A review of variables of urban street connectivity for spatial connection

Several studies on street connectivity in cities and towns have been modeled on topology, morphology, technology and psychology of people living in the urban environment. Street connectivity means the connection of streets that offers people alternative routes. However, there emerge difficulties to determine the suitable variables and analysis to be chosen in defining the accurate result for studies street connectivity. The aim of this paper is to identify variables of street connectivity by applying GIS and Space Syntax. This paper reviews the variables of street connectivity from 15 past articles done in 1990s to early 2000s from journals of nine disciplines on Environment and Behavior, Planning and Design, Computers, Environment and Urban Systems, Applied Earth Observation and Geo-information, Environment and Planning, Physica A: Statistical Mechanics and its Applications, Environmental Psychology, Social Science and Medicine and Building and Environment. From the review, there are four variables found for street connectivity: link (streets-streets, street-nodes or node-streets, nodes-nodes), accessibility, least-angle, and centrality. Space syntax and GIS are suitable tools to analyze the four variables relating to systematic street systems for pedestrians. This review implies that planners of the street systems, in the aspect of street connectivity in cities and towns, should consider these four variables.


Introduction
People use street as a space that involves action and reaction; space for a movement to find a place; space for a meeting to interact with others; space for a recreation activity; space for people to appreciate and remembers their historic. Thus, street is an important public space for human lifestyle in cities and towns. A street connects people from one point to a destination in any activities [1]. It is also identified as a space between blocks [2] and as the edge of block [3] in cities and towns. Studies related to space syntax represent streets as straight-lines connecting nodes such as junction to form a street network [3][4][5][6][7][8][9][10]. Thus, reflection of people movement through the street network for their wayfinding to the destination is influenced by its connectivity. 3 To whom any correspondence should be addressed. Good street-network design formed from the consideration of connectivity of streets. Connectivity exists when a subject linked to others in any form, for example, when people move from point A to point B, they need to pass through street A, street B and street C. The situation shows how street name of street A, street B and street C are connected. In the study done by Jiang and Claramunt [7], street connectivity is defined as the number of streets directly linked by a node. This means that connectivity can be formed by street function or character of the environment linked. For example, street connectivity form in intersection such as junction [9]. There are varies studies utilize street connectivity as their parameter, for example, researchers in planning and transportation used street connectivity as a key aspects for creating walkability indices. A study by Leslie et al. [11] reveals that connectivity of street determines the mean value for the high-and low-walkable neighbourhoods in Australia and North American regions [11]. Other study done by Tomko, Winter and Claramunt [3] found that street connectivity promotes the movement flow that capable to educate people to experience the streets and generate cognitive of spatial knowledge. In the view of spatial study, recent developments of the spatial information science have introduced the identification of street connectivity. Spatial layout of street is generated based on the psychological reaction of people mind to an object either in their interest or not to a feature in streets. Thus, the identification capable to determine the factors that might increase or decrease the probability of people being physically active according to selected spatial units of interest in connected street [10]. This shows a sign that spatial may connected in streets.
Nowadays, studies on street connectivity have achieved another level where the computer is used analyze the street connectivity of a city or town. Geographic Information System (GIS) and Space Syntax are applied to analyze street network. These applications have improved the studies on streets in terms of to standardize the analysis and the validity of the result based on the variables of street connectivity. However, there emerge difficulties to determine the suitable variables to be chosen in defining the most accurate result for a study on spatial connection in street connectivity [10]. The analytical review is required to reveal the appropriate variables for a study using GIS and Space Syntax analysis. Thus, the aim of this paper is to identify variables of street connectivity in cities and towns using GIS and Space Syntax. Hence, this paper offers a review to the variables of street connectivity that can assists researcher to determine suitable variables in studying street network in urban setting.

Methodology
Based on nine disciplines, the review refined the variables of street connectivity. The disciplines were Environment and Behavior, Planning and Design, Computers, Environment and Urban Systems, Applied Earth Observation and Geo-information, Environment and Planning, Physica A: Statistical Mechanics and its Applications, Environmental Psychology, Social Science and Medicine and Building and Environment. Fifteen collective articles were selected from years 1990s until 2000s which were conducted using online database from Science Direct, Scopus, Web of Science and Conference Proceedings. The review addressed the relationship between street connectivity variables and spatial connection served as the aspects that applicable to be used in GIS and Space Syntax software. The aspects taken into consideration were based on the most approaches benefits the street network planning and design.

Result
The study of street connectivity covers the aspects of street environment, users and structures. It includes understanding from the various perspectives in exploring the space.  Table 1.0 suggests that there is a pattern of regularity on street studies. The variables of street connectivity are related with (1) human and street, (2) form of human behavior, and (3) aspects of walkability, health, and psychology. In the aspect of human and street, Montello [12] found that connectivity means people identifying the location of a place. He tried to create a connection of human with the urban space that influenced the decision making to a target. In other words, direction and positioning of oneself on the street are the key factor of street connectivity. Focusing on street connectivity in street network, Hillier [20] reveals the idea of human behavior in space reflects to the revolution in analyzing street network through theory of space syntax. Studies on human behavior in quality of street connectivity are Peponis, Wineman, Rashid, Kim, and Bafna [13], Jiang, Claramunt, and Batty [14], Jiang, Claramunt, and Klarqvist [4], Jiang and Claramunt [1], Hochmair [15], Porta, Crucitti, and Latora [16], Porta, Crucitti, and Latora [6], Jiang [7] and Peponis, Bafna, and Zhang [2]. These articles used Hillier's theory of space syntax. Peponis, Wineman, Rashid, Kim, and Bafna [13] presented the human behavior in urban street in the form of maps. Furthermore, Jiang, Claramunt, and Batty [14] and Jiang, Claramunt, and Klarqvist [4] expanded the variables and them formalized in GIS and Space Syntax software. They have stepped up the variables to a different level where the result can be defined in computer set of analysis. Continuously, Jiang and Claramunt [1] develops a model for topological analysis of street network that refined the variables of street connectivity in form of topological network of a city or town. In other perspective, Hochmair [15] have developed novel variables by discovered Least-Angle Strategy in wayfinding approaches for unknown places. Then, new variables have been exposed by Porta, Crucitti, and Latora [6, 16,] by revealed the concept of centrality in street analysis study. These are the positive development of knowledge to the street connectivity study which potentially assist researcher to define the accuracy of street connectivity analysis in street network. Studies by Guo and Ferreira Jr [17], Borst et al. [8], Cutts, Darby, Boone, and Brewis [18], Hang, Sandberg, Li, and Claesson [19] and Lwin and Murayama [10] suggest that the street connectivity is related walkability, health and psychology of movement. For example, Borst et al. [8] defined the relation of street characteristics influence the route choice for elderly. They explain that the street connectivity is closely link with attractive features for walking that increase 'place value' and 'functional value' which project the decision making of elderly for route choice. Cutts, Darby, Boone, and Brewis [18] relate street connectivity in medication and health study where walking reduces obesity. Thus, the studies have opened the view that variables of street connectivity are related to wayfinding, place identity, urban morphology and even the economic sector. Integrating the variables is important in study of walkability. In summary, the appropriate variables for street connectivity study are presented in Table 2.

Discussion
According to Table 2.0, four variables influence the study of street connectivity: link, accessibility, least-angle and centrality. Street connectivity variable as link define as level of connection between street to streets, street to nodes or node to streets, and node to nodes which affect other aspects such walkability, wayfinding and spatial connection in streets environment. This implies that spatial connection can be measured through link as a variable of street connectivity. This is because connectivity existed in a street environment when a space is collaborated with street features in term of position, formation, and symbolism. Additionally, the level of connection produce by link may influence degree of spatial connection between streets. Besides, accessibility means the measurement and analysis spatial connection. It is the relation of a place to another place in terms of nearness and propinquity. This variable suggested because attraction in a street environment have influence on distance travel of the origin point to the destination and this widely used in other studies such as spatial analytic measure. Therefore, accessibility is a variable for study of street connectivity.
Another variable of street connectivity is the least-angle that influences people perception walking along a route. The least-angle is the acute angle between a destination direction and street to be selected in traveling to the destination. The angle to decide a route involved with percentage of perceived spatial quality in streets. This shows that least-angle is a fresh idea in measure and analyzes street connectivity for spatial connection.
The last street connectivity variable is centrality which derived from two concepts; central as being near others and central of being between or intermediary of others. Centrality looks to explain the relation between human and street elements such junctions or streets. This variable can be an analysis to determine suitable junctions' location in street base on human capability and limitation. Thus, centrality is suggested is a factor analyzing street network in a city or town. Therefore, all of the suggested variables are appropriate to measure or analyze street connectivity for spatial connection in streets environment.

Conclusion
The exposure of street connectivity variables, which are link, accessibility, least-angle and centrality, can assist researcher for the measurement and analysis of the research related with street connectivity. However, immensity and apprehension in deciding the selected variables for a research in street connectivity study is required to make sure the variables selected are appropriate to reveal the inquiry in a research. The variables are fit to be used for in wayfinding, urban appreciation, and historical investigation with street network. The analysis tools are space syntax and GIS can be used to analyze the four variables resulting to systematic street system for pedestrian.