Alain Bertaud Urbanist.ppt

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1、Alain Bertaud Urbanist,Module 2: Spatial Analysis and Urban Land Planning,The Spatial Structure of Cities: International Examples of the Interaction of Government, Topography and Markets,Summary,What are urban spatial structures?How do we define urban spatial structuresWhy spatial structures matter?

2、Is there a global trend in the evolution of urban spatial structures?,Section 1: What are Urban Spatial Structures?,What are Urban Spatial Structures? (1),Cities have complex shapes which seems to escape classification, and in a certain way every city is uniqueHowever, cities have a spatial structur

3、e which is not always visible from the ground but which appears when analyzing dataMany cities which appear very dissimilar from the ground share the same spatial structure,What are Urban Spatial Structures? (2),Understanding the spatial organization of a city allow planners to Predict the main dire

4、ctions of future developmentDevelop strategies which are compatible with the current spatial structureInfluence the evolution the current structure in a manner consistent with municipal objectives,What are Urban Spatial Structures? (3),Spatial organization could be described by 2 superimposed patter

5、ns: the spatial pattern of population distribution within the built up area and The pattern of population movement around the city during the day,Figure 1: Three dimensional views of population distributions in 7 cities represented at the same scale,Section 2: How do We Define and Measure Urban Spat

6、ial Structures?,How do We Define and Measure Urban Spatial Structures?,We can define urban spatial structures using a number of indicators. We will only focus on 3 main indicators:Land consumption (density) Density profileDegree of monocentricity (share of trips with central destination),Land Consum

7、ption per Person (density),Average density is the most common spatial indicatorIt is an indicator of land consumption, the higher the density the lower the consumption of land per personTo be meaningful, density should be equal to the city population divided by the built-up area Density measured by

8、dividing population by administrative area is not meaningful and do not allow cross cities comparisons,Urban Densities (1),Average density in built-up area varies by several order of magnitude from city to cityThere are no “correct densities”Densities reflect the complex interaction of land markets,

9、 topography, and government actionLand markets have a strong cultural components, densities are therefore very much linked to culture,Built-up Densities around the world (figure 2),Urban Densities (2),The built-up densities of figure 2 are showing the large variations in densities between cities (fr

10、om 6 p/ha in Atlanta to nearly 400 p/ha in Mumbai)All the cities shown on figure 2 are economically viable, large difference of densities are therefore compatible with viable economies and are manageable,Urban Densities (3),Densities seems to be more related to regions than to income, implying that

11、densities might have a strong cultural causalityThe high densities in Asian cities have not been “planned” but occurred because of cultural preferences expressed through land markets,Urban Densities in Chinese Cities,Chinese cities, which had been developing without land markets for about 40 years.

12、However, they have high densities because of : the relative underinvestment in urban infrastructure and housing during the same period cultural factors shared with other Asian citiesBetween 1947 and 1980 Chinese cities grew mostly through densification of pre-1947 built-up areaUntil about 1990 the d

13、ominant mode of transport in Chinese cities was the bicycle limiting the radius of city extension and therefore increasing density.,Urban Densities have Important Implications,The following example compare the built up area of 2 cities Atlanta (USA) and Barcelona (Spain)The 2 cities have about the s

14、ame population of 2.6 million (1990) but their density is very different: - Atlanta 6 p/ha- Barcelona 171 p/ha,Figure 3: The built-up area of Atlanta and Barcelona at the same scale,Urban Densities have Important Implications: Atlanta vs. Barcelona,Because of the difference of density, Atlantas buil

15、t-up area is 28 time larger than BarcelonasAs a consequence, the metro of Barcelona is only 99 km long and its stations are within walking distance to 60% of the populationBy contrast, to provide the same service to the same number of people , Atlanta would have to develop 3400 km of metro track!The

16、 lowest the density, the more expensive it is to provide public transport,Density Profile,Average density is a crude indicator of urban structureThe way densities are distributed within the built up area provide better information about urban structureIn the graphs of Figure 4 built up densities in

17、9 cities are measured within each successive rings from the center to the periphery,Figure 4: Comparative density profile between 9 cities in Asia, Europe and USA,Urban Density Profile,Urban density profiles of the cities shown on Figure 4 are unexpectedly similar for Asian and European cities Ameri

18、can cities profiles are following the same pattern of decreasing density from the center but are much flatter and much lower densities in the center than Asian and European citiesThis common density profile has not been created by design but by a self organizing mechanism resulting of the interactio

19、n of markets, infrastructure and regulations,Pattern of Daily Trips within the Built-up Area (1),Densities as shown by census data are densities during the nightDuring the day people are moving around the city and densities will keep changing with each hour of the dayThe pattern of daily trips withi

20、n the built up area can be monitored by traffic count and origin destination surveys,Figure 5: Schematic representation of trip patterns within a metropolitan areas,Pattern of Daily Trips within the Built-up Area (2),The pattern of daily trip follow one of the following pattern:Monocentric model Pol

21、ycentric model ,Urban village version (exist only in master plans not in reality)The polycentric model, random movements version (common in North American cities)The mixed mono-polycentric model (the more common),No city is purely monocentric, few cities are perfectly polycentric (with no dominant c

22、enter)A city can be considered dominantly monocentric if more than 35 % of all trips are to and from the central business districtTrips are only partially generated by jobs, many trips are for shopping, culture or entertainmentAsian and European cities tend to be more dominantly monocentric than Ame

23、rican cities,Pattern of Daily Trips within the Built-up Area (3),Section 3: Why Urban Spatial Structures Matter?,Why Urban Spatial Structure Matters?,Existing spatial structures limit the choice of urban strategies, for instance: low density dominantly polycentric cities cannot develop viable public

24、 transport systemHigh density dominantly monocentric cities cannot rely on private cars as the dominant mean of accessing the city centerHigh density monocentric cities tend to have higher housing costs and lower consumption of floor space than low density polycentric cities,Section 4: Is There a Gl

25、obal Trend in the Evolution of Urban Spatial Structures?,Global Trends (1),Cities tend to become more polycentric when they develop and when income are increasing, allowing individual transportDensities tend to decrease as income are increasing because people consume more floor space per capitaServi

26、ces tend to take more space in the city center decreasing population densities,Global Trends (2),Less trips are due to commuting to work and more trip to shopping, education and entertainment, as a consequence employment location become a poor predictor of trip patternsSuccessful cities tend to main

27、tain a prestigious city center containing high quality amenities while linking subcenters with rapid transit. Typically trip to and within the city center are made by public transport, while trips between suburbs use individual transport or small collective transports,Conclusions,Urban planners shou

28、ld analyze and monitor the evolution of the spatial structure of the city they are responsible to planPlanners should identify the type of urban structure which Is compatible with municipal strategy Could evolve from the existing urban structure Figure 6 shows the relationship between markets and government action in shaping cities,Figure 6: Schematic representation of the interaction of markets and government action in shaping cities,