Ground level permeability as indicator of walkability in cities

Abstract

For centuries, urban developments were dimensioned on a human scale. Cities had a limited number of inhabitants and a size which was not convenient to overtake so as not to exceed a balance point. Over time, after important discoveries in the field of transportation, these limits blurred. Mechanical transport systems made possible the expansion of cities because private cars allowed people to move easily and at a reduced cost. Then, the prioritization of vehicle traffic in streets grew at the expense of pedestrians. Street quality decreased gradually with this change, as authors like Jane Jacobs warned in the middle of the last century.

After decades of urban sprawl, academic and political institutions call today for a totally opposite model. A shift towards compact cities, with mixed use neighborhoods and walkable streets is aimed nowadays. The study presented in this paper is focused on the last point, the promotion of streets designed for people to walk, enjoy and feel secure.

Walkability is a concept that integrates several aspects related to street design with the aim of inviting people to move on foot. One of the aspects that affect walkability is the direct relationship between pedestrians and ground floors, both regarding the appearance and permeability of façades. Permeable ground level façades provide a connection between pedestrians and the surrounding buildings, making them feel secure and comfortable in the street.

Some institutions have developed urban sustainability assessment tools which evaluate walkability of urban developments, among other parameters. Usually, a score system quantifies the degree of compliance of some metrics related to walkability. In this study, two metrics from the TOD Standard 3.0 assessment tool were applied to a case of study, both referred to ground level permeability. The objective is double: testing the system itself by getting some reference values and finding out the relation between the values in detail and their position along the street.

The assessment was carried out in two streets in Gràcia neighborhood, Barcelona: Verdi and Bruniquer-Terol-Ros de Olano. The streets are perpendicular, 5-10 meters wide and 780 and 882 meters long. Gràcia is considered a vibrant, active, compact and mixed-use neighborhood, so that it would be expected to achieve a high score in ground level permeability.

The chosen parameters are metric 1.B.1, “Visually active Frontage” and 1.B.2, “Physically Permeable Frontage”. The metrics were applied to the mentioned streets in a field study following the procedure defined in the standard. A series of pictures and a floor plan of the zone were used to this end. The result of 1.B.1 metric was a 100% of visually active frontage in both streets, which means that all segments or blocks have more than 20% of transparent length. This value corresponds to the maximum value in the score. In the case of 1.B.2 metric, the number of entrances per 100 meters of block frontage is 17 in Bruniquer-Terol-Ros de Olano and 20 in Verdi. The required value to obtain the maximum score in the TOD Standard is 5 entrances per 100 meters, so both streets are much over the score. According to both metrics, walkability would be widely reached.

The analysis of each segment showed a relationship between ground level permeability and two features: plot distribution and the age of the building. Apart from this, an important question arose when applying the procedure, the influence of time; whether it be the time of day, the time of the year, or the situation of the premises on the ground floor according to market fluctuations. Depending on them, the data acquisition on field might change the result.

The application of walkability TOD Standard metrics in the field has resulted to be an easy and useful tool to analyze ground level permeability, though subject in some way to the criteria applied in the data acquisition. The values obtained have been much higher than the maximum required in the score, so compact cities may need another range of values to provide a subtler definition of walkability because the score seems to be out of scale. The application in this field study has shown that the score is bound to the context and that a tool may consider that the nature of cities is variable.