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Sunday, January 5, 2014


@StreetsblogNYC pic.twitter.com/tpVZjqTk28
The #sneckdown tag has been trending on Twitter. The term is a clever combination of the words "snow" and "neckdown." The tag is used with photos of snow at intersections where it has not been cleared by vehicles, leaving areas that closely resemble neckdowns (see p. 74 of NYC DOT's Street Design Manual). As it is currently trending, these generally have an implicit or explicit claim of demonstrating that space is wasted by vehicles and could be used instead for traffic calming and pedestrian space.

I initially saw some discussions about "sneckdowns" last winter, with useful observations about how the snow piles were acting as temporary traffic calming. This winter, I noticed #sneckdown making the rounds at the first snowfall here in New York. Rather than discussing the temporary effects or snow plowing practices, the focus seemed to have shifted to claims that the areas that had not been cleared by vehicles should be converted into permanent neckdowns.

I found the original observations and discussions about snow plowing very useful. The observations about roadway space that wasn't actively used after the snowfall also initially struck me as something that might inspire a closer look at specific intersections. However, as the popularity of the #sneckdown tag grows and more people jump on the bandwagon with their own photos, I am becoming increasingly uncomfortable.
There are many strong arguments that can be made in favor of neckdowns and other traffic calming treatments, but piling up examples of "sneckdowns" won't do a thing to convince anybody who needs convincing about the need for a permanent neckdown. Trying to illustrate operating needs based on unusual operating conditions is not credible, and could undermine the credibility of those advocating important improvements for pedestrians.

I can't deny the initial appeal. When you can clearly see that cars are not driving over portions of the roadway, it is very tempting to conclude that vehicles do not need it. Yet there are complications:
  • Differences in operating patterns when traffic volumes are higher
  • Requirements for less routine operating needs
  • Considerations of safety margins
  • The need for locations to store the snow
Just because a portion of the roadway went unused on a day when most people stayed home does not mean it isn't needed to accommodate normal traffic volumes. With higher traffic volumes, the vehicles would spread out into more lanes, and would use additional space for a range of bypassing movements. Moreover, none of the "sneckdown" claims have indicated how permanently reducing intersection widths could allow them to continue functioning after the further reductions when there is a large volume of snow plowed up.

At intersections, drivers proceeding straight ahead normally swing around turning vehicles that are waiting for pedestrians. This is necessary to maintain any normal movement. After a heavy snow storm, the lower traffic volumes may make it less likely through vehicles will be stuck behind a turning vehicle. The much lower volume of pedestrians would also tend to reduce the turning friction. The potential hazards and discomfort of driving onto the snow would motivate many drivers to wait for the turning vehicle rather than trying to clear a new path. Additionally, I expect people can be more tolerant of delays under infrequent circumstances, such as a major snowstorm.

Another bypassing movement involves standing/double-parked vehicles. Normally, any average New York City block will have a couple vehicles stopped at any given time (picking somebody up, running groceries into a house, making deliveries, etc.). I have seen nearly no double-parking since it snowed. This seems to be primarily a function of the few trips that are being made (most people are not going out on a weekly grocery trip right after the snow), but the reduced width of the roadway due to the snow plowed on each side reduces the ability to maintain an effective lane with any vehicles standing. Without the need to squeeze by on either side of double-parked vehicles, the traffic that enters intersections with snow will be more centered. I suspect that tracing vehicle trajectories when there is a normal amount of double-parking would show a greater use of the space in the intersection.

Next, it may be necessary to consider significant needs that may not manifest themselves with most of the vehicles on the road after a snow storm. For example, articulated buses were taken out of service until the roads were cleared; they sweep differently and may use space that is not worn down by other vehicles. The turning radius of fire trucks or other larger vehicles may also need to be considered, even if they did not travel through a specific intersection to clear the snow before a "sneckdown" photo was taken.

A consideration of safety margins is important when considering the constrained conditions imposed by snow piles. Without knowing if there are more collisions at intersections because drivers failed to make sharpers turns properly, it is difficult to conclude that the reduced geometry is actually appropriate, much less that a new, permanently reduced geometry would remain acceptable when it was further constrained by snow piles. Of course, an actual study of crashes with the snow-reduced geometry would be very difficult, if not impossible, given limited data and the compounding factors such as traction and possibly visibility. Dealing directly with geometric design seems clearly preferable to trying to discern patterns from temporary snow conditions.

If a review of constraining the geometry did, in fact, raise concerns about the additional constraints from future snowstorms, there may be a meaningful question of tradeoffs. Perhaps the improvements for pedestrian safety on typical days throughout the year would outweigh potential safety risks on the limited number of days affected by snow. A #sneckdown tweet reinforces uncritical advocacy positions that don't lead toward this type of problem-solving discourse.

Snow plowing that meets pedestrian needs
should be a priority (photo from last winter)
The snow piles themselves need to go somewhere. Looking at the place where the snow has been piled and assuming the space is "unused" is highly problematic. It is being used - it's storing the snow. Could the snow be plowed up over a neckdown? Would that still enable the crosswalk ramps to be cleared effectively at the corners? I'm not sure - but these questions would have to be answered. The snow pile issue might be something that could be addressed through design, but it is not the simplistic matter suggested by the #sneckdown tweets.

Observations about snow plowing practices should be a higher priority for pedestrian advocates on snow days than oversimplifying photo ops to make weak arguments in favor of permanent traffic calming treatments. I would regain my enthusiasm for the "sneckdown" observations if they returned their focus to reforming snow plowing to ensure crosswalks remain clear while forcing motorists to slow before entering slippery intersections.

Another problem that is particularly vexing with some of the "sneckdown" photos involves bicycle lanes. In some of the photos making claims for large neckdowns across extensive snow-covered areas, I have noticed some of the space often includes unplowed bike lanes. Since I am certain that nobody is advocating eliminating bicycle lanes for larger neckdowns, it is clear that this gimmick is just not accurate, and therefore not useful, for demonstrating the actual outlines of real traffic calming treatments.

As a final note, I do want to recognize that some of the "sneckdowns" people have posted probably do closely resemble the actual outlines of an appropriately designed neckdown for those specific sites. In many other cases, I strongly suspect a more detailed analysis would indicate that removing the snowy portion of the roadway could cause problems for normal street operations or create untenable complications for snow plowing.

There are great benefits from permanent neckdowns that clearly recommend more extensive use, but I am afraid the "sneckdown" framework does little to inform discussions. At best, "sneckdowns" can be a good observation to prompt people to start looking more closely at their local intersections. But spread uncritically across Twitter, this giddiness can just make it easier to write off proponents of street improvements as people who shouldn't be taken seriously.

For people who really are serious about making improvements on their streets, I would urge them to keep making observations. Focus on the snow plowing issues you can actually see. If you are inspired by what appears to be unused space at specific intersections, start looking more closely at how the real considerations for a curb extension would actually apply at those locations. The section on curb extensions in NYC DOT's Street Design Manual (see p. 74) is a good resource.

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