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Friday, December 20, 2013

Why One-Way Streets Work

The one-way operation on East 17th Street on the northern edge of Union Square allowed
NYC DOT planners to recapture space for a bike lane and new public space (photo NYC DOT)

One-way streets can provide many benefits for safety, sustainable transportation, and public space, when used properly under the right circumstances. Nevertheless, there are progressive advocates who believe that two-way streets are intrinsically superior. Their heart is in the right place in their effort to makes cities more livable, but I fear they are reaching the wrong conclusions because of an incorrect assumption and perhaps a bit too much nostalgia. This is an important issue for transportation and quality of life, so we should have some real discussion and communication.

This issue has been on my mind for some time, and a recent tweet by Brent Todarian brought it to the forefront:
One of the biggest city-making mistakes that continues to haunt & weaken downtowns is the abundance of one-way streets. .
From what I have seen, Todarian is a thoughtful planner with experience making substantive improvements in Vancouver. There are real limits to the complexity of an issue that can be conveyed in a tweet, so in all likelihood Toderian's thoughts have much more nuance. There are, however, vocal advocates who dogmatically take the position that one-way streets are fundamentally an anti-urban and illegitimate configuration.

First, it is always important to resist the formation of any sort of dogma when it comes to transportation or planning. Every location needs to be properly addressed based on its own unique conditions. There are situations where one-way streets are a better solution, others where two-way streets are certainly better, and some that have a wide range of tradeoffs that will require an extended discussion engaging both professional judgment and community preferences.

Next, before discussing some generalities, let's look at one example of one-way streets that have enhanced quality of life in a commercial, urban area. Consider the improvements New York City made by reclaiming space from Broadway north of Union Square, an existing one-way street, for a bike lane and new public space. As part of the same project, Union Square North was converted to a one-way to improve bicycle lanes, pedestrian conditions, and public space. The details are worth a look:
This is a case where a two-way treatment on either street would not have allowed the improvements to the streetscape, so it really illustrates the importance of one-way streets as part of a complete planning tool box.

The dogmatic view against one-way streets seems to follow from three or four main arguments:
    1. One-way streets have generally caused an increase in speed, which supposedly makes them more dangerous
    2. Advocates who favor more sustainable transportation argue that anything that makes driving less attractive changes mode choices
    3. Separating the bus stop locations for different directions on the same route makes wayfinding more challenging
    4. One-way streets require additional circulation due to indirect routes
The evidence that one-way streets have had a tendency to increase speed is fairly clear (although this is a condition that can be resolved or mitigated). It is also very well established that pedestrians struck at higher speeds have drastically lower chances of surviving a crash. Concluding that the higher speeds on one-way streets makes these streets more dangerous, however, is incorrect. What is missing in this logical leap is the likelihood of a pedestrian actually being struck by a vehicle.

One of the principal advantages one-way streets often provide, something that was explicitly considered by the traffic engineers who originally introduced these changes in street patterns so long ago, is the reduced number of conflicting movements. Fewer vehicles crossing paths with pedestrians and other vehicles limits the risk that they will collide. The matter was empirically tested decades ago. Advocates for two-way streets sometimes claim that the age of those studies invalidate their findings, yet there is a lack of any empirical research I have found, recent or otherwise, that contradicts either those old findings or common sense. Walk out to an intersection of any Manhattan avenue any day of the week and watch how easily pedestrians cross several lanes of traffic against the light, and then compare it to the relative challenges crossing the two-way streets with or without the light. The reason for this is straightforward; pedestrians have fewer places to look for oncoming vehicles, while the signal progression organizes the vehicles into waves that clear and then provide long, usable gaps.

Also consider driver behavior making left turns on two-way streets; their attention is focused primarily on gaps in oncoming traffic (out of sheer self preservation), and watching for pedestrians entering the crosswalk becomes secondary. When drivers are forced to choose between a possible collision with a head-on car or a pedestrian in the crosswalk, the person on foot is always going to lose. Pedestrian safety findings by the New York City Department of Transportation a few years ago were unsurprising: "Within Manhattan, a disproportionate number of pedestrian crashes occurred on major two-way streets."

The conflict between left-turning vehicles on two-way streets and pedestrians can be mitigated by providing a protected left-turn (preferably at the end of the cycle), but there are two problems with this. First, the signal timing for the separate turning phase must come at least partially at the expense of pedestrian crossing time. Second, space has to be dedicated for a left-turn lane that could otherwise be used for sustainable transportation, public space, or green infrastructure.

Concerns about speed are legitimate, but one-way streets may actually be more susceptible to reigning in excess speed than two-way streets when they allow for signal progression. The signal progression can be set to a slower speed; drivers quickly learn that speeding only forces them to sit and wait longer at the next signal light. Two-way streets, on the other hand, sometimes create an incentive for a driver to "gun it" to get through as many green lights as possible before the end of the countdown clock. This is likely to be highly site-specific, and other potential traffic calming treatments could address or mitigate speed on either type of street.

Reducing lane widths to discourage speed can often be more productive on one-way streets, since it is more likely the excess space will be sufficient for one protected bike lane (while the second bike lane that would be needed for two-way operation may not fit within the constraints of a fixed right-of-way).

The issue of splitting transit stops has some validity due to passenger confusion and the complexities it introduces for transit maps . The problem can be addressed in part through quality wayfinding and design treatments, but really should be considered within the context of the other tradeoffs. All else being equal, this could be the tipping point in favor of a two-way treatment, but I kind of doubt there are many cases that come down to this.

The related argument that splitting the transit stops results in longer walking distances is nonsense. Unless you have a very particular case where passenger origins and destinations are clustered strongly all on the same street, instead of spread more evenly between the two streets in the one-way pairing, the differences will average out.

There are complaints about negative impacts due to driving around the block. These seem to be grossly exaggerated, more a rationalization to prop up a favored position than a serious argument. In dense urban areas with limited parking, drivers are surely looking for parking before they would even begin circling the block, and they would likely be circling the block looking for parking regardless of the one-way street pattern. There may even be instances where one-way streets provide more parking because they can capture spaces at the corners that would be required for corner sight distance if the street operated in two directions. The question of extra circulation could perhaps be an issue in smaller cities with much more stagnant levels of activity, but until urban areas enable people to drive up and park right in front of their destination, this is a non-issue.

Related to this is an argument that is more serious; forcing cyclists to route around one-way streets is an inconvenience. That is true. This is somewhat less of an issue with shorter blocks, where there is less inconvenience from parking the bike at the corner or walking it down the sidewalk as an alternative to going around. There may be opportunities for two-way cycle tracks, although these can reduce some of the advantages of the simplified intersections on one-way streets, especially for pedestrians. In many cases, it will largely comes down to a question of the tradeoff between a better system of protected bike lanes and a little longer distances at some locations. My preference would generally be to create a full network of better protected lanes and more public space for everyone to enjoy their streets, with the small degree of inconvenience for myself and other cyclists to loop around a block or walk a little further. But this is where discussion with all community stakeholders becomes important, with an understanding of the specific tradeoffs in the local context.

Then there are less serious arguments that surface, like the question of storefront visibility. Let me just say that advocates that spend so much time decrying the "windshield perspective" do not sound very credible when they resort to claims that businesses rely on drivers having a good view of their awning when they drive by. The ever-growing usage of the internet as a guide for consumers should cast even more doubt on the seriousness of such claims. Hopefully we can at least all get on the same page about pedestrian-oriented retail downtown. To be fair, this issue may have more validity when it comes to arterial couplets outside of downtown areas.

With these sorts of issues, there are no absolutes. Each case must be evaluated based on the local conditions, and there most definitely will be instances where two-way streets will be preferable to pairs of one-way streets. Especially as block lengths get longer, the one-way streets may become less desirable. Arguments in favor of two-way streets are probably much stronger for arterial couplets, rather than downtown areas, given generally longer intersection spacing, less demand for dense pedestrian amenities, and greater parking availability. Wider streets may also favor two-way operations, since they can allow for the installation of refuge islands that enable pedestrians to make a safe, two-stage crossing rather than exposing them to a multiple-threat condition.

Even in downtown settings, there may be cases with enough reason to prefer converting a one-way street to two-way traffic. The problem is when there is a zeal to jump on the two-way bandwagon without a careful review of the various alternatives and tradeoffs. In many cases, I am sure people will find they have an opportunity to repurpose some of the roadway on their one-way streets for protected bicycle lanes, new public space, or green infrastructure, rather than introducing an increase in turning conflicts with a greater overall number of redundant traffic lanes.

Some additional reading:

"One-Way Streets Provide Superior Safety and Convenience" ITE Journal 

"Pedestrian Safety Guide and Countermeasures Selection System" FHWA

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