This article is from WeChat public account: Jizhi Club (ID: swarma_org) , author: Guo Ruidong
Introduction
Cities are a manifestation of human civilization. More than half of the world ’s population lives in cities. Nowadays, new cities are in full swing in China, and Xiong’an New District is a millennium plan in urban construction. Research has far-reaching and urgent practical significance. In a Nature communications paper, the researchers pointed out that the interaction between urban planning and hierarchical organizational structure will affect all aspects of people living in cities. Between the frequency and distance of travel, cities with strong hierarchical structure have better livability.
What is a city ’s hierarchical organization
For the city, the general impression is that the city will have one or more central circles, where the crowd is dense, while the rest of the city is relatively sparsely populated. A city has one ring, two rings and three rings, and the first ring will be more prosperous than the second ring and the third ring. When people travel, they will also give priority to downtown areas, so that the frequency and distance of the crowd will be level Clearly, this is a hierarchical organization of cities (hierarchical organization) Intuitive understanding.
However, if you are doing scientific research, you need to be able to have a clear and quantitative definition of the concepts that are commonly used in life and cannot be clearly understood. “When you can quantify what you are talking about and can describe it numerically, you have an in-depth understanding of it. But if you can’t describe it numerically, then your mind has not jumped to the state of scientific thinking at all.” (British physicist Lord Kelvin)
How to quantify the hierarchy of cities? This article will explain how the scientist uses a measure to define the concept of hierarchical organization of the city by reading this paper. Through this example, take you to realize how to think the path of data.
Quantitative definition of city ’s hierarchical organization
Researchers used the commuting record data of 3 million Google users who lived or traveled in different cities around the world recorded by their mobile phones in 2016 during the definition of city hierarchical organization. For example, from A to B, it is a record. Combine the starting position of commuting into a heat map, divide the city into small squares, and use different colors to represent the density of the commuting lines between a certain square, you can see the flow of people between different cities and starting points There is a clear difference in density.
Figure 1: Commuter heat maps of three metropolitan cities with populations of 10 million, Paris, Bangkok and Los Angeles. The red area indicates the starting point for commuting in this area Has a higher population density
In order to change the scale and degree of developmentThe cities are put together for comparison, and the data needs to be standardized (normalization) . The researchers use the total number of commutes in a city as the denominator to see the relative proportion of the starting points of commuting in different locations in the city to the total starting points of commuting.
Corresponding to the picture below, the proportion of different shades of blue in the picture is 10 ^ (-1) to 10 ^ (-7). The darker the color of the grid, the more often the area is used as a starting point or destination for commuting, and the hotter the area.
Figure 2: Relative connection density matrix between the realistic and simulated New York City commuting hotspots
In order to show that the laws behind the data are real, and the ruler used in the research can measure the strength of the laws in the data, the researchers compare the real data with the randomly generated data. As shown in the figure above, the researchers compared the randomly simulated New York commuter network with the real situation. Obviously, in the real situation, the city will present a hierarchical organizational structure.
Since the hierarchical organization of the city does exist, an indicator to distinguish the degree of urban hierarchy is needed. This standard should make the difference between the scores of real cities and simulated random cities marked by this standard obvious, and this standard should be applicable to cities of different sizes.
Figure 3: Schematic diagram of the relationship between urban hierarchical organization Φ and commuting popularity
Due to the hierarchical characteristics of urban commuting, it means that the proportion of people flowing between urban commuting hotspots is relatively higher than the ratio between hotspots and non-hotspots by one to two orders of magnitude. To quantify this observation, the quantification of the degree of urban stratification can be defined as the sum of the relative density of the areas on the diagonal, which is called Φ value in the text, which is the area surrounded by yellow circles .
Commuting popularity has L levels. The researchers first constructed the matrix T, Tij corresponding to the travel from the area of heat i to the area of heat j. According to the total amount of travel in the city, T is normalized. δij is the Kroneck delta function of the matrix T. If the travel occurs between areas of the same heat, then the value of δij is 1, if the travel is smoothly from high to low heat, or from low to high heat, that is, the heat is evenly distributed at different levels, then δij The value is 0. Therefore, a higher Φ value corresponds to a stronger hierarchy.
City hierarchy will affect travel choices
If a city does n’t have a center, or if it ’s next to the city center, it ’s a deserted wilderness. If you live in such a city, you will probably find the transportation inconvenient. Regardless of the former or the latter, you need to run a lot of wrong roads. The most direct impact of urban planning is the choice of people’s transportation methods.
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Figure 4: Changes in the proportion of self-driving, public transportation, and walking in different hierarchical organizations in commuting methods
Among the three modes of transportation such as self-driving, public transportation and walking, if the city’s stratification is relatively low, then self-driving will become the mainstream mode of transportation, and with the value of Φ representing the stratification of the city With the increase, the number of people choosing public transportation and walking will increase significantly.
The design of public transportation also needs to consider the hierarchical organization in the city. For example, choose to open more frequent flights to different hotspots. By reducing the number of stops, you can make public transportation faster and let residents choose to take public transportation or walk instead of driving.
The higher the level of the city, the more friendly it is to the environment
A large part of urban air pollution comes from the exhaust gas brought by commuting, so the difference in commuting methods indirectly affects the degree of urban air pollution.
After removing the impact of the city ’s different GDP, it can be found that the higher the level of urbanization, the smaller the air pollution per capita.
Figure 5: The relationship between air pollution per capita and the degree of urban stratification
The figure above describes the relationship between the number of nitrogen oxides, carbon monoxide and PM10 particles in different cities and Φ. It can be seen that as the level of urbanization increases, the amount of pollution per capita has dropped significantly.
In addition to air pollution, the level of urban organization will also affect the emission of greenhouse gas carbon dioxide.
Figure 6: The relationship between carbon dioxide emissions and the degree of urban stratification
As shown in the figure, the higher the degree of urban stratification, the lower the carbon dioxide emissions per dollar of GDP generated by the city. Note that the coordinates of the vertical axis change from 0 to 30, and the difference between the highest and lowest values is several times. This shows that the influence of Φ value on carbon dioxide emissions is more significant than air pollution.
Other impacts of urban hierarchy on urban life
If stroke patients can be treated in time, the probability of survival will increase significantly. The chart below compares the number of deaths due to stroke per 100,000 inhabitants, and the average distance between their residence and the hospital, and the number of deaths due to traffic accidents per 100,000.
Figure 7: Relationship between urban stratification and stroke mortality, average distance to hospital, and traffic accident mortality
It can be seen from the figure that cities with a higher urban stratification degree Φ value perform better in these three indicators, that is, the more convenient urban residents are to seek medical treatment, the greater the probability of traffic accidents in the city low. And the more hierarchical cities are, the more convenient it is to get medical treatment.
If you divide commuting into short-distance (daily commuting) and long-distance, you can also see that the city ’s hierarchy is divided into two categories The impact of commuting distance is different.
Figure 8: The relationship between short-distance (left) and long-distance (right) commuting distance and the degree of urban hierarchy
As shown in the figure, in the case of a high degree of urban stratification, the average distance of short-distance traffic is shorter, and the average distance of long-distance traffic is longer. For the residents of the city, it can also enhance their happiness. The former allows you to waste less time on the way to and from work, and the latter gives you the opportunity to explore more distant unknowns. For example, living in New York (NY) , the average distance to work commuting to Atlanta (ATL) is shorter, and for long-distance travel in the periphery of the urban area, most places can be visited in Biyatlan.
Summary
The larger the city, the faster the pace of life. The city is a continuously accelerating treadmill. This is the power law that no one can change. Everyone wants to live in a city with less air pollution, and hopes that we have a hospital near where we live, and the city’s builders also hope to increase the city’s public transportation utilization rate. Therefore, cities are more dense, especially more hierarchical, which is the future trend.
Whether building new cities or renovating old cities, you can consider the different levels of hierarchical organization brought by different plans, combined with methods such as subject modeling to make hotspot areas and hotspot areas adjacent, instead of taking the default star Type topology (hub-to-spoke) structure to improve the level of the city and make the city more livable.
Thesis title:
Hierarchical organization of urban mobility and its connection with city livability
Thesis address:
https://www.nature.com/articles/s41467-019-12809-y
This article is from WeChat public account: Jizhi Club (ID: swarma_org) , author: Guo Ruidong