One-Ways in Great Places (Pt 2)
by Michael Brown, PE, AICP
* Traffic Engineer, New Urbanist, Founder of Urban Innovators
One-Ways Underpin Our Strongest Activity Centers
One-way streets are a common thread linking America’s largest and most successful urban centers – Portland, Denver, Manhattan, New Orleans, and even Boise, Idaho. Each city has many examples of walkable, narrow, low-speed one-way streets. Alternative modes are vital in all of these, but their ecosystems still require efficient vehicle circulation for deliveries, construction, transit, and a great many private and shared autos. Their network of one-way streets ensures maximum efficiency. Here are some examples.
Convert these busy one-ways to two-ways, and you’ll create immediate gridlock. Gridlock increases the sense of autos everywhere, and hinders market interest in further development. (Same cars, but it feels like 3-times as many because they’re stacking up for blocks, never able to get out!) Congestion decreases Livability, and limits the market’s ability for continued development.
Auto-access may not sound like a critical component of Transit-Oriented Development, but it is. Developers know that even if alternative modes gain impressive shares, odds are most trips to/from their potential development will still be by vehicle. When congestion becomes chronic, as will happen faster with two-way systems, many developers will opt for further out but less congested sites. Thus one reason we sprawl outward is that congestion won’t allow us to grow inward and upward.
Two one-ways will provide more access and less delay, even at lower speeds, than two two-ways with the same total of lanes. Would-be Activity Centers may never reach critical mass for major transit investments until AFTER they first grow as dense as possible mainly via auto accessibility. One-ways provide better accessibility than two-ways.
Designs Using Couplets by World Famous New Urbanist, Peter Calthorpe
Peter Calthorpe once published a short white-paper titled “The Urban Network, a New Framework for Growth.” The primary diagram from that article is shown below. Notice that one-way couplets are a key feature of Calthorpe’s Town Centers and Village Centers. He notes that the couplets are a good way to provide significant vehicle access to the core centers, within a “slow-but-steady” very pedestrian-friendly framework.
Personally, I think Calthorpe’s entire framework is a stroke of brilliance and a very good model for Greenfield areas poised to urbanize in the near future. His paper also discusses the San Marcos Town Center, noted in Part 1.
Issaquah Highlands, by Calthorpe Associates
Calthorp’s paper also provides some analysis of a Town Center that uses crossing couplets at Issaquah Highlands just east of Seattle. The default plan would have resulted in a single huge intersection with 166 ft for pedestrians to cross. On the couplet, one street requires a 40 ft crossing, and the other just 28 ft. The couplets also allow on-street parking, which would have been impossible to allow with the previous two-way Stroad plan. Traffic engineers estimated 11% faster actual time with the couplet in spite of additional intersections, on-street parking, and reduced speed limit.
Summary of Pros and Cons
Advantages of One-Way Couplets
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Impressive vehicle capacity gains
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Less pavement = Complete Street space
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Shorter signal cycles
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Safer for autos and pedestrians
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Enhances and motivates TOD
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Little additional cost if planned from start
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Expands grid, enabling larger activity center
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If synchronized, low speed limits observed
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Narrow streets help pedestrian enclosure
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Proven: common feature of thriving Centers
Disadvantages
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Initial confusion for drivers
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Out of direction travel (offset by less delay)
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Opposition if 2nd street has incompatible uses
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Convenience stores may have initial loss.
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Transit boarding stop different than exit stop
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Risk of high-speed if designed poorly
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Change affects many people (education effort)
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Unwarranted stigma to overcome.