Saturday, March 26, 2011

The 50 Foot Rule

                Jan Gelh writes in his book “Life Between buildings” about how the limitations of the human senses effect the kind of places people enjoy being in and which encourage social activity. Based on his work I’ve constructed the following table:

I am particularly interested in the highlighted portion, the distance at which feeling and moods can be detected. It is within this distance that the transition from isolated to social individuals takes place.  It is at this distance that people decide whether or not they’d like to move to more intimate contact, whether it’s an acknowledging nod, a few words, or more involved conversation.  People who do not pass within 60-80 feet of each other do not then have the option of proceeding to the 3 to 10 feet distance.
Why does this matter? In American planning we have a tendency to construct streets that are far too wide. Much has been said about the physical aspect of this as it concerns pedestrians trying to cross the street. What I would like to address is the great imposition these distances have upon our ability to see each other and the details of each other’s faces. A corollary of this is that features on buildings particularly retail buildings are rendered less clear.  Jan Gehl suggests as a design principle for public spaces: “When in doubt, take space away.”  Here I would like to examine this principle in light of existing wide American streets and attempt to quantify the gains from narrowing the street and bringing the sidewalks on each side closer together
On any given street no matter how wide it is one can see people on the same side of the street in as much detail. The difference between a narrow street and a wide street is the amount of detail one can see on the opposite side or put another way, the amount of social space one can view.
In the bellow diagram the red shaded area is the “social space” of the sidewalk on the opposite side of the street. 

In the below graph I’ve calculated the this red area for different street widths. I’ve assumed a 10 foot sidewalk and that both the viewer and viewed are walking down the middle of the sidewalk on opposite sides of the street. The only variation is the street width, given here as the “curb to curb” distance.

Note that as one takes space away from a 60 foot street to a 50 foot street the amount of social space goes up by 40.3% From 50 to 40 feet the improvement is a more modest 18.6% and one sees declining returns from there. Also note that two people walking down a 60 foot street at 3.3 miles an hour have only 3.2 seconds of visual contact while on a 50 foot street they have 5.3 seconds of contact.
Going from a 60 foot street to a 50 foot street does have consequences for vehicular traffic, and these must be weighed against the gains in pedestrian traffic resulting from a more socially enjoyable environment.  Each lane of lost space constitutes a percent decrease in capacity. I’ve constructed the below graph to show the gains in social space and how they compare to losses in lane space.
Going from 60 to 50 feet constitutes a 16.6% loss of lane space but increases the social space by 40%.. Going from 50 to 40 feet has about equivalent losses to automobiles and gains to pedestrians.
All of which is to say there is a very strong case for a “50 foot rule” for street width in urban areas. Planners should consider retrofitting existing 60 foot streets in order to move the pedestrian ways on either side closer to each other. The benefits of pedestrian visibility also accrue to store fronts that are brought closer to each other. In general zoning should require store fronts to go right up to the sidewalk in order to be both visible and accessible to pedestrians. If planners follow this 50 foot rule for avoiding too large spaces then the resulting increase in walking and socializing will have positive impacts on our society’s physical and social health.


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