Font Size: 

Introduction

Quality of urban public spaces are measured by their usability; spaces that the user spends time in by choice, as opposed to obligation, are considered well designed [1]. Users’ spending time at an urban public space by choice or their staying in an urban space longer than necessary is directly related to their level of comfort. User comfort in public space can be linked to physical, psychological and social conditions [2]. Departing from Jane Jacobs’ “Eyes on the Street,” theory, this study presumes a direct relationship between a space’s visibility; one of the factors that effect the psychological conditions for user comfort; with the perceived and actual safety in a public space [3]. Based on this relationship, it proposes a computational model that calculates the visibility of an urban space based on its physical properties and usage patterns; and contributes in the design process.

Aim

The conditions that determine visibility of a public, open space can be considered as: the facade openings of the buildings surrounding an urban space and their distances to the space, the activity hours of these buildings, the physical conditions of the neighbouring open spaces and their usage patterns, density of pedestrian usage of the space, existence of visual obstructions and lighting conditions. This study aims to develop a model that computes the visibility of an urban space in the duration of a day, based on these factors. It is suggested that, through this model, based on the degrees of visibility in a public space, the prediction of users’ perceived safety and thus comfort can be possible. Based on the presumption also accepted in the studies of Whyte, Gehl, Kalay and Yan that users spend longer time where they feel comfortable, the relationship between the visibility in an urban space and the length of time a user spends in the space will be observed for assessment.

It is suggested that the module becomes a visibility-safety layer as an additional input for a behaviour simulation tool for the design of urban public spaces; a comprehensive tool similar in nature to Kalay and Yan’s and Ki Lam’s Behaviour Simulation Models [4][5].

Additionally, it is proposed that the visibility maps to be computed as an output of the tool developed, can be utilised as an input in the design processes of open spaces such as kindergartens, children's play areas, school courtyards, university campuses and elderly residential facilities.

Methodological Procedures

The study consists of three stages: development of the model for the visibility computation, the computation of visibility maps of an existing urban public space, and the assessment of the visibility maps through the comparison of observed user behaviours.

For the development of the computation model, Mc Neel’s Rhinosceros software with the Grasshopper Plugin was utilised.

For this study, visibility is defined as the possibility of seeing a point from another point with the naked eye.

In the first layer of the model, the percentage of an urban space’s visibility from the facade openings of the surrounding buildings is computed. For this, the urban space and it’s surrounding buildings are modelled. The space is then divided into a grid of 60cm by 60cm, similar to the resolution used in urban space simulation studies [4][5]. A line is drawn along the width of the facade openings of the buildings facing the space, 150cm high from the ground and 50cm inside the facades; and equidistant vision points are placed along these lines. The centre of each grid square is connected to each vision point with a line; the line is eliminated if it intersects an obstruction or is longer than a maximum visibility distance of 30m.

In the second layer, the popular pedestrian routes and commonly used neighbouring open spaces are determined and populated with vision lines, again 150cm above ground. Following a similar procedure, vision lines are created connecting these points to each grid cell’s centre in the assessed space. The lines are eliminated if they intersect with obstructions or exceed a predefined maximum length.

Eventually, the total number of lines that connect to a grid cell’s centre is calculated, and divided by the maximum number of lines that connect to a grid cell in the urban space. The grid is then given a gradient of colours based on the degree of the values assigned, demonstrating their level of visibility.

Since the model works in real time, design decisions can be made through testing various operations on the 3d model, observing the visibility values that update simultaneously.

Choice of an Urban Public Space, Creating the Visibility Maps Utilising the Model, Observation and Assessment

An urban square, in Istanbul’s Karakoy neighbourhood was selected. The reason for the choice of this square is due to the high level of difference in the use of the surrounding buildings, thus the expectation of similar changes in the visibility levels of the square. Additionally, the existence of several benches in the square would facilitate an easy observation regarding the usage patterns, such as durations of use and demographics of users at different times in the day.

A table of active use schedules was created for the three adjacent buildings as well as the main pedestrian routes and the ferry port neighbouring the square. Next, using the model developed, the visibility maps were created for each time frame that indicated a change in the active use schedule.

The square was observed and user behaviours were recorded in the month of May, for 5 minute intervals during 30 minutes in each time frame defined in the usage schedules. The observations were then compared with the visibility maps of the corresponding time frames.

As emphasised in studies focusing on user comfort [1], [2], [3], the demographics of the users; preference  of sitting, standing and passing by; as well as the duration of each users stay in the square was considered as critical indicators of comfort.

Results

While the activity schedules of surrounding buildings suggested a larger degree of difference in the visibility conditions of the space, the high level of pedestrian activity created greater visibility for a longer time frame during the day. It was observed that the pedestrian flow continued, due to the popularity of bars in the Kemankes Road and the sea side restaurants that stay open after midnight, even during the critical hours of 11.00 pm and 6.am. However, the demographics of users changed, both based on the hours and within the area of the square itself.

Some of the findings were as follows: The overall number of female users decreased significantly in the night hours. Female users, when alone, crossed the square at higher speeds, were less likely to sit at the benches as the visibility decreased, and if they did, were less likely to prefer the benches close to the edges in the evening hours, where the visibility decreased greatly. Users in groups and male users spent longer time in the square and were more likely to chose the benches at the edges at all hours of the day.

Discussion

Although perceived and actual safety can be linked to several other social, cultural, political or administrative macro conditions in an open, urban space; visibility can be considered as a significantly contributing and measurable factor contributing to the safety conditions in the micro scale. Thus it can become a valuable input and be utilised as one of the many criteria in the decision making process during the design of open urban spaces. The model developed in this study aims to make this possible through providing real time output as part of a commonly used 3d modelling environment.

Besides contributing to the design process of open urban spaces, the developed tool and its output can be valuable where visibility becomes an essential requirement of the designed environment. Some examples to such cases can be: kindergartens, schools and assisted living residences for the elderly.

It is suggested that this model is integrated into a comprehensive user behaviour simulation tool as a layer of input. The study is aimed to be developed and expanded with more comprehensive case studies, observations and comparative assessments.

simulation in design, decision support systems in urban design, visibility and safety in urban design, comfort based design