Exploring Enclosure in Urban Design: Beyond the 3:1 Ratio

Written by Sif Høg, 12/08/2023

NotJustBikes is a Netherlands-based youtuber creating videos about urban planning in the Netherlands from the perspective that there are more interesting aspects to the Netherlands than the popularity of bikes as a choice of transport. In the video Designing Urban Places that Don't Suck, they mention a rule of thumb often found in urban planning documents: that there should be no more than 3:1-ratio between street width and building height, and no more than a 6:1-ratio for public squares. This guideline appears in the U.S. Green Building Council guidelines for Walkable streets and Southampton's design guide that argues that enclosure with the 3:1 ratio makes routes "easier for people to visualize and remember," expresses "the importance of each street," and "avoid the gaps which can make places seem unsafe"(Ellison 2004, 3.2.5).

As NotJustBikes mentions in the video, there are examples of spaces with a great sense of place that deviate from these ratios, such as St. Peter's Square in the Vatican, where the ratio is 12:1. So, while these ratios are guidelines rather than hard rules, I am curious to dive into the research of them, and learn why they tend to work so well.

These ratios are based on the concept of a sense of closure as it is claimed by the urban design guidelines of the Scottish Government (Haile 2019), and the Urban Design Guidelines for Greenfield Neighbourhoods by the City of Ottawa defines a sense of enclosure as:

when buildings physically define public spaces particularly through proportions between height and width in an area to create places that are comfortable to pedestrians. (City of Ottawa 2023)

However, both Haile's blog post and a research article exploring and analyzing these ratios observe that these claims of the benefits of the ratios historically often lack supporting arguments or are based on the intuition of some urban designers (Haile 2019; Kim & Kim 2019, 1). Kim & Kim trace the discussion on ratios between building distances and heights back to the 19th-century urban theorist Camillo Sitte, who suggested a ratio between 1:1 and 2:1 as the perfect ratio area based on his gut feeling.

Kim & Kim discuss some of the issues with finding "the optimal" ratios. One issue is that different studies have judged the spaces on different qualities such as "openness, coziness, goodness, suitability for optional activities, suitability for informal social activities, and suitability for formal social activities." I want to question any study that tries to quantify very abstract qualities such as cosiness and particularly goodness. Another issue is that aesthetical preferences seem to vary between different groups differ in what ratio they prefer in an urban space. One study even shows different preferences between experts in urban design and laypersons (Kim & Kim 2019, 5. discussion; Ghomeshi & Jusan 2012, 952-64). In addition to all the claims and studies pointing in the direction of different ratios, Kim & Kim’s experiment concludes that their experiment participants preferred smaller ratios than those suggested by previous theorists when considering openness, cosiness, overall goodness, and suitability for three types of optional and social activities.

Considering all of this, I do not think it is very useful to try and find an optimal ratio. Instead, I think it is worth developing tools to qualitatively analyze a space to help us decide what ratio to aim for. I can agree that a certain ratio influences the sense of closure, and that closure can help make an urban space distinct. However, we also have to consider that a ratio is not the only factor playing into a sense of closure and that a sense of closure is not the only thing that can make space distinct, and closure is not necessarily a criterion for a successful urban design.

In the case of St. Peter's Square, NotJustBikes mentions the obelisk in the middle of the square that breaks up the space (NotJustBikes 2023, 1:55). While I do acknowledge that the obelisk plays a role in preventing the space from appearing empty, I do not find that the square is successful because the obelisk supports a sense of closure. Rather, I would hypothesize that it is the great openness to the Godly heaven above framed by sacred statues and ornaments, that makes the square successful for its purpose: to serve as a venue where masses gather for religious ceremonies, to act as an entry for the St. Peter's Basilica, and to represent the grandeur and significance of the Catholic Church. St. Peter's Square is well designed, not because it comforts to an ideal of a building ratio and a sense of closure, but because the design (including the ratio) supports the square's ability to fulfil its purpose.

It appears that the search for a single optimal ratio for urban spaces may not be practical or effective. Different preferences and contexts necessitate considering multiple ratios rather than a universal one.

So, what have we learned from this little study on ratios in urban design? I hoped to learn about the benefits of this widely accepted 3:1 urban planning principle. Instead, I found that there is no scientific basis for it. However, I did learn from the recent psychological articles attempting to determine an optimal ratio for citizen experience, that both the ratio itself and the level of enclosure are significant factors for the experience of the space. Rather than simply determining whether or not a space is enclosed and deeming it satisfactory if it is, I want to encourage that we consider if and how enclosure would benefit a space's overall quality considering the function of the space and the specific people that occupy the space.

References

  1. City of Ottawa. 2023. "Urban Design Guidelines for Greenfield Neighbourhoods." City of Ottawa. https://ottawa.ca/en/urban-design-guidelines-greenfield-neighbourhoods. Accessed 03-08-2023.
  2. Ellison, Mark. 2004. "Chapter 3: Continuity and Enclosure." In The Development Design Guide. Southampton City Council: City Development and Economy.
  3. Ghomeshi, Mohammad, and Mahmud Mohd Jusan. 2012. "Investigating Different Aesthetic Preferences Between Architects and Non-architects in Residential Façade Designs." Indoor and Built Environment 22(6): 952-64. https://doi.org/10.1177/1420326X12458513.
  4. Haile, Christ. 2019. "A Myth of Urban Design: The ‘Sense of Enclosure’ Theory." Chris Haile: Making Plans for Better Urban Futures, 18-04-2019. https://www.chrishaile.com/a-myth-of-urban-design-the-sense-of-enclosure-theory/. Accessed 03-08-2023.
  5. Kim, Jaecheol, and Seungnam Kim. 2019. "Finding the Optimal D/H Ratio for an Enclosed Urban Square: Testing an Urban Design Principle Using Immersive Virtual Reality Simulation Techniques." International Journal of Environmental Research and Public Health 16, no. 5: 865. https://doi.org/10.3390/ijerph16050865.
  6. NotJustBikes. 2023. "Designing Urban Places that Don't Suck." 30-07-2023. Youtube video, https://youtu.be/AOc8ASeHYNw.
  7. U.S. Green Building Council. Walkable streets: Location and Transportation. https://www.usgbc.org/credits/lt7. Accessed 03-08-2023.