Urban areas and management

Urban growth: population, area and the seven processes

Urban growth has two halves, and examiners look for both. One is a rise in the number of people living in urban areas, which is measured globally as the share of the world's population that is urban and locally as the population of a city. The other is a rise in the area of built-up land, which is measured by mapping the edge of the built-up zone, by the density of development and by the loss of farmland and woodland at the city's edge. The two do not always rise together: a city can add people without spreading much if it builds upwards, and it can spread a long way while its population barely changes if households are getting smaller and people are moving outwards.

Urbanisation is the increase in the proportion of a country's population living in urban areas, not simply the growth of cities. It is driven from both ends. The rural push factors are poverty, land fragmentation and uneconomic farm sizes, mechanisation of agriculture, drought and crop failure, and conflict. The urban pull factors are paid work, education, healthcare, electricity and water, and the expectation of a better life. Natural increase among a young urban population adds a great deal on top, and so does reclassification, when a village or district is officially redefined as urban because it has crossed a population or employment threshold, which makes the urban share jump without anyone moving.

Suburbanisation is the outward growth of the built-up area at the edge of the city, combined with the movement of people, especially better-off households, from the inner city to the new suburbs. It depends on transport: the railway and the tram in the nineteenth and early twentieth centuries, and the private car and the mortgage after the Second World War. The effect on the city is uneven. The suburbs gain population, owner-occupied housing and new shopping centres, while the inner city loses population and tax revenue and is left with an ageing housing stock and the households who cannot afford to move.

Urban sprawl is the low-density, often unplanned spread of the city into the surrounding countryside, usually in a fragmented pattern that leaps over land and leaves gaps. It is car-dependent by design, because densities are too low to support frequent public transport, and it consumes farmland, fragments habitats and makes services expensive to provide because the pipes and cables have to run so far for so few households. Counter-urbanisation is different again: it is the movement of people out of the city to smaller towns and rural areas, often beyond the commuting belt, and it is driven by congestion, pollution, house prices, crime, retirement and, more recently, broadband that allows remote work. Its effect on the countryside is the mirror image of sprawl, because the incoming households push up house prices, commute long distances and can price local people out of the villages they move into.

Re-urbanisation is the movement of people back into the inner city and the central areas, and it is normally a response to something having been done there: derelict land redeveloped, a waterfront cleaned up, a university or a cultural quarter expanded, or a transport link improved. The people who move back are usually young professionals, students and, later, older households trading a large suburban house for a flat. Urban renewal and urban regeneration are the two ways of making that happen, and they are not the same. Renewal is essentially physical: the old fabric is cleared and replaced, as in the slum clearance programmes that put up tower blocks in the 1960s. Regeneration is broader and works through partnership: it combines new buildings with jobs, training, transport, housing and environmental improvement, which is what happened in the London Docklands.

The examinable point is that these processes are not a queue that every city walks through in the same order. In high-income countries the usual sequence is urbanisation, then suburbanisation, then counter-urbanisation, then re-urbanisation as the inner city is regenerated. In a rapidly urbanising low-income country the dominant processes are urbanisation and sprawl, with the new growth appearing as unplanned peripheral settlements rather than suburbs. Within one country the processes can run at the same time in different places: a capital city may be re-urbanising in its centre while its outer edge sprawls and its surrounding market towns receive counter-urban migrants.

• Urbanisation: a rising share of the population living in towns and cities; rural push plus urban pull, natural increase and reclassification.
• Suburbanisation: the built-up area spreads outwards and better-off households move from the inner city to the new suburbs; depends on rail, tram and then the car.
• Urban sprawl: low-density, fragmented, car-dependent spread into the countryside; consumes farmland and makes infrastructure costly.
• Counter-urbanisation: people leave the city for smaller towns and rural areas beyond the commuting belt; raises rural house prices and creates commuter villages.
• Re-urbanisation: people move back into the inner city, usually after regeneration, and often young professionals and students.
• Urban renewal: physical clearance and rebuilding of run-down areas - a narrow, building-led approach.
• Urban regeneration: a wider partnership approach combining buildings with jobs, training, transport and environment - the London Docklands is the standard example.

Causes of urban growth and its consequences for urban and rural areas

Social causes. A young population already living in the city produces natural increase, which in many low-income cities now adds more people each year than migration does. Migration itself is social as much as economic: people follow relatives and people from their own village or ethnic group, because that network provides a bed, a contact and a job. Conflict and persecution displace people into cities, and so do environmental shocks such as drought and flooding, which are increasingly a rural push factor.

Economic causes. Industrialisation and the growth of service employment draw people in, and so does the agglomeration effect: firms want to be near their suppliers, their customers and a large labour pool, so once a city starts to grow it keeps attracting more firms, and each firm attracts more workers. Ports, airports, mines and oil fields create cities from nothing. A large informal economy also sustains growth, because it absorbs the people the formal sector cannot employ, which is why cities in low-income countries keep growing even when formal jobs are scarce.

Political and historical causes. Governments concentrate investment in capital cities: ministries, universities, hospitals, embassies, airports and the best roads all go there, which pulls in migration and private investment. Housing policy matters too, because a government that builds public housing or subsidises rents makes city living affordable, while one that restricts migration (through a registration system, for example) slows it. Historically, many large cities are where they are because of a colonial port, a railway junction, a mining district or a defensive site, and that inherited position still shapes which cities grow fastest today.

Consequences for urban areas. The environmental consequences are air and water pollution, solid waste that outstrips collection, loss of green space and farmland, the urban heat island, and increased flood risk because concrete and tarmac stop rain soaking away. The social consequences are housing shortage, overcrowding, informal settlements with insecure tenure, inadequate water and sanitation, disease, crime and widening inequality. The economic consequences cut both ways: agglomeration raises productivity and wages, but congestion, high land values and infrastructure deficits raise costs, and much of the informal economy is untaxed so the city cannot pay for what it needs. The political consequences are pressure on services, weak or fragmented local government, land disputes and evictions, and demands for decentralisation or metropolitan-wide authorities.

Consequences for rural areas. The most direct is the loss of farmland to sprawl and to the quarries, reservoirs and waste sites that serve the city. The most consequential is demographic: it is usually the young and better educated who leave, so the countryside ages, labour becomes short, farms are consolidated or abandoned and some services close. There are gains as well. Migrants send remittances home, urban demand creates a market for rural produce and raises farm incomes, and land near a growing city rises in value. Those gains are unevenly spread, and land speculation and land grabbing by better-connected buyers are a real cost of a nearby city.

The urban hierarchy, primate cities and world cities

A hierarchy of urban areas ranks settlements in a country by size and by the number and range of services they provide. At the bottom are hamlets and villages with a few low-order services used daily, such as a shop, a primary school and a post box. Higher up are market towns with a secondary school, a clinic and a bank, then cities with a university, a specialist hospital and a regional airport. Each level needs a threshold population to support its services, and each has a sphere of influence - the area from which it draws customers. The higher the level, the fewer the settlements, the larger the threshold and the wider the sphere. A country with a well-developed hierarchy has many small settlements and several competing regional cities; a country with a stunted hierarchy has one huge city and very little in between.

A primate city is the largest city in a country and is disproportionately larger than the second city, commonly taken as at least twice its population. It dominates the country's politics, finance, media, transport and higher education, and it usually contains the airport, the port, the stock exchange and the headquarters of national companies. The causes are historical and political as much as economic. Colonial administrations built one port or one administrative capital and ran the railways and roads out from it, so all subsequent investment followed the same lines. After independence, governments concentrated ministries, universities and industry in the capital, and urban bias in pricing, subsidies and infrastructure kept it there. Rural-urban migration, natural increase and the concentration of formal employment then made it grow faster than anywhere else.

The consequences for a country are mixed and examiners expect both sides. On the positive side, a primate city gives a country a single place with the agglomeration, infrastructure and international connections to attract foreign investment, and it acts as a gateway for trade and as a symbol of national identity. On the negative side, it produces regional inequality, because the rest of the country is starved of investment and its young people leave; it produces congestion, housing shortages and pollution at a scale the city cannot manage; and it makes the national economy vulnerable, because a flood, earthquake or epidemic in one city damages a large share of the country's output. Addressing it usually means deliberate decentralisation: new capital cities, growth poles, tax incentives to invest outside the capital, and devolved budgets to regional governments.

World cities are the top of a global rather than a national hierarchy, and they are classified in tiers - commonly alpha, beta and gamma - by the Globalization and World Cities (GaWC) research network, which ranks them by how much advanced producer services work they do. The categories are not about population size: Lagos is far larger than Zurich, but Zurich is the higher-order world city. Their causes are the control and command functions they host: headquarters of transnational corporations, international finance and banking, and the producer services that support them - law, accountancy, advertising, insurance, consultancy. Add media and culture, major airports and telecommunications hubs, political institutions such as the United Nations or the European Union, a large skilled migrant workforce and a language and time zone that make global business convenient, and the city becomes a place where global decisions are made. The consequence is a distinctive social structure: extreme income inequality, a hyper-diverse population, very expensive housing, and a large low-paid service workforce servicing the highly paid one.

• Hierarchy: settlements ranked by population and by the number and range of services; each level has a threshold population and a sphere of influence.
• Low-order services (shop, primary school) are found in many small settlements; high-order services (university, specialist hospital, airport) only in a few large ones.
• Primate city: the largest city, at least about twice the size of the second, dominating politics, finance, media and transport.
• Causes of primacy: colonial port or capital, radial transport, centralised post-independence investment, urban bias, rural-urban migration.
• Consequences of primacy: agglomeration and a gateway for investment, against regional inequality, congestion and national vulnerability.
• World cities: classified as alpha, beta and gamma by the GaWC network on the basis of advanced producer services, not population.
• Causes of world city status: TNC headquarters, international finance, producer services, media, transport and telecoms hubs, political institutions, skilled migration.

Horizontal and vertical structure, land-use zones, and place identity

Cities grow in one of two directions, and the choice shapes everything else about them. A horizontal structure spreads outwards at low density: detached and semi-detached houses, garden suburbs, industrial estates and out-of-town retail parks, all connected by roads and dependent on the car. It needs cheap land, weak planning control or a deliberate preference for low density, and it is typical of North American, Australian and many European cities. A vertical structure builds upwards at high density: apartment towers, offices and shopping centres stacked over transport interchanges. It needs scarce or very expensive land, strong public transport and, usually, a capable state that can assemble land and build at scale - Hong Kong, Singapore and New York are the standard examples. Most real cities are a mixture, with a high-rise core, high-rise peripheral estates and a wide sprawl of low-rise suburbs in between.

Inside either structure, land use is arranged in zones. The central business district (CBD) sits at the point of maximum accessibility - where the roads, railways and, historically, the port met - and it is dominated by offices, finance, government, high-order comparison retail, hotels and tourism, with very little housing. Around it lies the inner city, the zone of nineteenth-century industry and terraced housing, often with derelict land, warehouses and a mix of light industry, cheap rented flats and, increasingly, regenerated waterfronts. Beyond that are the inner and outer suburbs, mostly residential with schools, parks and district shopping, and then the rural-urban fringe, where the city meets the countryside and land uses compete: new housing estates, business parks, motorway junctions, retail parks, waste sites, airports and golf courses, alongside working farms.

Physical factors shape the pattern in ways students often forget. Relief decides where building is possible and where the expensive views are; a coast or a river limits growth in one direction and concentrates it in another; floodplains push housing onto higher ground and are often left as parks or sports fields; prevailing winds put industry downwind of the wealthy residential districts, which is why the industrial east side of many British cities still has the cheapest housing. Economic factors explain the broad arrangement through land values: competition for the most accessible sites pushes land values to a peak in the centre, and only activities that can generate enough revenue per square metre - finance, offices, flagship retail - can afford to be there, which is the logic behind the bid-rent curve. Social factors - income, class, ethnicity and the search for security - sort households between zones. Political factors include zoning, density controls, green belts, public housing allocation and the colonial practice of separating European and indigenous quarters, whose imprint is still visible in many cities. Historical factors work through inheritance: the route of a medieval street, a canal, a railway goods yard or a former factory determines what can be built where today.

Place identity is the set of characteristics that make a place recognisable and distinctive: its built form and skyline, its history, its cultural and religious life, the mix of people who live there, its economy, and the reputation it has. Identity is not fixed. It is made and remade by migration, investment, regeneration and the stories told about a place, which is why a regenerated dockland can acquire a completely new identity in twenty years. Perception of place is how different people see that identity, and the perceptions differ sharply: residents may value community and familiarity where outsiders see decline; investors see land value and risk; the media may fix a single negative image on a whole district after one incident. Those perceptions are not neutral, because they affect where people move, where firms invest, where banks lend and which areas governments decide to help. That is the justification for city branding and cultural regeneration, and also the reason residents often resist it.

The changing location of urban activities

The CBD is still the most accessible and most expensive land in most cities, and it still holds the activities that need face-to-face contact, prestige and a central address: finance, law, corporate headquarters, government, specialist retail, hotels and tourism. But it has changed. Wholesaling, warehousing and much manufacturing have gone, because they need space that the centre cannot offer and lorries that the centre cannot accommodate. Back-office functions have gone too, to cheaper sites with car parking. What is left is a smaller, more specialised core, and in weaker cities a good deal of it is vacant or converted to flats, which is why many CBDs now have a resident population they never had before.

Retail has moved outwards more than any other activity. The high street still serves convenience and comparison shopping, but the large floor-space retailers moved to out-of-town retail parks and regional shopping centres from the 1970s onwards, driven by car ownership, cheap edge-of-city land with free parking, planning policies that allowed it, and the congestion and parking charges of the centre. More recently, e-commerce has moved retail off the high street altogether and into warehouses on motorway junctions, from which vans make the last mile of the journey. The consequence for the centre is vacancy and falling footfall, and the response has been to convert shops to housing, to pedestrianise and to re-brand the high street as a place for food, leisure and services rather than for buying goods.

Manufacturing has decentralised for reasons that are easy to state and often badly stated. Inner-city sites are small, hemmed in by housing, and impossible to expand; they were built for canals and rail freight, not for container lorries; land values there are now higher for housing and offices; and the congestion and clean-air rules of the centre make heavy goods traffic difficult. So factories moved to edge-of-city industrial estates and motorway junctions, where plots are large, flat and cheap, access for lorries is direct, and there is room for parking and for expansion. Ports moved too, from inner-city docks too small for container ships to deep-water terminals downstream, which stranded the old docklands and created the derelict land that regeneration later reused.

Services have split in two directions. Producer services - finance, law, accountancy, advertising, design, consultancy - cluster tightly in the CBD and in a few adjacent districts, because they trade on face-to-face contact, reputation and the address. Consumer services follow the population outwards, so supermarkets, gyms, clinics and schools appear in the suburbs, and back-office processing moves to business parks and cheaper towns where wages and rents are lower. Public services follow the same logic, which is why hospitals and universities have relocated or opened campuses on the edge of cities.

The factors behind all of these shifts are the ones the syllabus names, and they interact. Population change - smaller households, an ageing population, inward migration - alters what is demanded and where. Cost of land and competition for space decide who can occupy the accessible sites. Transport provision decides what is accessible: a motorway junction or a new metro station can create a new retail or office location almost overnight. Investment - property capital, foreign direct investment, speculative office building - decides what actually gets built. Local and national planning decides what is permitted, through zoning, green belts, retail caps, enterprise zones and regeneration designations. And the rate of change varies: a post-industrial city in a high-income country changes through regeneration, retail decentralisation and office conversion over decades; a rapidly growing city in a low-income country changes through peripheral expansion and informal commercial development; a capital city or a port city changes faster than an inland regional city, because more investment is directed at it.

• CBD: retains finance, government, corporate headquarters and specialist retail, but has lost warehousing, manufacturing and back offices; many CBDs are now also residential.
• Retail: decentralised to out-of-town retail parks and shopping centres (car access, cheap land, free parking), then partly to e-commerce and motorway-side warehouses.
• Manufacturing: left inner-city sites (small plots, no lorry access, high land values, congestion) for edge-of-city industrial estates and motorway junctions; old docks were abandoned as ports moved downstream.
• Services: producer services stay clustered in the CBD for face-to-face contact and prestige; consumer and back-office services follow population and cheap rents.
• Drivers: population change, cost of land, transport provision, competition for space, investment, and local and national planning.
• Rate of change varies with location, economic classification and the balance between state direction and market forces.

Zonation of residential areas

Residential areas are not mixed evenly across a city; they are sorted into zones with recognisably different characteristics. The inner city typically has nineteenth-century terraced housing at high density, small yards rather than gardens, a high proportion of private renting, older and sometimes overcrowded stock, and a population that is younger, poorer and more ethnically mixed than the city average. The inner suburbs have denser terraces and low-rise flats built along tram and rail lines, with district shopping streets. The outer suburbs are dominated by owner-occupied semi-detached and detached houses with gardens at low density. On the periphery of many cities are large public housing estates, and in rapidly growing low-income cities, informal settlements built by their residents on unserviced land with insecure tenure. The rural-urban fringe and commuter villages hold the largest and most expensive houses, occupied by households with the longest commutes.

Income is the most powerful single cause, because it decides what a household can pay for space and for access. A household on a high income can buy more rooms and a garden further out and absorb the commuting cost; a low-income household is pushed to the cheapest land, which is usually the oldest inner-city stock or the most peripheral and least serviced sites. Supply of housing interacts with income: where the stock is old and private-rented, the poor occupy it; where the state has built large estates, the poor are concentrated there by allocation rules; where land release is restricted, prices rise and the sorting becomes harsher.

Ethnicity produces clustering that is partly chosen and partly imposed. New migrants often move to where people from the same origin already live, because that is where housing, work, shops, places of worship and childcare can be found through networks - chain migration. Clustering can also be imposed by discrimination in the housing market, by landlords, by estate agents, or by the legacy of colonial segregation, which reserved separate quarters for different groups and whose street pattern survives long after the law that created it. Clustering is not the same as segregation: the difference is whether households are there by choice or by constraint, and whether they can leave.

Planning shapes zonation directly. Zoning separates housing from industry, sets density and height limits, designates green belts that stop the outward spread of low-density housing, decides where social housing is built and who is allocated it, and licenses houses in multiple occupation. Planning can also consciously mix tenures through inclusionary quotas, or it can concentrate poverty by building all the social housing in one peripheral estate. Cultural and historical factors complete the list: the age at which each district was built, the housing ideals of the period (the garden city, the tower block, the gated community), inheritance patterns, the location of former industry, and the preferences of particular groups for particular kinds of housing. Zonation matters because it is not just a pattern on a map - it decides who gets the good school, the clean air, the park and the safe street, and who does not.

• Income decides what a household can buy in space and access; it is the strongest single cause of residential sorting.
• Supply of housing (age of stock, tenure, public housing, land release, rent controls) determines what is available to each income group.
• Ethnicity produces clusters through chain migration and networks (choice) and through discrimination and colonial segregation (constraint).
• Planning sets zoning, density, green belts, allocation rules and tenure mix, and can either mix or concentrate poverty.
• Cultural and historical factors include the housing ideals of each era, inheritance, former industry and the legacy of segregated quarters.
• The result is unequal access to schools, health care, environmental quality and safety, not just a different streetscape.

Sustainability: the concept, the difficulty of measuring it, and competing priorities

Sustainability means meeting the needs of people living now without compromising the ability of future generations to meet their own needs - the definition produced by the World Commission on Environment and Development (the Brundtland Commission) in 1987. It is usually broken into four dimensions that have to be satisfied together: environmental (within the limits of resources and ecosystems), social (meeting basic needs, equity, health, participation), economic (viable livelihoods, affordable services, growth that can be financed) and political or institutional (governance, enforcement, accountability). A scheme that is green but unaffordable, or affordable but built on contaminated land, is not sustainable; it has traded one dimension for another.

Measuring sustainability is genuinely difficult, and the exam wants the reasons rather than a list of indices. There are many dimensions and they can point in opposite directions, so any single number requires a judgement about how much weight to give each one, and different weightings produce different rankings of the same city. Data are patchy: informal settlements are often absent from official statistics, emissions and waste are estimated rather than measured, and cities define their boundaries differently, so a figure for one city is often not comparable with another. There is a scale problem, because a city can look sustainable while importing its problems - exporting its waste, or depending on food, water and electricity generated far beyond its boundary, so that its own footprint is invisible. There is a time problem, because the needs of future generations cannot be observed and are therefore assumed. And there is the question of substitutability: whether natural capital can be replaced by manufactured capital, or whether some of it - a stable climate, an aquifer, a species - cannot be replaced at all.

Sustainability in practice is a negotiation between groups with different priorities, and the examiner wants you to know whose interests are at stake. Residents want affordable housing, clean air, safe streets, green space and, above all, not to be displaced by the improvements. Landowners and developers want planning permission, high land values and a return on investment, and they will argue that without profit nothing gets built. Local business owners want footfall, delivery access and parking, and they fear that pedestrianisation, congestion charges or construction work will take their customers away. Community groups want green space, heritage, participation in decisions and protection from gentrification. Local government wants growth in the tax base, delivery against targets and re-election; national government wants growth, international commitments and visible success. These priorities genuinely conflict. A new tram line cuts emissions and congestion but disrupts traders during construction and raises land values along the route. A park improves health and cooling but takes land that could house people. Sustainable management is therefore not the search for a solution that pleases everyone; it is the search for a defensible trade-off, and the answer has to say who gains and who loses.

• Sustainability: meeting present needs without compromising the ability of future generations to meet theirs (Brundtland Commission, 1987).
• Four dimensions that must be satisfied together: environmental, social, economic, political/institutional.
• Measuring is difficult because the dimensions conflict, weightings are subjective, data are missing for informal areas, boundaries differ, and future needs cannot be observed.
• A city can export its unsustainability - waste shipped out, food and water imported, emissions generated elsewhere - so its own footprint looks small.
• Groups with a stake: residents, landowners and developers, local business owners, community groups, local and national government - each with a different priority.
• Conflicts are normal: a tram line cuts emissions but disrupts traders and raises land values; a park improves health but uses land that could house people.

Sustainability issues in urban areas: waste, transport, housing, green space and water

Solid waste disposal is a sustainability problem because the volume of waste rises with income and consumption, and the composition changes towards packaging, single-use plastics and electronics, which are bulky, slow to decompose and often hazardous. Collection is the first failure point: in many rapidly growing cities the municipal service covers the wealthy districts and barely reaches the informal settlements, so waste is dumped in drains, rivers and open ground or burned in the street, which blocks drainage and causes flooding, breeds disease vectors and produces toxic smoke. Where collection does work, the problem moves to disposal: landfill space near cities runs out and is opposed by nearby residents, and landfills produce methane and leachate that contaminates groundwater. Incineration recovers energy but needs expensive pollution control. Throughout the low-income city, an informal workforce of waste pickers does much of the actual recycling, usually unprotected and unpaid for the environmental service it provides.

Transport is the second issue and it has two halves. Private transport means congestion, which wastes fuel, time and money and makes journey times unreliable; air pollution from particulates and nitrogen oxides, which damages health, especially for children and the elderly; carbon emissions; road deaths and injuries; and the land taken by roads and parking, which pushes everything else further apart. Public transport in high-income cities is often underfunded and crowded, and in low-income cities it is frequently informal - minibuses and motorcycle taxis - which is flexible and provides work, but is unsafe, unregulated and contributes heavily to pollution. The equity dimension matters: households without a car depend on whatever public transport exists, so a transport policy that only widens roads is a policy that helps the better-off.

Housing quality and density is the third. Quality problems include overcrowding, inadequate water and sanitation, damp and poor ventilation, insecure tenure, and structural risk from fire, earthquake or landslide, all of which are concentrated in informal settlements and in ageing inner-city stock. Density is a genuinely double-edged issue, which is why it makes a good evaluation. Higher density uses land and energy more efficiently, makes public transport and services viable, and reduces emissions per household; but badly designed high density means overcrowding, heat, noise, a lack of private space and pressure on water, drainage and waste collection. Low density avoids those problems and creates sprawl, car dependence and expensive infrastructure. The sustainable answer is usually not more or less density but better-designed density in the right place, especially along transport corridors.

Green space and water features are the city's environmental infrastructure, and the syllabus expects you to say what they actually do. Vegetation and water cool the city and reduce the urban heat island, because they evaporate water and shade surfaces instead of storing heat. They intercept rainfall and let it infiltrate, which reduces surface runoff and the flash flooding that impermeable surfaces create. They filter air pollution, support biodiversity, and improve physical and mental health and social contact; a river corridor or a lake provides amenity, recreation and, if it is clean, an alternative to piped water for irrigation and a wildlife habitat. They also raise the value of nearby property, which is the problem: green improvements can trigger green gentrification and price out the residents they were meant to benefit, and parks that are not maintained, lit or overlooked become places people avoid.

Strategies for sustainable management: what works, what blocks it, and how to judge it

Waste strategies follow a hierarchy: reduce, reuse, recycle, recover, and only then dispose. In practice that means kerbside separation and collection charges that reward less waste, deposit-return schemes for bottles and cans, composting of organic waste, extended producer responsibility that makes manufacturers pay for packaging, landfill taxes and bans that make disposal expensive, and energy recovery from what cannot be recycled. The conditions for success are institutional rather than technical: reliable collection, somewhere for the separated material to be processed, a market for the recycled product, and - in cities where informal pickers already do the recycling - a formal arrangement that gives them protective equipment, a fair price and a recognised role instead of criminalising them.

Transport strategies aim to move people rather than vehicles. Bus rapid transit gives buses their own lanes, stations and priority at junctions, so for a fraction of the cost of a metro it can carry very large numbers of passengers quickly. Metro and light rail do the same at a higher cost and higher capacity. Integrated ticketing and fares across modes make a journey on public transport one journey rather than three. Demand management - congestion charging, road pricing, parking restraint and vehicle quotas - raises the cost of driving into the places where it does most harm. Park and ride, cycle lanes, pedestrianisation and transit-oriented development (building dense housing around stations) make the alternative practical rather than merely available. Cleaner fuels and low emission zones deal with the pollution that remains.

Housing strategies range from building to legalising. Slum upgrading improves conditions where people already live by granting secure tenure, laying water, sewerage, drainage and electricity, and paving streets - which works because it is cheap, it does not displace anyone, and secure tenure gives households a reason to invest in their own homes. Sites and services schemes provide serviced plots that households build on themselves. Public housing delivers large numbers of units quickly but can concentrate poverty if it is all in one place and badly connected. Inclusionary zoning requires private developments to include affordable units, rent regulation protects sitting tenants, and building codes with real enforcement decide whether the housing that is built will survive a fire or an earthquake. Densification along transport corridors is the current orthodoxy, because it reduces car dependence and makes services viable.

Constraints are what turn a good strategy into a failed one, and they are mostly about money, power and time. Finance is the first: infrastructure is expensive, the return is slow, and a city that cannot tax its informal economy cannot pay for it. Enforcement is the second: a building code or a zoning rule that is not enforced is a decoration, and enforcement is weakest exactly where the need is greatest. Land is the third, because schemes need land, land has owners, and compensation and title disputes can delay a project for years. Politics is the fourth: electoral cycles reward visible ribbon-cutting over maintenance, and a scheme that takes fifteen years to pay off is easy to cancel. Opposition is the fifth, because residents accept the need for new housing in general and object to it next door. Institutional fragmentation is the sixth, because the city government often does not control the transport, water and planning decisions that shape it. Incentives work in the opposite direction: grants and subsidised finance, tax relief, planning gain and density bonuses, public-private partnerships, international development and climate finance, legal targets and mandates, and the reputational reward of being seen to succeed.

Judging success is where the evaluation marks are. Ask five questions of any scheme: did it achieve its stated aim, measured how? What did it cost, and who paid? Who benefited and who was displaced or excluded? How quickly did it work, and will it keep working once the funding and the political attention move on? What unintended effects did it have - higher land values, gentrification, a new dependence, a maintenance bill nobody budgeted for? A defensible conclusion says that the physical interventions - buses, drains, pipes, parks, housing - are necessary but not sufficient, and that the schemes that last are the ones backed by an institution that can fund, enforce and maintain them, and by residents who were involved in the decision rather than moved out of the way by it.