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Town Center Intersections: Place-Making Split Intersections, One-way couplets & triplets Home
 
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The Sustainability Toolbox: Town Center Intersections

This Town Center Intersection set near San Diego is an excellent example of vision becoming reality. This layout can support far higher surrounding densities in a safer and more pedestrian-oriented way than a single intersection that might have otherwise occured here.

Traditionally, It's Either Form OR Function

All over the nation, planners and residents alike hope to reinvent languishing suburban retail corridors into sustainable, livable, transit and pedestrian-oriented "Places."  They hope to carve out a true Town Center within the midst of their stale, Anywhere USA sprawl.  But the exciting Visions created by well-meaning stakeholders, planners, and architects often bumps up against a hard reality: The streets in question often carry huge traffic loads and it is extremely difficult to acquire the space for desired "Complete Streets."

Concept for future Activity Center, with light rail in the center, high-volume arterial split into managable streams, and small cross-couplets.

Another sad truth is that even the best Transit Oriented Development will still generate a lot of vehicle traffic. If the roadways that serve proposed TOD are too congested, builders and city councils may be too reluctant to create TOD because it adds traffic to already intolerable congestion!
And engineers charged with managing the street simply will not accept form over function. In truth, to be truly livable and to sustain desired TOD densities, these intersections need to flow well. "Town Center Intersections" are a hot new design with excellent Place-Making qualities as well as exceptional ability to move a lot of traffic slowly but steadily and without congestion through sensitive mixed-use, multi-modal areas.

How do they work?

The TCI, also known as a Split Intersection, is an intersection where one or both of the streets involved are a one-way street.  Where two major arterials come together and would have normally formed a single massive, unwieldy intersection, the Town Center concept instead separates each arterial into one-way couplets, creating four small, efficient, easily manageable intersections of one-way streets that merge back to a two-way street a block or two upstream.  A TCI can also involve a "triplet", which is an alignment between the couplets, perhaps used as a pedestrian mall or for transit, bikes, and on-street angle parking.  

Conceptual suburban Activity Center, served by regional transit and localized circulation shuttles to peripheral parking so that the entire inner core remains accessible by auto, but with very little surface or garage parking in the core.

Since one-way streets have half the number of lanes, streets are narrower and safer for pedestrians to cross.  Even if speed limits are reduced when entering the Town Center area, and you might stop at two signals, average travel times will be faster! Why? One-ways make it easy to coordinate signals, and you'll probably stop two if not three times getting through a single overwhelmed intersection.

Hot New Mixed-Use Design, but what makes it Innovative?

San Diego, Las Vegas, Salt Lake City, and others have incorporated this new design into several mixes use projects - often paid for by developers. But what makes them Innovative?  Our definition of an Alternative Intersection is a design that successfully eliminates the need for left-turn arrows. One-way streets allow drivers both a free-right and a free-left.  Simulations comparing the same high traffic volumes in a single large intersection vs. passing through four simple intersections reveals that couplets will move more traffic more quickly, even if there are more signals.

It's Mother Nature's Way!

What if blood in our body's "arterials" tried to flow in both directions within the same "pipe?" The terms "chaos" and "high blood pressure" come to mind. It's an absurd way to run a body.  The more dense an urban area becomes, the more chaotic and absurd it is to operate the urban-organism's circulation system with two-way flow.  Nearly all of our highest density urban areas use one-way circulation.

Hot! - but not really so new


Denver, Portland, Boise, Manhattan - TCI's are "new" in that they're showing up in hot new developments, but in truth they are the common thread of our most intense, sustainable Downtown cores since the auto was first invented.

A tight grid of narrow one-way streets is at the foundation of Portland's ability to support quality multi-modal circulation.

Boise has crossing couplets that form the classic Town Center layout. Multi-modal circulation is enhanced by this layout, and it also makes it possible to use triplet alignments between the couplets as pedestrain areas.

Without these couplets, it is likely that every street in Downtown Denver would be in high demand for vehicles, making it far less politically feasible to ever reclaim 16th Street for their wildly successful pedestrian and transit mall.

Boulder's Pearl Street area can support high densities largly due to this "Triplet" configuration.

Palm Springs supports high traffic volumes moving steadily at slow peak speeds, but good average speeds, through an exceptionally welcoming pedestrian environment.

The numbers show that running couplets allows much more right-of-way to be released to uses other than just travel lanes.

Palm Springs - a quality pedestrian environment, brought to you by the letters T, C, and I!

Aren't One-Ways Killing Downtowns?

Some cities with struggling historic CBDs have been converting their one-way streets back to two-way, and have been pleased with the results.  But it is wrong to conclude that one-ways were a major factor in their past struggles. The conversion to two-way is often accompanied by a streetscape makeover, making it hard to tell if the rejuvenation came from the aesthetic upgrades or the two-way flow. Further, if the area has lost traffic and doesn't need one-way circulation, then why not go back to two-way flow. 
One-way streets can be safer and even more pedestrian oriented than the best two-way streets can be. And the more intense your Activity Center becomes, the more valuable one-way operation becomes.

More Lessons from Anatomy

If the blood flow to a child's growing limbs is restricted, the limbs will be stunted. Think for a moment of the life-cycle of our urban spaces that emerged largely after the invention of the auto. Areas with the best auto access attract popular businesses. Larger commercial areas emerge where there is good freeway access. 
In a world of cars, the infant urban body first "grows" it's most intense land uses (single-story businesses) where the cars are.  As that body reaches adolescence, transit-oriented development emerges in places that historically could serve a lot of cars.  And they grow even faster if those historic arterials happen to be close to a new freeway.  Then as there is market demand to intensify even more, multi-modal access can play an important roll in intensifying. But like the arm that will not reach its potential without an expanding blood supply, our car-oriented urban bodies may take an exceptionally long time to transition to a transit-oriented blood supply if it cannot also support more cars during that transition.
And even once it has made that transition, the ability to circulate huge numbers of vehicles is essential for the most sustainable environments. In addition to excellent multi-modal usage, urban centers in San Francisco, Portland, New York, and almost everywhere have a tight grid of one-way streets. Streets don't seem overwhelmed with vehicles, but there are so many through streets that in aggregate the area supports huge numbers of vehicles!
In other words, if any modern Greenfield or low-density area envisions its future with a lot of TOD, that same area has got to find a way to support a lot of cars, or the growth will happen at glacial speed even if it is served by high-capacity transit! It is true that the onset of gridlock will get more people onto dedicated transitways, but it will also discourage further development, which ultimately sees fewer people on transit.  Perhaps we sprawl outward in part because gridlock makes it too challenging to grow inward!
The Town Center Intersection and other Alternative Intersections are excellent strategies for creating the high-capacity vehicular circulation that is an essential ingredient for fostering TOD.

Advantages of TCIs

  • Up to 50% more capacity!
  • Level of Service B-D rather than E-F
  • Drive slower, travel faster
  • Narrower, Complete Streets
  • Safer for both autos and pedestrians
  • Supports very high densities
  • "Free" when integrated by developers
  • Easy access to retail
  • Expands grid within Activity Center
  • Better traffic progression

Disadvantages

  • Confusion in places with few one-ways
  • Challenge convincing retail and other stakeholder that this is good
  • Minor out of direction travel

 

TCIs are "Alternative Intersections"

Town Center Intersections, along with several cousins such as Quadrant Intersections, Through-Turn Intersections, Continuous Flow Intersection, and others are among a series of concepts collectively known as Alternative Intersections, Alternative Intersections, or even Unconventional Intersections. The key trait that links all Innovative Intersections is that they successfully eliminate the "left-turn arrow" phase, which otherwise reduces intersection efficiency considerably.

Where can I learn more?

TCIs and other Alternative Intersections can all be found at www.alternativeintersections.org, where you can search for every Alternative Intersection that exists or has been planned anywhere in the world (or at least those that our members are aware of).  This site as well as www.alternativeintersections.org, are sponsored by www.metroanalytics.com.

Links

FHWA's Joe Bared has for years sponsored groundbreaking research on a number of Alternative Intersections. FHWA's latest findings on the DDI and other concepts, can be found at:
http://www.fhwa.dot.gov/publications/research/safety/09060/
 
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