1. Introduction
Vitality is defined as the degree to which a settlement form supports life functions, ecological requirements, and human capabilities [
1]. Urban vitality refers to the frequency and diversity of various economic, cultural, and social activities in a city, arising from the various social activities undertaken by residents in public spaces and serving as an important indicator to measure the level of urban development and competitiveness [
2,
3,
4,
5]. Within cities, areas with high vitality offer numerous advantages. Economically, these areas tend to be business hubs, hosting multiple employment opportunities and a range of comprehensive service facilities [
6]. Culturally, these areas often offer a variety of activities that bolster the efficiency of residential interactions, thereby fostering innovation and entrepreneurship [
7]. Socially, these areas are characterized by frequent interpersonal communication and strong social cohesion [
8]. In recent years, rapid urbanization has propelled the level of urban development. However, it also poses a risk to urban vitality [
9,
10], particularly manifested in the gradual decline of vibrant traditional urban streets [
2]. Street vitality, as a part of the urban space, is an important aspect of urban vitality [
11]. Street vitality encompasses the extent of diversity and frequency of street activities, including shopping, commuting, socializing, and cultural engagements [
2,
5,
12]. Streets with high vitality are typically highly attractive to residents, and can bolster happiness and community cohesion [
8,
13,
14,
15]. Street space is now widely recognized in the field of urban planning, and street vitality has emerged as a key issue in urban development. Urban planning theories, including New Urbanism and smart growth, emphasize the importance of street vitality [
16,
17]. Since 2000, various countries have implemented urban street design guidelines to steer and standardize the construction of local streets in alignment with local conditions, with the goal of creating more attractive streets [
18].
Urban construction in China has shifted from incremental expansion to enhancing existing infrastructure, and urban renewal has emerged as a pivotal approach to urban development [
19]. During urban renewal, the vitality of urban public spaces, particularly street vitality, plays a vital role in accommodating residents’ needs at various levels, and is essential for optimizing urban development [
20]. Pedestrians are the main group of people who use streets for various activities. The number of pedestrians on the street and the frequency of their activities directly reflect street vitality; more pedestrians indicate more social interactions, commercial activities, and cultural exchanges, i.e., higher street vitality [
2,
3,
21]. Recently, cities such as Shanghai, Wuhan, and Xi’an in China have introduced street design guidelines that prioritize pedestrian needs and strive to balance traffic efficiency with pedestrian experience. Consequently, it is important to precisely assess the vitality of urban streets, investigate the determinants of street vitality, and devise strategies to invigorate them, thereby enhancing the quality of urban public spaces and achieving superior urban development.
Research on urban street vitality measurement has evolved from subjective to objective approaches, from qualitative to quantitative methods, and from traditional survey data to leveraging big data. Initially, studies typically obtained data through field observations [
22], interviews [
23,
24], and questionnaire surveys [
25] to measure street vitality. While these techniques can directly capture citizens’ subjective perceptions and assessments, they are inefficient and unsuitable for sustained large-scale research. The onset of the big data era has prompted researchers to use multiple datasets, including real-time user datasets (RTUDs) [
26], mobile phone data [
27], and social media check-in data [
28], to assess street vitality. Big data is characterized by its real-time nature, wide scope, and high accuracy, and allows for integrated spatiotemporal analysis, enabling more extensive and prolonged research.
Building on this foundation, scholars have conducted research on the determinants of street vitality, with a primary focus on locational conditions and built environment. Considering locational conditions, streets in prime locations—namely, those proximate to service amenities, including metro stations or commercial entities—are deemed to possess greater vitality [
29,
30,
31,
32]. Regarding the built environment, small-scale and compact streets are viewed as more dynamic because of their moderate construction density and appealing design [
3,
33,
34,
35]; simultaneously, urban streets that offer a variety of functions and mixed uses are perceived as more vibrant [
2,
16,
23,
36]. Moreover, streets that facilitate efficient commuting and those with a more transparent ground–floor interface are considered livelier [
22,
34]. Ultimately, to invigorate street vitality, urban planners have enacted measures for city development, including the integration of diverse functional zones, the expansion of pedestrian areas, and the enhancement of transportation access. In fact, along with the dimensions of location conditions and built environment, residents’ subjective perceptions also significantly influence street vitality. Relevant studies demonstrated that residents’ subjective experiences with urban streets, including comfort, security, enjoyment, and satisfaction, influence their selection of streets for diverse activities [
37,
38,
39,
40,
41,
42]. However, the impact of residents’ perceptual factors on street vitality has not received sufficient attention. Therefore, unlike previous studies, the dimension of residents’ perceptions was taken into account based on location conditions and built environment. Four variables related to residents’ perceptions were selected for this study as potential influences on street vitality, making the indicators of influence richer and more comprehensive.
Contemporary research on street vitality has employed two scales. The first is the block scale, which typically examines a specific street or cluster of streets, concentrating on street interface elements and residents’ particular activities, albeit with a limited scope and sample size [
13,
43]. The second is the urban scale, which encompasses the city’s entire street network and often utilizes large-scale grids, such as a 500 m grid, to delineate streets and assess vitality [
44,
45]. However, these grid units exceed the street scale, posing challenges for the precise capture of street vitality and its critical determinants. To strike a balance between accurately capturing street-scale changes in vitality and the operability of data processing and analysis, a small-scale grid of 100 m was selected as the study unit. This study first uses a 100 m grid for the analysis of street vitality, primarily because it strikes a balance between capturing subtle spatial changes and keeping data processing and analysis tractable. The 100 m grid coincides with the street scale and provides a more accurate picture of street vitality while avoiding excessive computational complexity.
In summary, this research utilized the old city of Nanjing as a case study, using a 100 m fine-grained grid as the analysis unit. Initially, the RTUDs were employed to assess temporal and spatial patterns of street vitality in the old city of Nanjing. Subsequently, a range of determinants was analyzed using travel cost data, road network data, point of interest (POI) data, street view image (SVI) data, and social media commentary data. By integrating a spatial autoregression model (SAM) and a multiscale geographically weighted regression (MGWR) model, this study investigated the factors influencing and spatial variances of street vitality, to yield precise insights. This study addressed three key questions: (1) what are the characteristics of the spatial distribution of street vitality in the old city of Nanjing; (2) what are the factors impacting the vitality and the spatial variance in terms of their quantitative influence; and (3) what strategies can be employed in urban planning to enhance urban vitality?
4. Discussion
4.1. Spatial and Temporal Distributions of Street Vitality
This study revealed a distinct, single-center spatial pattern of street vitality within the old city of Nanjing, China. Vitality was highest in streets near the primary urban center, Xinjiekou, and there was a clear clustering effect, suggesting that high vitality in one area can positively influence the vitality of neighboring regions. The concentration of commercial and office spaces around Xinjiekou draws substantial foot traffic for diverse activities, intensifying street vitality and establishing Nanjing’s central activity zone. A comparison of working day and weekend vitality measurements revealed that the tertiary business districts of Hunan Road and Fuzimiao and the quaternary business districts of Ruijin Road and Zhongyangmen had higher vitality than other areas on weekends, underscoring the significance of CL in influencing street vitality during weekend periods. Furthermore, vitality on weekends was more unpredictable than on working days, indicating greater diversity in public activities during weekends.
4.2. Factors Influencing Street Vitality
Overall, FD, TL, and RAR were positively correlated with enhanced street vitality. Streets endowed with ample amenities, prime positioning, and accessible commuting options typically offer a higher caliber of services, thereby attracting more residents and leading to greater vitality, as indicated in previous research [
23,
27,
29,
30,
36]. FD was strongly associated with street vitality and was more concentrated on working days and more scattered on weekends, possibly due to the more routine nature of resident activities on working days as opposed to weekends [
62]. In the proximity of very few large entities and parks, FD was negatively associated with street vitality. This association can be attributed to the stable pedestrian traffic in large built establishments, and expansive parks draw substantial crowds to adjacent zones, resulting in pronounced vitality, albeit with a typically limited number of POIs. TL had a significant positive impact on street vitality generally, and on working days this impact was evenly distributed within the study area, while on weekends only areas near business districts and some metro stations exhibited a positive impact, suggesting that residents have a greater preference for commercial services on weekends. The negative impact of TL on street vitality was particularly pronounced around Zhongyangmen and Sanpailou on weekends, possibly due to the large bus terminal and wholesale market near the central gate, attracting a large number of people even though these areas are some distance from the metro stations in the study area. In addition, the established residential community in the Sanpailou area was serviced by comprehensive amenities, diminishing its reliance on metro stations.
The SSI emerged as a strong correlate of street vitality, indicating that streets capable of delivering superior services hold greater appeal for residents. Recent research has demonstrated that residents’ evaluations of their experiences significantly influence waterfront vitality [
42]. The regression coefficient for this index was particularly high on weekends, indicating that residents have a higher demand for service quality during their leisure time. Previous studies have noted that higher visual complexity equates to a richer visual experience, enhancing the likelihood of lingering [
39,
40]. VCI was positively correlated with street vitality, indicating that visually diverse streets are more enticing to residents. RD was positively correlated with street vitality, and this association was more pronounced on working days than on weekends. The same relationship was observed for TL and RAR, indicating that commuting activities are significant indicators of street vitality [
66]. Under the time constraints of working days, residents preferred streets that facilitated easy commuting, as per previous research findings [
34,
37]. The MGWR findings showed that EI was positively associated with street vitality on working days and weekends, perhaps because a higher degree of enclosure facilitates certain social activities that require a sense of bounded privacy, thereby attracting residents to linger and engage in the area [
57].
Research on the association between CL and street vitality has been relatively limited, and the majority of findings suggest that streets closer to business districts possess higher vitality [
31,
32]. The present study showed that CL was positively associated with street vitality on weekends and negatively associated with street vitality on working days. People may be more inclined to spend their weekend leisure time in commercial areas when they have more time and energy to utilize the commercial services [
66], whereas on working days, they likely opt for neighborhood businesses or online shopping to meet their basic needs. This finding reflects the diversity of residents’ shopping and consumption choices.
FM was positively correlated with street vitality on working days but was not correlated with street vitality on weekends, suggesting that residents prefer streets with diverse functions when leisure time is limited. Given that this study encompasses both the street and its surrounding areas, the increased presence of residents in residential areas on weekends compared to working days may render the influence of this metric on vitality less pronounced [
67].
The observation that GVI was negatively correlated with street activity differs from previous studies [
38,
39]. There are two potential reasons for this phenomenon: (1) Pertinent research indicates that a GVI of 15% represents a threshold in residents’ perceptions of urban greening and that beyond this point, urban greening enhances the psychological and physiological well-being of residents [
68]. Once the GVI surpasses this threshold, the correlation between the extent of greening and its advantages assumes an “inverted U-shape,” with approximately 24% being optimal for resident well-being [
69]. In the present study, the average GVI in the old city of Nanjing was 23.58%, with 66.84% of the grids exceeding 15% and 42.25% exceeding 24%, suggesting that urban greening in the old city of Nanjing is quite advanced and that the GVI is not a primary driver of residents’ selection of streets for activities. (2) An overabundance of urban greenery might constrict informal commercial spaces, such as the street economy, thereby diminishing street vitality [
70,
71]. (3) Streets with high GVI require more resources for maintenance and management. If poorly managed, this may result in cluttered green areas, which, in turn, can affect the aesthetics and the experience of using the street [
72].
Additionally, PR was not strongly associated with street vitality, contrasting with an earlier empirical study in Osaka which found that permeability along the street interface positively impacted pedestrian activity [
13]. However, that study focused on areas adjacent to commercial complexes, whereas our research encompassed streets across the old city without differentiating between street types. Consequently, we suggest that permeability only affects the vitality of streets of a commercial nature.
4.3. Strategies for Enhancing Street Vitality
As forerunners in modernization efforts, old cities should seize the opportunities presented by urban renewal to enhance street vitality and urban spatial quality, thereby meeting the diverse needs of residents. Strategies to enhance street vitality should be approached in a number of ways.
Firstly, increasing functional density is crucial. The attractiveness of streets can be significantly enhanced by adding a wide range of facilities such as dining, retail, entertainment, and cultural facilities, particularly in areas with high pedestrian flow. The flexible utilization of vacant buildings and sites to create temporary markets, exhibitions, and event spaces can also increase street vitality. Furthermore, street vitality can be enhanced by increasing the number of mixed-use neighborhoods that offer a variety of services and activities, especially in areas with high weekday pedestrian traffic.
Secondly, urban planning should draw inspiration from the principles of New Urbanism [
16,
17] by employing a transit-oriented development (TOD) model to bolster the construction of rail transit stations. Such a model advocates for compact development, integrating mixed functions with small block construction and dense road networks [
33,
34]. This approach aims to reorganize street and alley spaces to enhance the connectivity and accessibility of street networks, create social interaction spaces on the streets, and improve traffic efficiency. Additionally, in neighborhoods farther away from metro stations, the focus is on the improvement of facilities to neutralize the negative impacts of traffic locations.
Thirdly, while promoting the prosperous development of the city center (such as Xinjiekou), this approach should also stimulate the growth of other commercial centers at various levels. The construction of a multi-tiered, systematic commercial service facility system is proposed to meet the diverse needs of residents and foster social interactions within the region. Simultaneously, by increasing street furniture and vignettes and encouraging diversified architectural designs, the visual complexity of streets can be increased, thus enhancing vitality.
Fourthly, appropriate street greenery is conducive to street vitality, supporting the development of the street economy and promoting related commercial policies. However, it is important to moderate street greening to ensure sufficient space for street-level commerce.
Finally, focusing on residents’ subjective experiences is beneficial for human-centered planning. Thus, construction of urban street facilities should meet qualitative as well as quantitative targets. To ensure the adequacy of facility numbers, service quality should be gradually enhanced. A feedback mechanism for residents should be established to improve services in a timely manner and enhance user experience. The goal is to build a city with a “sense of warmth”, thereby strengthening residents’ sense of belonging.
4.4. Limitations
Although we conducted an in-depth study on street vitality and its influencing factors, this study has several limitations. Firstly, the concept of vitality was initially intricate and expansive. Street vitality, as referenced herein, encompasses both the street’s inherent vibrancy and that of its contiguous lands, and the vitality of adjacent areas may affect indicators such as FM. Secondly, the Tencent EasyGo data used in the study do not delineate pedestrian pathways. In addition, situations in which children or other vulnerable groups do not use the Internet services provided by Tencent were not considered. It is recommended that further research on this topic involves integration of data from diverse sources, including heat maps, mobile phones, and social media check-ins. Where possible, field research methods should be incorporated to obtain comprehensive vitality measurements. Moreover, in
Section 4.2, we speculate that the influence of GVI on street vitality shows an “inverted U-shaped” relationship, and that PR only influences the vitality of streets of a commercial nature. These speculations need to be explored further. Lastly, the SAM and MGWR models used in this study also have some limitations. The results of the SAM can only reveal the extent of the independent effects of factors on street vitality, and future studies may consider using a geographical detector model to explore the comprehensive effects of different combinations of factors. Although the MGWR model can reveal the spatial heterogeneity of street vitality, it also varies temporally. Therefore, combined with more temporal data, the geographically and temporally weighted regression (GTWR) model is required to obtain a comprehensive analysis of the temporal changes in street vitality.
5. Conclusions
To gain a comprehensive understanding of the characteristics, influencing factors, and enhancement of street vitality in an old city, this study applied a refined scale of analysis, using smaller grid units than previous studies and high-resolution RTUDs to achieve a detailed measurement of street vitality. A more comprehensive set of influencing factor variables was selected from the perspectives of three categories: locational conditions, built environment, and residents’ perceptions. Additionally, the SEM and MGWR models were employed to ascertain the factors associated with street vitality and their complex spatial differences.
Several conclusions were drawn from this study. Firstly, street vitality in the old city of Nanjing is monocentric during both working days and weekends, with high vitality areas focused around Xinjiekou, and with greater regularity and predictability in street vitality on working days than on weekends. Secondly, factors such as TL, RD, FD, FM, RAR, EI, VCI, and SSI are positively associated with street vitality. Among them, the association of TL and RAR with street vitality was more pronounced on working days, whereas FD and SSI were more strongly associated with street vitality on weekends. Thirdly, GVI has a negative impact on street vitality, which may be due to the fact that the green environment of streets in the old city of Nanjing has reached a high level, and GVI is not the primary factor affecting residents’ choice of streets for their activities. Fourthly, because CL has a positive impact on street vitality on weekends and shows the opposite result on working days, a big difference is observed between residents’ shopping demands on working days and on weekends. Therefore, it is necessary to build a multi-level and systematic commercial service facility system. Finally, the influence of different factors on street vitality varied according to spatial location; TL was most strongly associated with street vitality in the southern part of the old city, while FD was most strongly associated with street vitality in the Xinjiekou area.
In summary, against the backdrop of urban development transitioning from expansion to intensification, policymakers and urban planners should recognize the significant role of old city streets in urban spaces. Comprehensive revitalization of the vitality of old city streets can be achieved by adopting a public transportation-oriented development model and constructing mixed-use, compact neighborhoods with dense street networks, ensuring appropriate street greening to support the street economy, and enhancing the quality of urban street service facilities, thereby promoting high-quality urban development.