Estimating pedestrian volume using Street View images: A large-scale validation test
Introduction
Recent developments in urban big data and machine learning have provided us new data sources and an interdisciplinary approach to understand urban phenomena (Ruppert, 2013). For instance, in the past, information on perceived of built environment characteristics (e.g., urban greenery and aesthetics) or residents' travel behavior (e.g., walking, cycling) were often manually collected through surveys or field observations. Recent urban big data have provided effective means to collect such environmental and behavioral data on a large geographical scale. Hence, urban big data can advance urban studies, especially in the areas of walkability and healthy cities (Rzotkiewicz, Pearson, Dougherty, Shortridge, & Wilson, 2018).
Rapid global urbanization over recent decades has led to a fundamental change in people's lifestyle and a rapid expansion of urban populations. The United Nations estimates that nearly 70% of the world's population will live in cities by 2050 (United Nations, 2018). Moreover, urban residents have experienced rapid declines in physical activity levels, with roughly one-third of urban adults in the world being physically inactive (Hallal et al., 2012). Compelling evidence demonstrates that regular physical activity, such as walking and cycling, has an array of health benefits, including the reduced risk of obesity and chronic illness, and improved physiological and psychological health (I.-M. Lee et al., 2012; Sallis, Floyd, Rodriguez, & Saelens, 2012; Wang et al., 2019).
As the most common form of physical activity, walking can be easily incorporated into daily life, and it has additional environmental and social benefits, such as reducing private vehicle use, mitigating traffic congestion and air pollution, and encouraging social interaction (Giles-Corti et al., 2013; Lu, Xiao, & Ye, 2017; Nazelle et al., 2011; Yin, 2017). Many seminal urban planning theories aims to facilitate walking and pedestrian activities, by well-designed city image (Lynch, 1960), streets (Jacobs, 1961), and public open spaces (Whyte, 1980). Recent planning theories, such as smart growth and neourbanism, explicitly aim to promote walking and active living through various design strategies, such as mixed land use, compact development, well-connected streets, and the provision of pedestrian destinations (Durand, Andalib, Dunton, Wolch, & Pentz, 2011; Giles-Corti et al., 2013).
To provide sustained support for researchers and planners who aim to create walkable and healthy cities, it is necessary to assess pedestrian volume, and other walking behaviors constantly. Assessing pedestrian volumes in different streets or areas can help researchers to evaluate walkability and to discern how built environment characteristics affect walking behaviors (Ewing & Clemente, 2013). In the past, pedestrian volume has been typically collected with pedestrian counts on sites. However, field observation is inherently subject to significant limitations, such as high demand for manpower and cost, and small study areas. Street View image services from companies such as Google, Baidu, and Tencent provide high-resolution, geocoded, streetscape images in many global cities (Rzotkiewicz et al., 2018). Researchers can use Street View images (SVIs) in conjunction with machine learning techniques to extract new built environment characteristics and behavioral data (Shapiro, 2017; Ye et al., 2018; Ye, Zeng, Shen, Zhang, & Lu, 2019). For example, this automated approach has been used to assess levels of social and physical disorder (Badland, Opit, Witten, Kearns, & Mavoa, 2010; Rundle, Bader, Richards, Neckerman, & Teitler, 2011), the presence or absence of public facilities (Clarke, Ailshire, Melendez, Bader, & Morenoff, 2010; Kelly, Wilson, Baker, Miller, & Schootman, 2013; Wilson et al., 2012), and the degrees of urban greenness (Li et al., 2015; Lu, Sarkar, & Xiao, 2018). Recent research has suggested that SVIs is a promising and effective alternative for assessing pedestrian volume (Yin, Cheng, Wang, & Shao, 2015). To our knowledge, the accuracy of this new method has not been rigorously tested yet. Therefore, it remains unclear whether SVIs and machine learning techniques can offer a reliable approach to assess pedestrian volume with acceptable accuracy.
In this study, we conducted a comprehensive validation test by comparing the automated pedestrian volume extracted from SVIs with field observation for 701 street segments in Tianjin, China. More importantly, we identified the optimal parameters of SVIs and street features to achieve the highest level of agreement with field observation data. The SVIs parameters we considered included image source, collection time, and image quality. The street features included street length, pedestrian volume, and street connectivity. Our study contributed to the development of an innovative and efficient method to collect pedestrian volume data for any location covered by SVIs worldwide. More importantly, this study can help researchers and planners to create healthy cities and promote physical activities by identifying which areas have high or low pedestrian activity, and by highlighting which built environment characteristics facilitate or hinder walking in the long run.
Section snippets
The importance of walking
The last few decades have witnessed a substantial decline of engagement in physical activity by urban residents, and this decline has serious effects on the population's health (Koohsari, Badland, & Giles-Corti, 2013; I.-M. Lee et al., 2012). A global survey indicated that 31% of the world's urban population fails to meet the recommendation of 150-min moderate to vigorous physical activity per week (Hallal et al., 2012). Many developing countries such as China and India have undergone rapid
Study area
This study was conducted in Tianjin, China, one of four municipalities directly administered by the central government. It is a large city in northern China, with a population of 15.5 million in 2015 (Liu, Gao, & Wang, 2018). Tianjin remains a monocentric city, and this study focused on several parts of the downtown area (Fig. 1). The “street segment” was used as the unit of analysis, which was defined as the portion of street between two adjacent street intersections. The lengths of these
Results
The descriptive statistics are presented in Table 1. Most of the sampling street segments were covered by Tencent service in 2014. However, the coverage of Baidu Street View was more inconsistent in different years. A total of 4507 sampling points from 701 streets were used in this study. As a result of SVIs availability, 4840, 3440, 4134, and 7376 images were collected from Baidu 2013, 2015, 2016, 2017 datasets respectively. A total of 9012 images were collected from Tencent 2014 dataset.
Discussion
Previous studies have verified the feasibility of using SVIs to conduct environmental audits, such as audits of general neighborhood environments (Charreire et al., 2014; Rundle et al., 2011), street greenery (Lu, 2018), and open skies (Yin & Wang, 2016). However, most of these studies have focused on static environment features, rather than dynamic information such as pedestrian volumes, which constantly fluctuate over time and space. In this study, we sampled 701 street segments in Tianjin
Conclusion
The traditional method of assessing pedestrian volume with field observations is often time-consuming, labor-intensive, and with limited study areas. In this study, we have investigated the accuracy of an automated method for assessing pedestrian volume. This method uses Tencent and Baidu SVIs with machine learning techniques, to overcome the limitations of field observations. Our results demonstrated that overall, the new method provided acceptable or good levels of agreement with field
Declaration of Competing Interest
None.
Acknowledgements
The work described in this paper was fully supported by grants from the National Natural Science Foundation of China (No. 51778552), the Research Grants Council of the Hong Kong Special Administrative Region, China (No. CityU11666716).
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