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Why do inner cities tend to have a higher density than outer suburbs?I apologise in advance if this answer sounds too Western (if not European) centred. I realised that several weeks after having started it and did my best to correct that during the writing process. Yet, I must admit that my knowledge about Asian, African or south American cities, societies and history is too limited. Let’s go!Asking about the spatial organisation of a territory is to wonder about the individuals who dwell in it. Be it the economic production, social framework or political systems, a population's life is closely related to the place it belongs to (Indovina, 2009, p18).History, and people of the past left us a fantastic legacy: our land. Not only did they transmitted it in its original condition, but they worked on it, they cultivated it. They grew forests, crops, villages, towns and cities. They built roads, ports, waterways, bridges, between them and created homes, mills and thousands of other wonders.Generations of men and women, receiving this legacy kept working on improving their environment and adapted it to their way of life. As a result, we can consider contemporary cities as these successive societies’ product.This is why the Coliseum became a touristic hotspot after being a stadium and a stone reserve during generations. At the end of the day it ends up on tourists' keyring. Likewise, Toledo went from Visigoth kingdom's capital to a touristic town in central Spain.Many territories that reconverted or kept their functions over centuries to fit their contemporary needs can be mentioned as examples. Some ancient cities kept in their role of hubs for centuries and the regions where they were located remained local powerhouses. Memphis, Cairo have been in the Nile Delta for millenias/centuries. The cities in the Rhine valley made it a trade centre since middle ages. Londinium is still standing while Florence remains Tuscany’s capital. Colonial cities are still the main conurbations in Brazil, the US or Australia. Paris has been around and important since a while. In Middle East, Tyr and Jerusalem amongst others were located in an early densely populated area. Trier and Istanbul have still marks from their Roman inheritance. Kyoto or Kolkata are no longer capital cities but remained major cities. All these example show us that important city can go through ages and remain important. They just need to adapt to fit new organisations of their world.As cities are nowadays where the majority of mankind lives, focusing on them is looking at the environment of the majority. The people living in these cities are very diverse and as such, produce various environments. Cities have been major theatres of human's History and as such often have feats that enlightens us on both the world history and their century for the past centuries. Many big cities will have ancient remnants, a colonial town, industrial areas and a modern CBD coexisting as a footprint of their past. Very often these areas were reconverted and fully integrated in the new agglomeration.What is the question here?I would start with questioning the question and its terms. What is an inner city and what is an outer suburb?According to Collins dictionary, an inner city is “the parts of a city in or near its centre, esp when they are associated with poverty, unemployment, substandard housing, etc” . From the Cambridge dictionary “the central part of a city where people live and where there are often problems because people are poor and there are few jobs and bad houses” .The terms “especially” and “often” indicate a feature (here poor people, bad houses, few jobs) that is related to inner cities, but is not systematic. In other words, the latter elements of the definition are incidental to the first half which is the “central part of a city”. So the part of a city near or in its centre seems to be a good starting point of a reliable definition for inner city.On the other hand, a suburb is, according to the Collin's dictionary, “A suburb of a city or large town is a smaller area which is part of the city or large town but is outside its centre” . I imagine an outer suburb would be farther from the city centre than a simple suburb.I've looked on Wikipedia and could see that both terms inner city and outer suburbs are found in the inner city's page, so it’s probably where the question originates from.As we can see, both terms are closely related to the notion of city centre. So what is a city centre? It is defined as “ the busiest part of a city, where most of the shops and businesses are” from the aforementioned dictionary .The density is a measure of a quantity in regards to a surface. It can be expressed in “something”/ square meter. In this context it can be the population, shops, services, infrastructure... It’s quite vague but we can work this out together. You can refer to my blog’s post about density, meant to be used for my answers.Finally this question could be rephrased as “Why do some fractions of a city feature higher concentrations of people, jobs, services and infrastructures whereas others are sparsely inhabited/occupied?”As populations drag with them services and economic activities, I’ll focus on the population density. I could have used stats about workplaces, cultural equipment and such but they are harder to collect and less uniform according to the country.I think the question was more why some parts of a city are like Manhattan whereas others are “little boxes on the hill side”.What does density imply?The population density of an area is related to the number of homes and the number of persons in these homes, in other words the households size. A household size can be explained by social reasons (are the grandparents living with their grandchildren, prevalence of divorce, fertility rate...), or by the size of the homes (is a part of a city composed of small homes, large ones etc...). So behind the population density, there can be a whole range of situations all very diverse.Is it really so?Before going further, I would like to check the validity of the question's premise: Are the centres really denser than the suburbs?Cities can be seen as concentrations of population and activities, their centres and suburbs stand out on population density maps:Berlin and the surrounding Brandenburg - In purple: Very high density area - Pink: high density areas - Orange: Medium density area - White: Low density area - Source: ESPON population Grid 2011, manually simplified, no scaleIn this overview of Berlin, we can see the centre of the city and the close suburbs appear as very dense. The area with high or medium density decrease as we get farther from the centre, with the exception of small local hubs, Potsdam for example.Torino and the surrounding Piedmont region - In purple: Very high density area - Pink: high density areas - Orange: Medium density area - White: Low density area - Source: ESPON population Grid 2011, manually simplified, no scaleAt a regional scale, Torino appears to be very dense, and just like Berlin the density decreases as we get deeper in the countryside. Between these two extremes, the suburbs have a medium population density.For anyone here, it’s easy to see the urban core in its regional environment as well as the suburbs, as they are totally different in terms of population density. We can zoom in and check the actual numbers for the population density in a few cities in Europe, America and Asia.The institutional view of cities seen through greater cities presents some bias, an inner city can for example be at the edge of the greater city and have other suburbs outside its limits. Some would argue that the cities, as a physical object (urban areas) or a social one (metropolitan areas) often go far beyond. But for data availability I'll chose them as the territory of comparison when possible.The following figures show the average density of territories from the city centre, starting at 0-5km to 45-50km away. The chart’s design is simple: the centre of a city is marked. It can be a landmark, place, train station.. from this point, we draw a 10km circle from it. Then “doughnuts” shaped areas from 5km to 10km etc ... Until we reach 50km.Each city’s part within the different doughnuts is calculated and an equivalent part of the population is calculated and attributed to the doughnut. For instance a city has 2,000 inhabitants on 10km2, with 50% of its surface included in a 10-15km from the centre doughnut and 50% in the 15-20km from the centre doughnut. So two zones of 5km2 will be added to the doughnuts with 1,000 inhabitants each. The total area and population within each doughnuts are then calculated and give an average density for the entire doughnut.This underlies each territorial unit is uniformly populated which is obviously wrong, but simplification is the price to pay in order to use statistics. I could have used a finest technique, which would be using the built up area of each city instead of their total land (+ water) area. This is however harder to use, and the data is quite difficult to find and requires a ridiculous amount of work. This is why I chose not to use this method.Population density from city centre for a selection of Italian, English, and American cities. Source: Istat (2016), UK (2016) office of statistics and the US census (2012- Tiger products), GIS processing and drawing: JMThis chart will be broken down per countries below. All this aggregated data show a trend, city centres are actually more densely populated in most of the cases than their suburbs. Of course the selection of cities and countries is limited. I tried to take various examples with first the “country of suburbia”, the US. I also took an early industrialized country that experienced a abrupt urban growth more than 100 years ago and remains characterised by several cities close to each others, the UK. Finally I chose a Mediterranean country with a quite old urban culture, Italy.Population density from city centre for a selection of Italian cities. Source: Istat (2016), GIS processing and drawing: JMItaly definitely is a country where cities matter. It is made of several medium size cities that can be very old. Few large cities can be found there as well. I took a few example of several sizes to show that every greater city follows the same trend no matter its size. The boundaries are the Citta metropolitana, former provinces reorganised as such for a handful of cities across the country. They are not perfect and are simply a jurisdictional boundary. But for convenience I chose this Italian unit. Sometimes these administrative units are very small hence Naples’ figure stopping before the other ones.Municipalities population has been divided by their graphic land area. This explains that Venice and Reggio Calabria, both encompassing water or natural areas have a low density in their centres. If I isolated their actually urbanised area and excluded these not built lands I would probably have got very high densities as well.With a high density within its centre and one of the lowest in its suburbs, Bologna show the starkest difference between centre and suburb. On the other hand Venice shows a line way flatter than its counterparts due to both a low density inner city and relatively high density suburbs.Milan and Naples have a functional area spreading well beyond their administrative boundaries, with their outer suburbs being indeed outside of it. This means that even from a certain distance from the centre, the density remains quite high.Population density from city centre for a selection of English cities. Source: UK (2016) office of statistics, GIS processing and drawing: JMIn the UK, I took the ceremonial counties as a boundary and the wards’s population and graphic land area. The selected cities list does not include London due to its specificity. Other cities are from large to mid-range size in population.There is globally the same trend, however you can find that density rises again further away from the city centre (after about 40km) in some cases. This may be because of another metropolitan area that is intertwined with the analysed one or large cities have secondary centres that are denser than a classic suburb and can develop beyond a certain radius from the inner city. They can encompass several services necessary for suburbs and rural areas too far to benefit from daily services available in a urban centre. But overall the density far igher within the 0-5km radius.Population density from city centre for a selection of American cities. Source: US census (2012- Tiger products), GIS processing and drawing: JMIn the US I choose the Miami Dade county for Miami, and the Maricopa county for Phoenix. I took the Metropolitan area for the three other cities. Overall I tried to take cities of various sizes and locations. The data is calculated from the census tracts population and their graphic land area.Like for Italian and English cities the American city centre is densely populated with the suburbs having a very low population density. Yet, the population density in the centre is below their European counterparts’ figures. Atlanta’s population density at a 5km radius from city centre is comparable to Leeds-Newcastle at 5-10 km from their centres, or Milan and Naples at 10-15 km from their centres.Let’s now see how the inner cities fit so many people within their boundaries, and how outer suburbs allocate so much space to their occupants. We’ll see three means for that : The architecture, the layout and the public space.Architecture:The architecture is very often the first thing that strikes a person when walking through a city. There are several ways to increase density with architecture, the most obvious being the height. High rises can generally fit more people or activities than a low rise building, because for the same footprint there are more storeys therefore more available area.It’s also a matter of type of building. Single detached house will take more land than row houses for the same number of inhabitants. In the same way for the same land use, an apartment building would use even less land area.City centres often have high rises in their CBD, mostly for offices but also for homes. High rise buildings have a symbolic value and also allow their owner to use the land most efficiently, improving the floor area ratio. Medium rises are also very common in city centres and specially in Europe where old city centres were preserved. The mix of new and old buildings depends on the local policies and some old centres can be exclusively occupied by old buildings, filled of modern architecture (Stuttgart, many Asian cities). Brussel is infamous for the cohabitation between historical and modern cities (On a personal note: I briefly visited Brussel and think it is an amazing city in any regards you can take, especially architecture!).In some areas, high rises can’t be built in some parts because of the soil (New-York for instance) or because the land value that is not high enough to erect buildings higher.Jakarta's city centre, Source: Google image, Realization: JM - Population density (overall municipality): 14,464 inhabitants/km2, Source: http://Worldpopulationreview.com, 2018Yaletown neighbourhood, in Downtown Vancouver, Source: Google image, Realization: JM - Population density (Downtown density): 16,764 inhabitants/km2, Source: City of Vancouver, 2016The suburbs are generally covered with single detached houses. From time to time some collective buildings can be there, or row houses but it remains rare. These houses are often standard model replicated through the development.America is characterised by single level houses, as those of the Broadacre city from Frank L Wright , whereas in Europe a first floor is common, even more sometimes. Those houses can be pastiches of several styles than can be amalgamated together. Timberhouses, tiled roofs, columns, pediments... You could find regional styles far away from their cradle. I found good article that you can read on this matter even if it represents a professional standpoint, I think it is quite relevant. It will be added to the bibliography.Aerial view of Surrey, near Vancouver, Source: Google Earth, Realisation: JM - Population density (For the overall municipality): 1,637 inhabitants/km2, Source: City of Surrey, 2016Public spaceInner cities tend to have well designed public space. Bins, benches, parks, flowers, sidewalks, bike lanes, places and public squares. This public space is collective infrastructure that is used everyday by people living there or passing by. Being collective, they are a perfect theatre for social interaction and events of all sort. You’ll also see that many old city centre have been designed before the invention of the car, and have narrower streets and are generally designed at a pedestrians scale.These pedestrian roads allow someone walking through them to see on both sides of the street, or at least more easily than through a many lanes large modernist avenue. The result is that it is easier for shops to thrive in such environment independently from the motorised traffic.A pedestrian square in Belfast, Source: Google Streetview, Realization: JMIn suburbs the public space is first of all meant for cars. This is why streets are wide, and do not always have sidewalks. Those development being first of all commercial investment everything is designed to minimize the costs and maximize profits. On one hand the public space is as small as possible (being reduced to streets), with little equipment (no sidewalks, no bike lanes). On the other hand what can be sold, the plots are as large as possible. Thus, they include a garden that will encompass a parking lot, benches and other equipment that is generally collective in a city centre’s public space.It is also a bargain for city councils that don't have loads of public space to upkeep. This is the owner of the plot who mows the lawn and invest in his own equipment, not the local authority.Some place in Belfast’s Suburbia, Source: Google Streetview, Realization: JMThe land use is optimized in case of mutual public space compared to the individual garden on each plot. This is one of the reason why developments with collective public space are on average denser than those with private gardens as you can see on the example below.Sants neighborhood in Barcelona. Realisation: JM - In Black: Buildings - In Green: Private land - In White: Public spaceThe plan above represents Sants Neighbourhood in Barcelona, Spain’s second city. As you can see on this cadastre most of the private land is built. The rest of these plots consists in the blocks’ cores that consist of some collective space, it can be for some facilities like rubbish storage but it can also be a small collective space or even a private square.The buildings end at the limit with the public space, thus they delimitate it. Between the buildings, squares and places are collective and free access amenities. This neighbourhood has a (very) high density of 37,150 inhabitants/km2 in 2016 (source: Municipality of Barcelona).Alella near Barcelona. Realisation: JM - In Black: Buildings - In Green: Private land - In White: Public spaceAlella is a town nearby Barcelona. The neighbourhood in the cadastral map above is an typical of the residential low density suburb as it can be found across the globe. The municipality’s overall density is 1,054.7 inhabitants/km2 in 2016 (source: IDESCAT).You can see that there is private land between the homes (mainly gardens) and roads are the only type of public space in the area (no parks, square, etc...). Private land is delimited with fences, bushes or not delimited.Central Cádiz, in southern Spain. Realisation: JM - In Black: Buildings - In Green: Private land - In White: Public spaceWith 9,593 inhabitants/km2 on the overall municipality in 2017 (Source: http://juntadeanalucia.es), Cadiz is one of the most densely populated municipalities across Spain. The city has lost many inhabitants over the past decades, which mean the density may have been much higher in the past.The shape of this city is typical of organic urban growth, with places of irregular sizes delimited by buildings, enlargement of roads creating new places. It is a modified grid layout that we’ll see later in this text.Vistahermosa neighbourhood in El Puerto De Santa Maria, near Cadiz in southern Spain. Realisation: JM - In Black: Buildings - In Green: Private land - In White: Public spaceThis cadastre shows a residential development in Puerto de Santa Maria, near Cadiz in southern Spain. Once again we can see the same pattern of building at the centre of a plot surrounded by private land (mostly gardens). The public space consist primarily in roads/streets.The overall municipality population density is 554 inhabitants/km2 in 2017 (Source: http://juntadeanalucia.es). The central town of Cadiz is seventeenfold more densily populated than this satellite town.The layout:Most city centres are based on a Grid layout, or a modified Grid layout. The first one is very often resulting of a global coherent plan, whereas the second is more due to an organic growth that occured step by step.This allows density as the massive flows of traffic (motorised or not) are spread over various itinerary combinations possible to get from a point A to a point B. This also facilitate the pedestrian moves as the many road options allow them to efficiently travel to their destination. Easing the pedestrian traffic fosters density in the sense that for the same number of travellers much less road area is needed than for cars. This leaves more space for other necessities. Grid layouts is a better connected environment thanks to their limited number of dead-ends and their many crossroads.This type of layouts can often be found in the denser city centres. Old cities are most of the time denser as a way to compensate slow transportation at the time they were built. And very often old towns are city centre’s cores. We’ll see that later.The grid layout is seldom “pure”. In some cases it is associated with large diagonal avenues (Barcelona, or Washington). In other cases, either older or newest developments are erected according to other types of layout (Valencia, Vienna, Torino). Moreover, some modern infrastructures or natural elements can disrupt the grid, following other logics. Such elements can be rivers, railways or motorways.Bruges’ City center, Belgium - Source: Openstreetmap - Black: roads - Red: motorways - Brown: RailwaysAbove the city centre of Bruges in Belgium, an old city that has been a commercial hub and a major port in Europe since Middle Ages and may have reached 50,000 inhabitants for the first time in the late XIIIth century (Chandler).This plan shows a good example of modified Grid layout. Their are very few dead ends and every block if very well connected. This is a city that has grown organically since medieval times when it was a cultural hub. This means it has been built and planned piece after piece, according to several urban doctrines and needs over the time.The old city centres as such aren’t the easiest to drive through. Actually even officials do not recommend tourists to drive in it . The many curves, intersections, one way streets (often because the streets are too narrow) are as many difficulties driver may experience there. It may be hard or one who’s used to a grid pattern but after getting used, any pedestrian can enjoy the efficiency of this type of layout.The municipality of Bruges has a population density of 850 inhabitants/km2 in 2016 (source: StatBel), which is pretty low for an inner city. This can be explained by the municipal boundaries going to the sea and the Zeebrugges port, encompassing several farmlands. The city centre has a density of 4,841 inhabitant/km2 in 2016 (source: Statbel).Part of Toronto's city centre – Source: Openstreetmap - Black: roads - Red: motorways - Brown: RailwaysToronto, Canada has been settled first in the middle of the XVIIIth century. It grew a lot during the XIXth century and the XXth. Today it is the centre of the largest metro in Canada, which is also one of the largest in Northern America.The city centre runs along a classic grid layout. You can try to zoom in the picture, choose a point A and a point B. Then see how many possibilities you have to go from one to another without taking an unreasonably long way. You’ll see many possibilities.The grid is aligned to the lake’s shore and the motorway/railway are at the edge of the city centre. As I said, the layout is not pure and you can see few dead ends in it.The density throughout Toronto’s Municipality is of 4,334 inhabitants/km2. You can see here a fraction of the current Ward 28 of Toronto. The overall ward has a population density of 8,300 inhabitants/km2 in 2016 (Source City of Toronto), and it includes some less urbanised land so it may be even higher for the grid only part of the ward.Part of Johannesburg’s city centre – Source: Openstreetmap - Black: roads - Red: motorways - Brown: RailwaysThis is Johannesburg’s city centre in South Africa. It is the most populous municipality in South Africa and is nearby the capital, Pretoria. The city has been created during the XIXth century with gold mining activities. It s a quite recent city.As you can see the city centre is a grid layout with the adjunction of more recent infrastructure such as the railway or the motorways that break the regularity of the original grid. Overall the city grid has been well preserved event though many landmarks have been replaced over decades.Johannesburg itself has a population density of 2,860 inhabitants/km2 (source: Stats SA). The municipality is divided into electoral wards. In the city centre there are a few ones. The ward 60 a population density of 6,835 inhabitants/km2 (Source: http://Wazimaps.co.za), the ward 123 is denser with 19,494 inhabitants/km2 (Source: http://Wazimaps.co.za). The ward 63 has a small size but a quite high population. The result is a population density of 83,844 inhabitants/km2 (Source: http://Wazimaps.co.za)!The suburbs can also be organised with a grid pattern, but more often they will have some dead end and feature completely different layouts. This reduces the different itineraries and to get from a point A to a point B there is little choice. The pedestrian can have to walk around a lot for itineraries that could be more direct. These parts of the city are often designed for cars and take little care of the pedestrian were not the priority during their planning. They can be built by developers who designed them as an economic good to be traded rather than a urban artefact that will go through ages, as I already explained before. Therefore they have reduced the cost at their maximum by building as little road as possible and commercialising as much land as possible to keep the not profitable public land as small as possible. The general layout is organised in “Pods” that are wide groups of streets having very few accesses.They are oriented towards themselves as the access to the different homes can be done only after getting inside the pod and not from the outside. Very often these pods are mono-functional. By monofunctionnal we mean featuring a limited range of urban functionalities such as public services, economic production, trade, housing... this brings us to the next point, a city’s functions and how it is zoned.Midrand, a Johannesburg’s Suburb – source: Openstreetmap - Black: roads - Red: motorways - Brown: RailwaysMidrand is a suburb north of Johannesburg. It features typical pods oriented towards themselves with few accesses to bigger roads. It has also several dead ends which complicates transit. Their is not a coherent thinking over this area. It’s more a succession of developments according to the opportunities. Nothing to compare with a grid that has been thought and planned before building the city. The population density on the whole Midrand area is 570 inhabitants/km2 in 2016 (source: Stats SA), on the ward 96 that covers a part of the map below the population density is 454.4 inhabitants/km2 (Source: http://Wazimaps.co.za).Part of Brampton, a Toronto’s suburb – Source: Openstreetmap - Black: roads - Red: motorways - Brown: RailwaysBrampton is a suburb of Toronto. It is made of pods that are part of a wide grid system. Those pods, as you can see, consist of many dead ends. This makes the car a necessity and makes it hard for pedestrians to move from one point to another. Those pods are integrated in a grid layout, with wide perpendicular lanes. The population density is of 2,229 inhabitants/km2 in 2016 ( Source: Statistics Canada) I couldn’t find the data for the ward 10, appearing on the previous map.Sint Andries, a Brugges’ suburb – source: Openstreetmap - Black: roads - Red: motorways - Brown: RailwaysSint Andries is made up of several neighbourhoods turned towards themselves. Something interesting here is the discontinuity between them. Each area is built according to commercial opportunity and not according to a global master plan. Discontinuity is also something very common in suburbs compared against the city centres that could fill most of their empty spaces over the time, reconverting their brown fields. The population density in Sint-Andreis is 814 inhabitants/km2 in 2016 (source: StatBel).The city’s functions through zoning policiesModern cities have been developed according to (or under the influence of) the zoning method. This planning technique is based on dividing the city in different zones, each having a specific function. There are various types of zoning, some regulating the land uses themselves, others on the urban form or on both.“Intrusions [...] and the impacts of rapid growth added urgency to the calls of reformers for zoning restrictions separating residential, commercial and manufacturing uses”From NYC city planning department’s website, under “City planning history”“Sono considerate zone territoriali omogenee, ai sensi e per gli effetti dell'art. 17 della legge 6 agosto 1967, n. 765:A) le parti del territorio interessate da agglomerati urbani che rivestano carattere storico, artistico e di particolare pregio ambientale [...]B) le parti del territorio totalmente o parzialmente edificate, diverse dalle zone A)[...]C) le parti del territorio destinate a nuovi complessi insediativi, [...]D) le parti del territorio destinate a nuovi insediamenti per impianti industriali [...]E) le parti del territorio destinate ad usi agricoli [...]F) le parti del territorio destinate ad attrezzature ed impianti di interesse generale”From the Decreto interministeriale 2 aprile 1968, n. 1444 , which describes what types of zones the local planning documents in Italia can plan.“Les destinations de constructions sont :1° Exploitation agricole et forestière ;2° Habitation ;3° Commerce et activités de service ;4° Equipements d'intérêt collectif et services publics ;5° Autres activités des secteurs secondaire ou tertiaire.”From the article R152-27 from the Code de l’Urbanisme, in France. This also states the various types of zones that can be planned within the local planning documents.Le Corbusier was for instance designing cities with superblocks, each having a specific function. There were the homes, including parks, local daily shops, sports fields, and the industrial areas having their own design, separated from the homes by a large road . Earlier Unwin and Howard were calling for working in the smoke and living the countryside . Each garden city diagram shows well that the industries are strategically gathered in such way it would reduce the disturbance for the homes . Tony Garnier was also dividing the city in zones for each main function the city needed .This doctrine resulted in factories located on one side of a city, then homes and public services at a totally different one. Brasilia that has a few main roads dedicated to each urban function if a perfect example for this, but I could take also examples of commercial zones located close to the city motorways and with few inhabitants in spite of thousands or hundreds of jobs (Wrexham industrial estate in Wales is a good example, Avignon Nord in France is another one).Planners, architects and engineers imagined this type of city during the beginning of the 20th century at a time where industries has a nasty impact on their inhabited surroundings, with smokes fostering deceases growing amongst the working class communities. This kind of `business’ disamenities were such, that they could have impact on the moths’ colours . They are, in some ways relevant for the problems planners of that time were tasked to solve.Zoning relies on a city's inhabitants high mobility. They will have to commute, whether for a few kilometres or dozens, to work, leisure or shopping. The space one may go through during a day totally changes going from narrow and continuous (a daily life taking place in one neighbourhood) to wide and fragmented (working, living and socializing is several areas all separated by kilometres). This can disrupt the classic local community as a backbone of the social life in a city, partially replaced by other forms of sociability (F. Ascher, 1999).This new pattern of spatial behaviour lots of infrastructure, and especially road and parking lots, sized according to the rush hours. Parking lots of each zones are sized according to the maximum affluence’s need. Therefore each zone cannot take benefits of an other zone’s infrastructure because they’re too distant to be efficient to other than their own users. You can visit a mall on an evening and will see the parking lots are quite empty because cars are parked in front of their owner’s homes. This is why mixed use in a city centre way is more efficient, houses and business share the same infrastructure.This very extensive land use contributes to the low density in the suburbs since more space is needed to accommodate the same amount of people and jobs. We just explained why zoning policies have an impact on land use and density.During the second half (perhaps even before for the US) of the XXth century, many cities’ outskirts were urbanized according to these policies and mixed use areas became rarer in these outer parts of cities. This means that instead of a building an apartment building with shops at the ground level and homes on the other floors, one Mall with few levels of shopping area with 0 inhabitants was built, and a classic residential neighbourhood was built elsewhere. Suburbia can be described as such in many countries.Of course, the current challenges cities face across the world are changed since modernist ideas were elaborated and took the lead of planning policies. Traffic congestion, social exclusion led to public powers more prone to promote mixed use zones in their zoning policies. Still, an important share of our territories have been developed according to these ideas in the past, and in some cases, zoning policies still prevail.Private agents and market’s dynamicsMany space consuming activities have been relocated outside of the inner cities over the time, for two reasons: They were pulled in by suburbs and pushed out by inner cities. They were pulled in because transport infrastructure (especially but not exclusively motorways) made these activity viable in the suburbs for lower prices. Pushed out because the land value in city centres increased, and congestion made logistics complicated in these actual bottlenecks city centres are.This market dynamics is the other way around for other types of economic activities. Actually city centres are arguably stronger than ever. They concentrate many high added value services. Offices can dwell within high rises a higher density can be reached. This makes them less sensitive to land value. Therefore these business can look for more synergies with other neighbouring activities, high mass transit systems accessibility and symbolic status by living in the urban core.For instance, the Car factory in Brampton will definitely take more land than an office building in Toronto’s Downtown, although they can have the same number of employees. The car factory was implemented in a suburbs probably for the accessibility to some type of infrastructure (fret train stations, motorways) that may be better out of the centre. As they would need a massive plot, the land price may be a hindrance in city centre whereas it is way cheaper in a suburb. The office will focus more on optic fibre access, good transportation for their workforce and proximity of other services like restaurants for their employees, law firms... City centre would seem more appropriate for them. To sum this up, the former needs to have an efficient access to a gateway from the city, when the latter needed to be within a central area.Public city managementLet’s now imagine you are a mayor in a large city. You need to implement a building to host some essential service that would be useful to the major part of the population. You have two possibilities: The first location is in the outskirts of your city, where people have generally low access to services because the area has a few public transportation and few infrastructure in general. The second is the centre where there are many inhabitants have already the highest access to services and an extensive transportation system. As a mayor choosing the second possibility may be the most rational. The new service implemented would be more accessible to most of the city’s inhabitants (and voters). This is a decision many city councils do and this also explains why public office buildings can be within city centres.This primacy of city centres retroactively draws public councils to improve transportation to centre. This gives better access to all these amenities and reinforces their attractiveness.Private and public initiatives lead to dynamics that agglomerate high density activities in city centres (so the inner cities) and spread activities on a city outskirts (so the outer suburbs).But this is intertwined with WHY city centres and suburbs developed in such way and this is the topic of our second chapter.We can now see why cities developed this way.Now it is time to see why inner cities became denser than suburbs. I suggest to start looking at a timeline various cities across the globe. Inner cities often match with original urban cores while suburbs really developed quite recently. This is why the suburbs will appear in this part quite late.The limit to this method is that cities all over the world all have different stories, and followed different paths. Usually urban studies focus on a particular city through time or a particular period across the globe. In this case, I would suggest to take this timeline as a suggestion of elements that successively played a role in the cities’ formation and as examples. Some of them may apply to many other cities, other to only a few ones.I prepared this reading most of all about Norther American, European and Chinese cities. I tried to add some from the Indian subcontinent, Africa and South America.The birth of cities or how density and great scale can coexistIt is quite uncertain why men started dwelling in large, high density settlements. But there is a consensus (you can even find it here on Quora) to say that it has been allowed by agricultural surplus and high crops productivity. This is also suggested by the location of first cities, Mesopotamia (Tigre and Euphrate’s valleys), Yellow River, Yangtse river, Nile Valley, Mesoamerica... Eridu is believed to be the first one with 6,000 to 10,000 inhabitants in 3700BC. Uruk became the biggest city in the world by the beginning of the thirtieth century BC and other cities ranked amongst the largest in the world appeared from eastern Asia to Mesoamerica by the fifth century BC (George Modelski, 1997 ).From: Meredith Reba, Femme Reitsma and Karen C. Zero, 2016 .Six most populous cities' location in 3000BC, source: Google Maps, Realisation: JMFrom: Meredith Reba, Femme Reitsma and Karen C. Zero, 2016.Six most populous cities' location in 2500BC, source: Google Maps, Realisation: JMFrom: Meredith Reba, Femme Reitsma and Karen C. Zero, 2016.Six most populous cities' location in 500BC, source: Google Maps, Realisation: JMMany of these areas encompass very fertile lands. With sufficient agricultural surplus, a part of the population was able to stop farming and start other activities, at various levels of specialisation. Economic specialisation means dramatic increase of exchanges. Instead of having a farmer community with each household almost auto sufficient, there was people crafting tools, clothes, plates... Resident needed to sell their production to their fellow citizens and to buy goods, tools, food or services to them to survive. Durkheim called this a shift from a mechanical to an organic solidarity .Ancient cities were built in a way to ease those essential exchanges as much as possible. There were squares where people could meet, streets to move from an area to another. To minimize the costs of exchanges and maximize their number, a good way is to get everybody living close to each other. A potter needed to walk a few minutes to go to the forge to build new tools, and could sell his pots to the large marketplace the city is. Farmers from outside brought their products to the market where people from the town were able to buy them...With the development of trade, administration appeared as well. The appearance of cities and their development since ancient time correlates with their inhabitants’ specialization and the development of new activities (Hanson JW, Ortman SG, Lobo J., 2017 ).With the productivity improvements, workforce can be used to build infrastructure, that helps grow more crops (through irrigation for instance) and also increases other activities’ productivity (roads or ports can improve trade). In any case, the successful city will grow and become a trading centre for farther and farther rural communities.The point of this part is to show that density is, since the urban phenomena origins, the cities’ very essence. They are a way to concentrate population in a small space and maximize their interactions whether it is between its own inhabitants, or with the people from outside it and who will gather in the city in order to provide and obtain goods, services and amenities to/from the city. Older parts of cities being oftentimes at their centres they remained the denser areas.The city Walls, one more constraint to urban growthThese first cities very quickly (or simultaneously?) built walls in order to protect themselves from outer foes, to whom such wealth concentration was a bait. Walls are a very common feature in ancient cities.A defensive wall is a high cost investment and it can’t be repeated many times. Thus, before a city could extend and enlarge its fortifications, it needed to be filled as much as possible. In other terms the wall extension was started when the city had no other option for further development. A good example of this is the history of Florence’s city walls . This ensured that most of the urban growth was contained within the city walls.During these times, there could be some faubourgs that emerged from city gates along the main roads. Some of them would be incorporated to the walled city as the city walls were enlarged. These new neighbourhoods would become denser and denser with the development of the city until the walls need an extension. These faubourgs were some kind of early suburb.Change in nature of conflicts, military technology made walls less relevant. Depending on the country these walls have been removed and the city started to spread at a much faster pace. London is famous for having removed the walls quite early, being on an island probably helped being that early. Paris removed its walls quite late, and still to this day Paris within its old protective walls’ perimeter remains much denser (21,067 inhabitants/km2 in 2014, source: INSEE) than London (11,839 inhabitants/km2, source: London Datastore - Greater London Authority).Freed from their walls, cities could expand more freely with much more available land.What is meant here, is to show that city walls limited the urban sprawl at that time. The urban population increased through infill so the population density grew in these strictly closed spaces until an extension was needed. When a city was starting a new flourishing period, a wall extension was undertaken. Sometimes, a smaller walls could be built inside the previous ones like in the case of Florence.For centuries, ruling classes and wealthy families owned rural residences near urban cores. You can see that from the roman villae, to the hunting residence temporarily inhabited by medieval lords, to the industrial captain’s country mansions. These could be, in some way the earliest form of suburb with people working in town and living (as much as it was possible considering slow transportation) outside the city.I am currently reading about Chinese cities story. It seems they followed a much different storyline and even though they had walls, the lack of a specific administrative status for cities and towns led to a situation where city walls were less determinant on city development (Yinong Xu , 2000, p54).Alongside the protective walls, other natural feature could also limit urban growth. Many cities were located near rivers, like Paris (a Seine’s island easy to defend), London (on the Thames), Lyon (Confluence of the Saone iver and the Rhone river), Glasgow (located on the Clyde river banks).There is a is a double edged relationship between human settlements and the site where they are built on. The environment allows the settlement to grow, with access to water, high crops productivity, access to raw material, and is a condition for local development. On the other hand natural disaster are also caused by this natural environment with areas regularly flooded, or parts of the territory that aren’t constructible. Dikes, swamp drying, building techniques, such evolutions get more space available for new development as they come up. This results in cities always less restrained so they have more available lands where to develop.The industrial times: a urban revolution?In a significant part of the western hemisphere, the industrial revolution brought many changes during the XIXth in the entire society, changes that also impacted the spatial framework of cities.Division of work, steam engines and other disruptive innovation made industry the main source of wealth for newly developed nations. Using local resources such as coal or iron, many towns turned into bustling cities. With these new resources, manufacturing developed to an unprecedented extent and technology improved at a faster pace.Many agricultural workers were dragged to the new industrial centres where they became the new industrial workforce. They would cluster in working class neighbourhoods that grew alongside their number. In the UK and Benelux, raw houses were the classic type of accommodation for these populations in other countries it can be more collective buildings, like in Paris or Madrid. New systems will be needed to support the new activities as well as the new habits of the population.Coron du Grand Condé in Lens, Northern France, source: Google Image, Realisation: JMMassive infrastructure were built at cities peripheries, such as railways, waterways and ports, gigantic factories... These new facilities weren’t fitting within the traditional packed cities therefore they were placed on the outskirts of the cities.Allowing smooth transportation within these new populous cities became crucial and mass transit developed besides of fret infrastructures to allow workforce to commute and to access facilities in both the centre and the periphery. On the other hand, new developments appeared nearby new train stations. Population, urbanism and transportation were more bonded than ever.In reaction to these social, technological and economic changes, new urban theories emerged. New developments were erected according to them, such as the garden cities in UK or the ciudad lineal in Madrid.Some theorist, witnessing the drastic growth of cities, expressed the will to limit their extension. Rather than walls, space around cities was allocated to Green Belts. Those sparkled all over the UK for instance.This was also the hygienist period. Architects and planners have developed the idea that free space and sunlight would prevent miasmas from infecting urban crowds. Large roads, blocks requalification, parks, public sport fields... These ideas would be a first step before the modernist architecture that would become prevalent later on. Good realizations of this thought in France are open blocks from some HBM housing, Haussmann’s large roads of early XXth city extensions.In the US, the first skyscrapers appeared, Chicagoan and New Yorkers are still arguing about where exactly. Lifts and elevators lead to an increase of storeys where it was economically achievable, generally in the city centres.The tallest building throughout the years, source: Wikipedia - Realisation: JMIn short, the city centres grew higher, the cities exploded beyond their traditional boundaries and were enlarged with industrial areas, and developments were erected remotely from the cities to meet the new needs caused by infrastructures, the population growth, the recent industrialization of the economy and the new social structure. These events have also sparkled a major feature of present urban landscapes: The suburbanization, with garden cities and other urban utopias, new towns, more or less remote industrial neighbourhoods...The XXth century – The advent of suburbsThe XXth century saw a new wave of social, economic and urban changes. The population in cities kept growing. Inhabitants became able to move faster and further from a part of the city to another thanks to many technological improvements.Cars allowed individual commuting hence fostered urban sprawl and the individual houses type of dwelling. This also allowed to live farther from the workplace. This has been possible with the rise of the middle class in the western countries with a high purchase power.Number of cars in France since 1956 - Data From Michel Freyssenet , 2013 - Formatting: JMI can’t resist stressing the importance of the fridge as a condition to this evolution. The fridge allowed households to keep food for longer therefore daily groceries became avoidable. This allowed the separation between dwellings and shops by reducing the number of shopping sessions necessary.Thus, cities saw massive shifts of population from centres to suburbs as the middle class and technology grew. This middle class was attracted with home ownership, more space, living closer to nature (Jean-Michel ROUX, 1976). Local authorities could see this as an opportunity, since rural communities were drastically deserted during industrial times, they got a rejuvenated with new families from medium to high income rushing to these new standardized single houses or high end estate developments.French population within large agglomeration according to the type or urbanisation - Source: INSEE, pôle Analyse territoriale - Formatting: JMFrench population’s growth within large agglomeration according to the type or urbanisation - Source: INSEE, pôle Analyse territoriale - Formatting: JMI can’t stress enough that mobility is a crucial contributor in shaping urban landscapes. As we said, city centres remain the best connected pieces of land thanks to their historic mass transit systems.To support the growth of car ownership, new motorway rings around city centres or giant motorways going through city centres themselves were erected . This is why they remained relevant as entire urban regions cores. Besides, the suburbs were provided with motorways to allow faster and greater commuting fluxes. The development of suburbs have relied with this of the car (but not exclusively). Moreover, city centres became more powerful than ever, becoming cores of territories broader than ever.Road and railway networks in Leeds, United Kingdom - From dark red to yellow : Main road to secondary roads - In Black : Railway - Blue : Water area - Source: Openstreetmap, design: JMRoad and railway networks in Jakarta, Indonesia - From dark red to yellow : Main road to secondary roads - In Black : Railway - The dark line on the bottom left hand corner represents 5km - Source: Openstreetmap, design: JMRoad and railway networks in Ahmedabad, India - From dark red to yellow : Main road to secondary roads - In Black : Railway - Blue : Water area - The dark line on the bottom left hand corner represents 3km - Source: Openstreetmap, design: JMBesides the trends in housing distribution, the suburban spaces saw also commercial malls, megastructures (such as airports or hospitals), logistic facilities emerging. If suburban areas are nowadays multifunctional as a whole, these functions’ repartition remain very distinct. As we said earlier, shopping malls aren’t encompassing many homes.City centres lost a part of their local daily grocery stores and saw other types of shops emerging (clothing, electronics...). Many of those shops are franchises in Europe. You’ll find in France a Zara shop in most city centres, with a Macdonald’s, a Subway... Some people argue that city centres have lost their identity. Mixed land use is way more common in city centres than suburbs.Based on the shops’ names, which cities are these? (hint: it’s in the UK)It’s Manchester and Birmingham (Source: OpenstreetMap)We just explained why suburb could develop and why city centres didn’t collapse. But why these new outer developments around urban centres did not develop in a such dense way?One may say this is because they are still young pieces of cities. It could also be said that their relatively poor dotation in services does not create a land value high enough to foster profitability for denser housing and that it’s rather more cost efficient to spread further for developers.It could also be agued that what makes city centres’ attractive is their high density and what’s attractive in suburbs is the low one. People moving there are looking for low density estate and want to have this “closer to nature” landscape, hardly compatible with high rises and high commercial density in general. This also explains the strong NIMBY in these areas, where home owners don’t want to see their asset losing value nor their quiet neighbourhood turned into a messy and hectic piece of town.Old social/urban structures weren’t destroyed. If they have been sometimes wrecked by the moves of history, they remained well equipped and attractive to certain populations. Rather than a new form of cities, new framework emerged integrating these new villages in a closer relationship to the mutated urban cores. This reflects what we said in our introduction about new social structures coping with their old assets and creating new ones.ConclusionI hope this modest contribution will trigger your curiosity on this matter and push you to look for more information (the references used could be a good start) or that it would give you a new prospective on the topic.As another invitation to dig further this topic, I would like to introduce you a couple of concepts and dynamics that invites us to question the premise of this initial question. Inner cities may not always be denser than their suburbs. I’ll take here two examples, but I am sure there are many others:Declining city centres and suburban densificationAlthough they have been once the thriving core of entire regions, city centres can sometimes shrink, and lose population, economic activities and shops. In the case a city centre loses its population but the urban area grows (not through out growth though) there might be some point where this contrast inner city/suburbs is not so stark.I invite you to check the evolution of Detroit’s population density per census tract as shown on Datadrivendetroit’s video you’ll find within the references . It shows a good example of suburbanisation and centre shrinkage.Moreover politics, urban theories, ecologists activists, architects and planners have shown a certain aversion towards suburban environment. The reasons are many for this, and more often I consider them as legitimate. This lead to many laws/bills/initiatives in favour of dense suburban development as the green belt and new towns we have mentioned previously.PolycentrismThe European union seems to consider the polycentrism is the best territorial organisation, whether at a continental scale or at the city scale. More than really reducing a centre’s importance, the idea is to see other places in the city with centre like features (high concentration of population, services...).Moreover, in large metros the last decades saw the blooming of edge cities. Edge cities are new centres that grow at strategic locations such as interstates and motorways intersection, train stations and such... outside city centres. They have been coined as cities featuring a high concentration of jobs . 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