Friday, July 2, 2010

How Savannah Generates Diversity

An Odonomy of Savannah VII: A Tale of Two Grids

~ part i ~
how savannah generates diversity

Four-way intersections really do make an active city.
A round of interesting posts on the blogosphere lately concern the attributes and forms of American city grids.  While some have questioned their utility or highlighted their shortcomings (at least those of prominent precedents), there are some really good reasons for valuing city grids.  I'd like to point to another (which is more important to me): grids multiply route options and thus help generate land use diversity in open conditions that contribute to a city's economic prospects and resilience.  How well different grids functionally and efficiently connect people is the important attribute that needs measurement and observation.  Perhaps, Jarrett Walker brings up, we avoid the topic of travel efficiencies because Urban-ist discourse tends to devalue so prominently the virtues of "purposefulness", seeing as we American flaneurs all tend to prioritize the qualities of places with the complexity of texture that invites spontaneity, slow and not-so-purposeful wandering.  Speed and directness of travel are goals that we have learned all too well tend to erode the design quality of our urban places.  But, I see Jarrett's point.  The fact is, the well-designed network is functioning best when it works to meet and preserve multiple functions.  Some of those functions may include speed and "cold" efficiency.  How grids find ways to meet these cross-purposes in mobility and land use economy simultaneously -- all important factors in the life of cities and many of which are seemingly at conflict on paper and in much of our urbanist discourse -- may just be one of the most important of their attributes.

Jarrett's vantage points have resonance with my experience.  My formative travel and collective graduate school experience taught me that cities, even their forms, always find their sweet spots.  The cultural endeavors of mankind (including languages and their products, written works) tend to have this inside quality.  They serve their peculiar and diverse cultures in ways that are often hidden from direct observation.  However, such observation is possible if you have the patience of a insider or a philologist.  A philologist inspects form, the transmutation of words or their use, to tease out the cultural development of meaning.  Observe the city well, Jane Jacobs (that philologist of urban form) suggested, and you will discover why many well-intentioned interventions mistranslate the city and often miss their mark.  The underlying purpose of the grid, similarly, is to hit one of the sweet spots of cultural work that actually generates urban vitality.  This work of the grid, like language itself, may be invisible at our scales of experience and to generational observation often.  Such transmutable vitality of the bones is what Miss Jane called the inherent "diversity" of cities.  She pointed out that local interventions into grid functions often frustrate the "generators of diversity", some of these traveling at the scales of time so slow they are epochal, and we just haven't taken notice of their dynamic economy.  An undisturbed, well-connected grid will naturally advance and balance the simultaneous, diverse pulls of the urban economy.  Savannah's Historic District form, more than any other grid I know, crystallizes this fact and, in fact, employs it to generate commercial and civic vitality in manner we can plainly read.

In fact, Savannah does this well and transparently, because, almost unique among cities, its historic fabric does not contain one grid, but two.  The second major division in the Odonomy of Savannah's Historic District is the division between Savannah's two grids.  The 3/4 mile x 3/4 mile Historic District is a composite grid created out of two overlapping grids. Understand the virtues of these two grids and you apprehend one of the generators of urban land use diversity in Savannah's grid.  I call these complementary grids the "Fast Grid" and the "Slow Grid" due to the relative speed of vehicular traffic on them.

Conceptual Map of Savannah's "Fast Grid"
The Fast  Grid, or Ward Grid, is a simple squarish grid composed of Savannah's "primary" loading streets and avenues, which divide up the 3/4 mile square district into a 6 by 5 array of modules averaging 680x820 feet. These modules are the signature wards of Savannah.  At the center of the majority of these modules are thus the signature garden squares (depending on what you include, 22 squares presently exist, but some might want to include the Colonial Park Cemetery as the honorific 23rd). This Fast Grid, really, represents what most grided cities have typically for their gridiron.  Usually, the interstitial circulation inside this primary grid is discordant, discontinuous and un-patterned or simply serves parking or service uses, if it even exists at all.  With most cities, including my own, the story of the grid stops here, and that is where it also stops conceptually, by precedence, in the minds of planners, discerning designers, public officials, lawyers and not-so-discerning developers.

Broughton Street, the "Main Street" of Savannah
The most important streets in Savannah are the east-west loading streets of the Fast Grid, including Broughton Street at left.  They are the two-way streets depicted in teal in the 9x9 "conceptual map" here for the Fast Grid.  They are spaced between 735 and 875 feet (centerline to centerline) and get progressively more distant from one another further south.  These streets are valuable for two main reasons.  Firstly, they are the streets in the district with the widest right-of-way and that represent the most direct and/or prioritized routes for the important regional east-west direction of travel, the direction of regional commerce between the bay areas to the east and the access points to the coastal commerce routes (including I-95) on the west.  Secondly, they are the only streets in the district that are continuously fronted by buildings on both sides of the street.  These two attributes together makes them valuable for commercial activity, real estate investment, and overall use vitality. While these streets are in many ways similar, they often exhibit dramatic differences from one another in terms of their activity, use-mixture, transportation and environmental qualities.  Each has a peculiar valence that is not coincidentally tied to local conditions and to the subtle and manifold adjustments traveling through the grid or creating disruptions to its normative fabric.  I don't have time to get into the details now, but be assured that this fascinating order of land use diversity generation will get further treatment down the road.

A car stops for a cyclist on Drayton St. ..Is this any barrier to crossing?
Savannah's primary avenues (the purple streets in the conceptual "Fast Grid" map) represent the north-south streets of the Fast Grid, and these are spaced 620-730 feet apart, with the widest Ward "columns" being in the center. Unlike the primary loading streets, these streets often have very narrow rights-of-way. The perimeter avenues of the Historic District, MLK on the west and Broad Street on the east, are wider two-way streets, creating natural terminations to loading street travel, while the central Fast Grid avenues are two lane one-way streets (like Drayton Street in the foreground of this image).  The fact that these central avenues in the interior of the grid are one-way is handy since one-way, two-lane travel poses comparatively little hindrance for east-west crossing traffic of all modes. Traffic on one-way streets tends to "platoon" in bunches and so offers plenty of opportunity for traffic and pedestrians to cross with little, if any, wait times.  The one-way travel also allows easier turning oppurtunities and to thus avoid queuing.  To me, these one-way avenues are the second-most important asset for fluid travel in Savannah's grid (we'll talk about the top-most later). Vehicular travel on these one-ways is effortless, relatively unhindered (except at the intersections with the primary loading streets) and fast, as it should be for one-way grid streets - an important factor since Savannah is oriented north-south geographically and therefore these streets serve their function well as conveyances of cross-town and internal commuting traffic.

Since these primary Fast Grid streets (both loading streets and avenues) are the most heavily traveled by vehicles in the grid, the intersections of the Fast Grid streets represent the largest conflict points for vehicular travel. The majority of signalized intersections in Savannah (which, by the way, occur far fewer times per square mile than in a typical grid, like Charlotte's) are therefore at the four-way intersections of the Fast Grid streets. Since the Fast Grid loading streets usually have two to three extra traffic-controlled intersections to navigate across the grid and have lots of pedestrians and opposing turning movements to deal with at the intersections, these streets can stack appreciably and can congest at times. Because of the heavy east-west traffic volumes sometimes produced by the "cross-bay" traffic, they can present formidable challenges for pedestrian and bike crossing, and, at places, such travel along them. Thankfully, with the exception of Jones Street, they are given much attention in terms of traffic control.  Only one of the streets, median-less Bay Street (at the top near the waterfront), represents persistent pedestrian and traffic problems around the clock. Bay Street is easily the greatest traffic deficit of Savannah's grid, especially since the right-of-way is very narrow for its volumes and much of the cross-bay and industrial/port trucking traffic is forced to route through this bottleneck. But many waterfront boulevards in touristy and busy port cities usually are a bit congested after all (and perhaps should be).

As I mentioned, all of the primary loading streets are given priority except for Jones Street. Avenue traffic does not stop anywhere for Jones Street. The fact that you only have only up to five potential stopping points, thus, between Bay Street and Gaston Street, the southern-most Historic District street, really makes travel on those avenues unhindered, especially as you move further south away from the most urbanized portion of the district near the waterfront. Despite covering the same rough distance (actually, a little bit greater distance) than the east-west Historic District streets, you can usually manage to travel the entire district from Bay to Gaston in a car during rush hour in about 3 minutes, by far the fastest average travel times I recorded among all of the grid's through streets. (Yes, I timed this in my car, keeping pace with the traffic, timing runs between 7:30-9:00 am and 4:30-6:30 pm on most of the district's through streets at least twice). In normal rush hour traffic, I was usually a full one to two minutes slower on the primary loading streets getting across the grid east-west from Broad Street to MLK.  (See...These kinds of useful facts are what I wish people recorded somewhere about grids--hopefully, real-time GPS data might some day prove to be a boon for this kind of research.)

Conceptual Map of Savannah's "Slow Grid"
Almost all of the intersections along the Fast Grid streets are four-way intersections (most exceptions happening at the streets on the perimeter of the Historic District). However, the second grid of Savannah, the Slow Grid (which I shall nickname the Southern Grid!), is distinguished by its three-way vehicular intersections and its consequent predilection for more aimless, prevaricating, and unburdened travel. This grid is composed of the streets that all create T-intersections with the sides of the garden squares.

I like to think of the Fast Grid avenues as creating a "pressure drop" in the grid, pulling out traffic circulating in the Slow Grid, much as anteater snouts would suck termites out of a mound (or "java chips" through a pick your metaphor).  The garden squares help this process by introducing centrifugal forces to vehicular traffic in the Slow Grid.  They induce "high pressure" in the vehicular flow network.  (Of course, some drivers like to travel in the "high pressure" zones nonetheless.  They just like driving around the squares, enjoying the scenery.  That's ok!  Savannah gives them that option.  You can be a slow poke in Savannah just fine.  ...There are your slow grid people and your fast grid people everywhere, but, normally, they just don't travel separately.)

Not really a barrier to pedestrian mobility (a T-intersection on a square) 
This relationship, however, is exactly inverted for pedestrian traffic.  Pedestrians, as well as cyclists, bike taxis, horse buggies, and the like instead gravitate to the squares, for a multitude of reasons, including shade, attractive uses, slower traffic (sometimes including foot traffic on the travel way), very wide and generous streets (reserved for aforementioned slow traffic and "jaywalkers")... and the much greater ease and safety of crossing streets (partly the reason for the ubiquitous jaywalking), ...just to name a few.  3-way or T-intersections, by the way, when they limit pedestrian movement to only three directions do discourage pedestrian travel...NOT so the case in Savannah!  Around Savannah's squares, T-intersections just discourage walking in only four directions (therewise folks even feel free to push their strollers along the travel way).

The garden squares and "low pressure" avenues exert two forces that create the land use diversity that is relatively evenly dispersed in Savannah's historic district:

Along Drayton Street.  ...A city in flux.
1. First, they divert vehicular traffic away from the garden squares and therefore direct it to the Fast Grid.  This helps commerce along the loading streets, by giving their frontage more passing eyes to attract users, and creates a need for commercial frontage serving convenience (stop and go) services to concentrate on the avenues, where frontage is not as important to preserve and thus vehicular access can be accommodated.  For such convenience commerce to take a hold on the avenue, however, it has to creatively make space for parking and compete directly with civic and residential uses to do so.  Thus, it has to make a lasting case for itself.  In the image here (and in my banner at the top) is an old garage site on Drayton Street, one of Savannah's one-way avenues.  As you can see, this garage is trying hard to re-open a case for itself.  I can't help but to notice how it is now trying to do this.  To me, it represents a city in flux.  Like my blog name, it is an image I place on my banner in order to posit it as a question: what is the "proper scale" to understand this?  (I chose to name this blog "Proper Scale" in order to query the starting assumptions of transportation and land use discourse...and to be open to reconfiguring my own, especially for my Southern context).   Unfortunately (or fortunately?) for this garage, now a Vespa dealership, it is on an in-bound avenue, not an out-bound one, where I suspect its original use could have fared better.  Perhaps, it should have been a donut shop instead.  In Savannah, these things matter.  With a form like Savannah's, they can't help but to matter.

2. Secondly, they pool pedestrian and slower traffic to the interior of the wards.  The garden squares thus support amiable uses geared to attracting walking and more leisured traffic.  They also invite the frontage of civic uses that are not always open around the clock, but, nonetheless, need a ceremonial front addressing a public place with foot traffic.  The uses on the Trust Lots east and west of the gardens are therefore perfect for civic uses since they are tied directly to both kinds of traffic, creating a presence on both the Fast Grid avenues for the convenience of users and the Slow Grid squares for their ceremonial frontage.

A corner use on Bull Street
On the north and south sides of the garden squares, the corners created by the intersecting north-south Slow Grid avenues tend to support street-level commercial uses, since these are important funnel points for pedestrian traffic.  The uses best supported here are corner coffee shops, galleries, cafes, museums and shops catering to leisure, ...not convenience.  The attractive pull between the central waterfront area and Forsyth Park to the south on Gaston Street makes Bull Street, the central north-south avenue connecting the garden squares between, the most important concentrator of pedestrian-geared uses in Savannah.

Because of its form, Savannah attracts a remarkable density of civic uses, but not even Savannah can support so many, so, where civic uses are less supportable, other uses, typically gorgeous Southern mansions, take their place on those prominent trust lots. The garden squares along Bull Street especially attract a high concentration of civic uses.  Going away from Bull Street east to west, the garden square character progressively becomes more quiet.  The squares on the perimeter form the communal heart of attractive little urban neighborhoods that, surely, still must preserve something of the original serene quality of Oglethorpe's early Savannah.

But the connection to Forsyth Park is not the only reason higher intensity uses concentrate on Bull Street and its squares.  Simply, Bull Street is in the center.  Uses in the center have more access to other uses and thus more users.  Uses on the perimeter have less.  Intensity of use steps higher in two directions in Savannah.  Intensity steps higher going north towards the waterfront; this is the response to the pull of the attractive edge.  And, intensity steps up going to the center in the east-west loading direction; this is the pull of centrality.  So Savannah adapts to its regional shape, as well as its modular dynamics.

SCAD saw low overhead in Savannah's old uses. The economic relationship reciprocated.

Savannah's form, as one can hopefully begin to see here, provides simultaneous-countervailing centrifugal and centripetal pulls that ensure a distributed land use diversity, stepped in intensity and navigating transitions of use dictated by internal form, greater geography and a patterned but variegated transportation network.  You can read Savannah like a book, and see stories within stories just walking square to square, without even needing to purchase a guidebook.  You just need to learn her alphabet, her grammar and her thought patterns of time.  For a primer on that language, here is a good starting point.

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