The numbers behind transect classifications

Understanding the numbers behind transect zones helps communities plan better and create places.

The numbers behind transect classifications

Creating quality places requires quality form.  Form is not just how one builds and places buildings.  It is also dependent on how one builds streets as they come together to create blocks.  Just like with buildings, there are key design principles (how long, wide, a block is, its perimeter distance) that go into creating a successful place.

One of the tools commonly used in placemaking in Michigan is the transect.  The rural to urban transect is divided into six transect zones based on intensity of the built environment and physical and social character.  One of the underlying principles of the transect is that certain forms and elements belong in certain environments.  For example an apartment building belongs in an urban setting and a farm belongs in a more rural or working lands setting. As transect zones become more urban they also increase in complexity, density and intensity. These zones in the transect are often referred to as natural (T1), rural (T2), suburban (T3), general urban (T4), urban center (T5), urban core (T6), and special district (SD).

But how does one determine the difference between urban and suburban or center and core.  Block and street sizes are one of the indicators used to differentiate between transect zones.  Another indicator typically used is looking at building height.  Natural and rural are typically not measured with the above indicators, but are defined by character. 

Road miles per square mile (measured at the centerline)

Equivalent block length

Building height

Transect zone

12

800

1-2

T3 Suburban

14

700

1-2

T3 Suburban

16

600

2-3

T4 General urban

19

500

2-4

T5 Urban center

23 & greater

400 and less

4+

T6 Urban core

 
These measures are important because they have a direct impact on several factors.  Block length affects lot size which affects density, and with lower density (fewer units per square mile) you get less diversity of uses that can be supported. Road miles and block length effect connectivity which impacts walkability.  The more road miles and shorter block lengths lead to a  denser and more compact structure within the block.

The purpose of a denser and more compact structure within a block is to maximize connectivity and access.  One of the most historical and successful block patterns is Savannah, GA.  Laid out by General Oglethorpe, there were rules for streets, lots, and buildings that resulted in a pattern repeated multiple times over and intersected with broader, tree lined boulevards as thoroughfares through the neighborhood.  The street network is laid out with one acre blocks.  Intersections occur every 350 feet (block length).  Greens and plazas are interspersed within the network. This design provides a lot of terminal vistas. Savannah’s pattern places it squarely in an urban transect zone.  Many older communities were laid out in a similar grid pattern but with rectangular blocks. 

Building height helps create enclosure of the space. For a more detailed discussion on enclosure see the article by Michigan State University ExtensionGetting enclosure right: creating a comfortable public room”.

When coordinated with building setbacks from the street the building’s height creates the sense of privacy that helps shape character of a neighborhood.  Lower building heights and larger setbacks create more of a sense of privacy for the structures.  Conversely, higher buildings and smaller setbacks create more interaction between buildings and the public space.  Suburban areas are characterized by lower building heights and larger setbacks while urban areas are more enclosed with taller structures and smaller setbacks. 

Understanding the numbers behind transect zone classification can help communities get beyond subjective measures and move toward using numbers to help plan and code for place and community. 

For more information transects and placemaking as part of community planning  contact Michigan State University Extension or contact a Land Use Educator for more information on these issues facing communities. 

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