs to go in the future. Perhaps a crucial point lies in the choice of VGA as the analytic basis for calculating accessibility. VGA is a really nice idea (although speaking as its originators we would say that), however we have now been looking at it for nearly 7 years and still in urban street systems axial mapping significantly outperforms VGA as an explanatory variable of pedestrian movement flows in urban systems (Turner, 2003). The reasons why this might be the case are the subject of considerable research at the moment (see Penn, 2002 and Bill Hillier's paper in the Space Syntax Symposium next week (The Architectures of Seeing and Going). However, from a pragmatic point of view, where predictive power and empirical evidence are the most important criteria, it is clear that axial mapping and high resolution axial mapping could form a much more powerful basis for a multicriteria regression model. It might also be an idea to look more generally at the available land use data since the previous studies found that office space also contributed significantly to pedestrian movement, and that significant differences at different times of day were best explained by looking in detail at a full land use breakdown. Development density is yet another of the significant factors and should ideally be incorporated in a model of this sort. Perhaps most important however, was the finding that average axial radius-3 integration was powerfully related to average movement flow rates in large aggregated areas. We suspect that any model of this sort will need to take account no only of the specific local spatial properties, including those that are measured in terms of the global network, but will also need to address neghbourhood properties that describe the degree of grid intensification in different parts of the urban area. In conclusion, whilst it is highly embarrassing that a paper in this state has been published on a UCL website, there is significant data and a worthwhile technique published within it, and we look forward to its republication in a revised and more academically responsible form. Alasdair Turner, Alan Penn & Bill Hillier The Bartlett School of Graduate Studies Faculty of the Built Environment University College London References: Braaksma, J P, Cook, W J, 1980, "Human orientation in transportation terminals" Transportation Engineering Journal 106(TE2) 189 203 Chang, D., & Penn, A., 1998, "Integrated multilevel circulation in dense urban areas: the effect of multiple interacting constraints on the use of complex urban areas," Environment and Planning B: Planning and Design, 1998, Vol. 25, 507-538, Pion. De Floriani, L, Marzano, P, Puppo, E, 1994, "Line-of-sight communication on terrain models" International Journal of Geographical Information Systems 8 329 342 Harvey, D, 1973 Social Justice and the City (Edward Arnold, London, UK) Hillier, B, 2003, "The Architectures of Seeing and Going", 4th Space Syntax Symposium, London June 16-19th 2003. Hillier, B, 1996 Space is the Machine (Cambridge University Press, Cambridge, UK) Hillier, B, Hanson, J, 1984, The Social Logic of Space (Cambridge University Press, Cambridge, UK) Lee, D B, 1973, "Requiem for large-scale models" Journal of the American Institute of Planners 39 163 178 Penn, A., 2003, "Space syntax and spatial cognition: or why the axial line?" Environment and Behaviour, (2003) 35 (1) 30-64, Sage Publications, ISSN:0013-9165 Penn, A., Hillier, B., Banister, D. & Xu, J. 1998, "Configurational Modelling of Urban Movement Networks," in Travel Behaviour Research: updating the state of play, Juan de Dios Ortuzar, David Henshar and Sergio Jara-Diaz (eds), Pergamon press, Elsevier, Amsterdam, 1998, Ch19, 339-362. ISBN 0-08-043360-x Penn, A., Hillier, B., Banister, D., Xu, J., 1998, "Configurational modelling of urban movement networks," Environment and Planning B: Planning and Design, Vol 25, pp 59-84, Pion, 1998. Stonor, T., Campos, M.B deA & Smith, A., "Towards a Walkability Index", Proceedings of teh 3rd International Conference, Walk 21, Barcelona, 2002. Turner, A, 2003, "Analysing the Visual Dynamics of Spatial Morphology", Environment and Planning B: Planning and Design, 2003, vol 30, (in press). Turner, A, Doxa, M, O Sullivan, D, Penn, A, 2001, "From isovists to visibility graphs: a methodology for the analysis of architectural space" Environment and Planning B: Planning and Design 28 103 121 Wasserman, S, Faust, K, 1994, "Social Network Analysis: Methods and Applications" (Cambridge University Press, Cambridge, UK)