Editing IPCC:AR6/WGIII/Chapter-10 (section)

=== 10.2.1 Urban Form, Physical Geography, and Transport Infrastructure ===

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The physical characteristics that make up built areas define the urban form. These physical characteristics include the shape, size, density, and configuration of the human settlements. Urban form is intrinsically coupled with the infrastructure that allows human settlements to operate. In the context of the transport sector, urban form and urban infrastructure influence the time and cost of travel, which, in turn, drive travel demand and modal choice ( [[#Marchetti--2001|Marchetti and Ausubel 2001]] ; [[#Newman--2015|Newman and Kenworthy 2015]] ).

Throughout history, three main urban fabrics have developed, each with different effects on transport patterns based on a fixed travel time budget of around one hour ( [[#Newman--2016|Newman et al. 2016]] ). The high-density urban fabric developed over the past several millennia favoured walking and active transport for only a few kilometres (km). In the mid-19th century, urban settlements developed a medium-density fabric that favoured trains and trams traveling over 10 to 30 km corridors. Finally, since the mid-20th century, urban form has favoured automobile travel, enabling mass movement between 50 and 60 km. Table 10.2 describes the effect of these urban fabrics on GHG emissions and other well-being indicators.

'''Table 10.2 | The systemic effect of city form and transport emissions.'''

{| class="wikitable"
|-
| '''Annual transport emissions and co-benefits'''

| '''Walking urban fabric'''

| '''Transit urban fabric'''

| '''Automobile urban fabric'''

|-
| Transport GHG

| 4 tonnes per person

| 6 tonnes per person

| 8 tonnes per person

|-
| Health benefits from walkability

| High

| Medium

| Low

|-
| Equity of locational accessibility

| High

| Medium

| Low

|-
| Construction and household waste

| 0.87 tonnes per person

| 1.13 tonnes per person

| 1.59 tonnes per person

|-
| Water consumption

| 35 kilolitre per person

| 42 kilolitre per person

| 70 kilolitre per person

|-
| Land

| 133 square metres per person

| 214 square metres per person

| 547 square metres per person

|-
| Economics of infrastructure and transport operations

| High

| Medium

| Low

|}

Source: [[#Newman--2016|Newman et al. (2016)]] ; [[#Thomson--2018|Thomson and Newman (2018)]] ; [[#Seto--2021|Seto et al. (2021)]] .

Since AR5, urban design has increasingly been seen as a major way to influence the GHG emissions from urban transport systems. Indeed, research suggests that implementing urban form changes could reduce GHG emissions from urban transport by 25% in 2050, compared with a business-as-usual scenario ( [[#Creutzig--2015b|Creutzig et al. 2015b]] ; [[#Creutzig--2016|Creutzig 2016]] ). Researchers have identified a variety of variables to study the relationship between urban form and transport-related GHG emissions. Three notable aspects summarise these relationships: urban space utilisation, urban spatial form, and urban transportation infrastructure ( [[#Tian--2020|Tian et al. 2020]] ). Urban density (population or employment density) and land-use mix define the urban space utilisation. Increases in urban density and mixed function can effectively reduce per capita car use by reducing the number of trips and shortening travel distances. Similarly, the continuity of urban space and the dispersion of centres reduces travel distances ( [[#Tian--2020|Tian et al. 2020]] ), though such changes are rarely achieved without shifting transport infrastructure investments away from road capacity increases ( [[#Newman--2015|Newman and Kenworthy 2015]] ; [[#McIntosh--2017|McIntosh et al. 2017]] ). For example, increased investment in public transport coverage, optimal transfer plans, shorter transit travel time, and improved transit travel efficiency make public transit more attractive ( [[#Heinen--2017|Heinen et al. 2017]] ; [[#Nugroho--2018a|Nugroho et al. 2018a]] ; [[#Nugroho--2018b|Nugroho et al. 2018b]] ) and hence increase density and land values ( [[#Sharma--2020|Sharma and Newman 2020]] ). Similarly, forgoing the development of major roads for the development of pedestrian and bike pathways enhances the attractiveness of active transport modes ( [[#Zahabi--2016|Zahabi et al. 2016]] ; [[#Keall--2018|Keall et al. 2018]] ; [[#Tian--2020|Tian et al. 2020]] ).

Ultimately, infrastructure investments influence the structural dependence on cars, which in turn influence the lock-in or path dependency of transport options with their greenhouse emissions ( [[#Newman--2015|Newman et al. 2015]] ; [[#Grieco--2016|Grieco and Urry 2016]] ). The 21st century saw a new trend to reach peak car use in some countries as a result of a revival in walking and transit use( [[#Grieco--2016|Grieco and Urry 2016]] ; [[#Newman--2017|Newman et al. 2017]] ; [[#Gota--2019|Gota et al. 2019]] ). While some cities continue on a trend towards reaching peak car use on a per-capita basis, for example Shanghai and Beijing ( [[#Gao--2020|Gao and Newman 2020]] ), there is a need for increased investments in urban form strategies that can continue to reduce car dependency around the world.

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