## 2 Literature Review  
### Perceived Spatial Quality of Urban Streets  
Street space consists of streets and the various elements along them. The capacity of street space is the ability of the street to sustain a particular scale of people passing, staying, and performing other activities, which is the premise and foundation of all action [12]. Early studies on urban street space were mainly based on urban macro theory. _The Image of the City_ categorized urban spatial elements into five categories: roads, boundaries, zones, nodes, and markers [13]. In the early days, the study of urban streets was closely related to urban roads, and these features were considered to be part of the community [14]. [PERSON], proposed that streets and squares are the main factors in city composition and that other facility functions are arranged around them [15]. In terms of discussing urban streets and their related elements, [PERSON] mentioned that the active factors of the street landscape can increase the appeal of street space [16]. At the same time, the sense of spatial enclosure is also closely related to the oppressive perception of pedestrians in the street [17]. Inthe field of urban morphology, streets constitute important public spaces. In the relevant description of the term street (cadastral) model, the space between blocks is the public space network, while the urban street network model uses different categories. For example, rule grids composed of geometric laws are generally planned. With the organic development of urban roads, the organic or deformed grid becomes another form of street grid. Perception is the result of human process filtering, which is due to the environment inspiring emotions and providing people with more information than human processing power can handle at once [18]. Whether the city streets are interesting or not can reflect whether the city is interesting or not. Therefore, the quality perception evaluation of streets can reflect the influence of the environment on people and can also be used to understand the spatial quality of a city. In the macro dimension of urban activities, street spatial perception research based on GIS, which utilizes road network data, POI data, etc., is more commonly used as the basis of perception research [19]. Some scholars have also constructed a street walkability measurement model including feasibility, accessibility, safety, comfort, and pleasure by combining street view maps and Arc GIS [20]. On the pedestrian scale, some scholars have calculated urban spatial measurement indicators such as the street aspect ratio, green viewing index, and sky visual factor, and then explored their relationship with spatial perception [21; 22; 23]. The most direct manifestation of pedestrians' perception of street space comes from their vision. Visual appropriateness is important for the places most likely to be visited by people from wider and various backgrounds [25], and, obviously, urban street spaces are such places. [PERSON] pointed out that, besides being a natural element of the city [26], streets are also a social factor, inseparable from the social, economic, cultural, and historical background. It is noteworthy that research on street perception varies depending on the cultural context. Western and Eastern nations have differing starting conceptions of urban planning, which affects the layout of urban streets and how people perceive space. Due to varying environmental experiences and personal evaluation standards, it is conceivable that different social groups will have diverse opinions about the same location. In the West, street space is an important space for communication and gathering. On the contrary, in Chinese history, under the long-term influence of the ritual system, people possess a strong sense of the boundary between the inner and outer urban spaces. As a result, street space was not valued for a long historical period, unlike Western street space which has diversified functionality and high vitality.