Policy makers, urban planners, architects, sociologists, and economists are interested in creating urban areas that are both lively and safe. But are the safety and liveliness of neighborhoods independent characteristics? Or are they just two sides of the same coin? In a world where people avoid unsafe looking places, neighborhoods that look unsafe will be less lively, and will fail to harness the natural surveillance of human activity. But in a world where the preference for safe looking neighborhoods is small, the connection between the perception of safety and liveliness will be either weak or nonexistent. In this paper we explore the connection between the levels of activity and the perception of safety of neighborhoods in two major Italian cities by combining mobile phone data (as a proxy for activity or liveliness) with scores of perceived safety estimated using a Convolutional Neural Network trained on a dataset of Google Street View images scored using a crowdsourced visual perception survey. We find that: (i) safer looking neighborhoods are more active than what is expected from their population density, employee density, and distance to the city centre; and (ii) that the correlation between appearance of safety and activity is positive, strong, and significant, for females and people over 50, but negative for people under 30, suggesting that the behavioral impact of perception depends on the demographic of the population. Finally, we use occlusion techniques to identify the urban features that contribute to the appearance of safety, finding that greenery and street facing windows contribute to a positive appearance of safety (in agreement with Oscar Newman's defensible space theory). These results suggest that urban appearance modulates levels of human activity and, consequently, a neighborhood's rate of natural surveillance.
@cite_41: A traveler visiting Rio, Manila or Caracas does not need a report to learn that these cities are unequal; she can see it directly from the taxicab window. This is because in most cities inequality is conspicuous, but also, because cities express different forms of inequality that are evident to casual observers. Cities are highly heterogeneous and often unequal with respect to the income of their residents, but also with respect to the cleanliness of their neighborhoods, the beauty of their architecture, and the liveliness of their streets, among many other evaluative dimensions. Until now, however, our ability to understand the effect of a city's built environment on social and economic outcomes has been limited by the lack of quantitative data on urban perception. Here, we build on the intuition that inequality is partly conspicuous to create quantitative measure of a city's contrasts. Using thousands of geo-tagged images, we measure the perception of safety, class and uniqueness; in the cities of Boston and New York in the United States, and Linz and Salzburg in Austria, finding that the range of perceptions elicited by the images of New York and Boston is larger than the range of perceptions elicited by images from Linz and Salzburg. We interpret this as evidence that the cityscapes of Boston and New York are more contrasting, or unequal, than those of Linz and Salzburg. Finally, we validate our measures by exploring the connection between them and homicides, finding a significant correlation between the perceptions of safety and class and the number of homicides in a NYC zip code, after controlling for the effects of income, population, area and age. Our results show that online images can be used to create reproducible quantitative measures of urban perception and characterize the inequality of different cities.
@cite_52: Social science literature has shown a strong connection between the visual appearance of a city's neighborhoods and the behavior and health of its citizens. Yet, this research is limited by the lack of methods that can be used to quantify the appearance of streetscapes across cities or at high enough spatial resolutions. In this paper, we describe 'Streetscore', a scene understanding algorithm that predicts the perceived safety of a streetscape, using training data from an online survey with contributions from more than 7000 participants. We first study the predictive power of commonly used image features using support vector regression, finding that Geometric Texton and Color Histograms along with GIST are the best performers when it comes to predict the perceived safety of a streetscape. Using Streetscore, we create high resolution maps of perceived safety for 21 cities in the Northeast and Midwest of the United States at a resolution of 200 images square mile, scoring 1 million images from Google Streetview. These datasets should be useful for urban planners, economists and social scientists looking to explain the social and economic consequences of urban perception.
@cite_24: 
@cite_23: The Death and Life of Great American Cities was written in 1961 and is now one of the most influential book in city planning. In it, Jane Jacobs proposed four conditions that promote life in a city. However, these conditions have not been empirically tested until recently. This is mainly because it is hard to collect data about "city life". The city of Seoul recently collected pedestrian activity through surveys at an unprecedented scale, with an effort spanning more than a decade, allowing researchers to conduct the first study successfully testing Jacobs's conditions. In this paper, we identify a valuable alternative to the lengthy and costly collection of activity survey data: mobile phone data. We extract human activity from such data, collect land use and socio-demographic information from the Italian Census and Open Street Map, and test the four conditions in six Italian cities. Although these cities are very different from the places for which Jacobs's conditions were spelled out (i.e., great American cities) and from the places in which they were recently tested (i.e., the Asian city of Seoul), we find those conditions to be indeed associated with urban life in Italy as well. Our methodology promises to have a great impact on urban studies, not least because, if replicated, it will make it possible to test Jacobs's theories at scale.
@cite_5: This article traces the movement of the concept of 'defensible space' from New York City in the 1970s, where it was developed by the Canadian architect planner Oscar Newman, to London in the 1980s and into design interventions in British public housing in the 1990s, through British geographer Alice Coleman, who acted as an especially powerful transfer agent. In focusing on this urban design 'concept' on the move we contribute to existing scholarship on policy mobility and city building in a number of ways. First, we explore an instance of the movement mobility of a planning concept in a historical period (the recent past) largely overlooked to date. Secondly, we demonstrate that this movement was the result of a disaggregated series of expert knowledge transfers and localized translations of pre-policy expert knowledge, generated through university-based research work and networks. We theorize this instance of urban planning mobility by way of the interlinked insights offered by the sociology of science and policy-mobilities literatures. As this is an instance of university research shaping public policy it also offers an opportunity to reflect upon the meaning of 'evidence-based policy' and the impact agenda in contemporary higher education.
@cite_25: Abstract To improve community appearance, planners need to know how the public evaluates the cityscape: their evaluative image of the city. This article presents research aimed at uncovering this information. In two U.S. cities, Knoxville and Chattanooga, Tennessee, we interviewed 220 residents by phone and 180 visitors face-to-face, asking them to specify areas they liked and areas they disliked visually and to describe the physical features accounting for their evaluations. From each interview we prepared an evaluative map of the city. We then overlaid these maps to produce for each city and for residents and visitors separately a composite map—the evaluative image of the city. The evaluative maps suggest effects of city structure and experience, and they indicate five desirable features—naturalness, upkeep, openness, order, and historical significance. By showing the identity, location, and likability of visual features, evaluative maps provide a basis for a visual plan.
The connection between urban perception and human activity speaks primarily to two streams of literature. The first one is the stream of literature focused on the environmental factors contributing to crime, which has a long tradition in criminology and urban sociology. While our paper does not focus on crime per se, the connection between the physical appearance of neighborhoods and natural surveillance suggested by Jacobs and Newman makes our results relevant to that stream of literature @cite_23 @cite_5 @cite_24 . The second one is the stream of literature using surveys, and more recently, computer vision methods to quantify people’s perception of urban environments @cite_52 @cite_25 @cite_41 .