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Thread: Sustainability metrics - block length and perimeter length

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    Cyburbian donk's avatar
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    Sustainability metrics - block length and perimeter length

    A few municipalities I do work in have adopted a sustainability program with various metrics. One of the metrics they are going to measure is block length and perimeter.

    I am having a very difficult time understanding and quantifying how a block length and perimeter contribute to sustainability. It is even hard to understand when the values chosen exceed the lengths in some of the most walkable areas in adjacent andn bench mark communities..

    Other than having piggy backed off of LEED and New Urbanist philosophy does anyone have any insight into how block length and block perimeter contribute to sustainability, and if they do what is a realistic value?

    For discussion purpose block length is 250 m and block perimeter is 550 metres.
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    Cyburbian Cardinal's avatar
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    Walkability would seem to be the aim, but at the same time, don't small blocks also increase the length of streets and other paved areas in a neighborhood? And what about conditions dictated by the terrain and features of a site? Because you are reducing the developable area, you would also need to develop housing more densly to achieve the same number of units that could be created in a development with fewer streets.
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    Cyburbian
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    Quote Originally posted by donk View post
    A few municipalities I do work in have adopted a sustainability program with various metrics. One of the metrics they are going to measure is block length and perimeter.

    I am having a very difficult time understanding and quantifying how a block length and perimeter contribute to sustainability. It is even hard to understand when the values chosen exceed the lengths in some of the most walkable areas in adjacent andn bench mark communities..

    Other than having piggy backed off of LEED and New Urbanist philosophy does anyone have any insight into how block length and block perimeter contribute to sustainability, and if they do what is a realistic value?
    IMHO Portland's blocks are too short - this has several effects on real estate value/development and parking. Too many cities to list have blocks that are too long, but perhaps parts of Atlanta near the Olympic Park can be used.

    Nonetheless, maybe what you are looking for is node or intersection density rather than block length for a decent metric, because as Cardinal mentioned, terrain may be an issue making regular grids problematic. As Cardinal also insinuated, you choose your metric to measure something. What purpose(s) would block length serve to measure?

    And are you choosing your policy because it is easy to measure, or are you choosing policy according to desired outcome, and you adjust your metric accordingly?

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    Cyburbian donk's avatar
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    The municipalities think that shorter blocks will lead to a more interesting pedestrian environment and that the permeability it will create will increase walking and cycling as there are more route options.

    With respect to intersection density, that is another metric in the model. In thinking about if, it makes little sense and needs to be weighed against the loss of lots and the contribution to affordability that an extra 4 units per intersection create. Also, municipalities seem to forget that increased lengths / area of roads reduce soil permeability, increase their costs (maintenance and replacement) and increases the lands that will create a heat island.

    More comments and thoughts greatly appreciated and welcomed. It is the rationale and justification I am most curious about.
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    Cyburbian dvdneal's avatar
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    I think it may be a similar metric to number of intersections in an area. Number of intersections can usually tell you if an area is more urbanized or suburban. Somewhere there is an article on a sprawl index that uses intersection count as one of many indicators. I think you need to have some intersections or other feature to break up downtown buildings, but every downtown is different. I like the idea of what can you walk to with 5 minutes more than block length. Then again large unbroken blocks will reduce what people can walk to.
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    Cyburbian Cardinal's avatar
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    How about longer blocks that are bisected by pedestrian walkways?
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    A city block is a central element of urban plan and design. The sizes can vary according to the design and structure planned.

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    Cyburbian
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    I would ask the question what you are trying to achieve. At the end of the day, your normative theories about the nature of the public realm and the requirements of the market will inform your views on ideal block sizes. If you want each block to be self-contained, with either a single building (at high densities) or a limited number of smaller buildings (at lower densities) or open space and not both, then tiny Portland blocks may make sense. That leads to a particular sense and function of space. If, on the other hand, you want each block to contain multiple multiple uses arranged in different ways, with, say, a streetwall to the outside and a campus-like feeling or parking to the inside, then larger blocks with urban design requirements to put pedestrian cut-throughs and drop-offs may be a better way to go. The market is another issue. If your program has an office building component, and you're in a market that requires offices to be arranged in campus-like settings, as opposed to single buildings with separate lobbies, then you should look at larger block configurations. Each case is different. New urbanists don't understand this.

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    I'm just a student, so I'm here for learning as much as anything. But, I would say it has a lot to with walkability and bikeability, as well as reducing driving, assuming you buy into people in cities use less fuel.

    Since ~400 ft (~122 m) in length is so prevalent for old town/city main street blocks that one can deduce there is something about that length that pertains to pedestrians. Beyond that length, people seem to be far less likely to walk, and thus will look for some sort of alternative mode of transportation to get from A to B. That's where bikeability comes in, where the distance people are willing to travel increases significantly; 1-2 miles (1.6-3.2 km), I believe.

    So, if you're looking for a city layout that is conducive to walking and biking, while also pissing drivers off to the extent that they'll be willing to consider walking more, using a bikeshare, or even the bus or rail (if available) instead, medium sized blocks with most necessities available within 1-2 miles, designed in a way that creates a lot of stop-and-go traffic seems to be the ticket.

    In short, it's about making walking and biking more attractive while, in a way, discouraging the automobile.

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