Scaling up and scaling down

New York bicycling advocate Steve Faust has stated that some ways of accommodating bicycling do not “scale up” — that is, they work with small numbers of cyclists, but less well with larger numbers.

His central complaint is that use of roadways with no special bicycle facilities, according to the conventional rules of the road, does not scale up well.

I might put that a bit differently. After all, more cyclists need more room. Mass rides such as New York’s own 5-Borough Tour avoid special bicycle facilities and occupy the entire width of Manhattan’s multi-lane avenues. Motor vehicles are excluded while these rides pass through. Interaction within the group of many thousands of cyclists is for the most part according to the conventional rules of the road, and falls short only in that many of the participants are inexperienced.

On roadways carrying both cyclists and motorists, cyclists inconvenience motorists when the motor traffic could go faster — that is, when there are many cyclists and few enough motorists that they could travel unimpeded, if not for the cyclists. Motorists inconvenience cyclists when motor traffic is congested, and stopped or traveling slower than cyclists might want to go. Level of service always declines as a road becomes more congested, and it declines faster when vehicles have differing speed capabilities.

On the other hand, there also are situations in which operation as intended does not scale down to smaller numbers.

Motorists are more likely, for example, to yield to a crowd of pedestrians than to a single pedestrian.

Another example is the leading pedestrian interval: the walk signal goes on a couple of seconds before motorists get the green light. The leading pedestrian interval is intended to get pedestrians moving out into the intersection before motor traffic can begin to turn across a crosswalk, encouraging motorists to yield to the pedestrians. The same approach is used sometimes on bicycle facilities, for example on the Boulevard de Maisonneuve bicycle sidepath in Montréal, Québec, Canada. But a leading interval only works if there is someone waiting to cross when the signal changes. With smaller numbers, so the first pedestrian or bicyclist reaches the crossing after the motorists get their green light, the leading interval’s only achievement is slightly to reduce the capacity of the intersection.

The same issue can occur with any “conflict zone” with poor visibility as users approach, including the “bike box” or bicycle waiting area ahead of the stop line for motorists at an intersection. Once one cyclist is in a “bike box”, a motorist is unlikely to move forward, because that would require running over the cyclist. Therefore, the bike box is then safe for the entry of other cyclists, at least into the same lane in which the first cyclist is waiting.

The”bike box” works as intended when there are large numbers of cyclists so the first one arrives well before the traffic signal turns green.

If there are few cyclists, so the first one is likely to arrive just as the traffic signal turns green, then there is the potential for a right-hook collision, or a motorist’s colliding with a cyclist swerving into the bike box.

Safety requires that there be enough cyclists that early-arriving ones block the way of motorists, or at least alert the motorists that others may arrive. This safety factor does not scale down to small numbers.

Research in Portland, Oregon shows that only 5% of bicyclists swerve into the bike box when they are first to arrive; about 35% if they arrive later. The reluctance of the first-arriving cyclist reflects risk avoidance to some extent, due to not knowing when the traffic signal will change, but also that the swerve lengthens the cyclist’s trip — none of the Portland bike boxes are designated for left turns. The later-arriving cyclists are to some degree protected by the arrival of the first one, but also they either have to wait behind or move over to the left of that cyclist, into the bike box.

“Safety in numbers” claims become rather interesting when such issues are considered.

The design challenge is to achieve efficiency and safety of all travelers, regardless of whether numbers are large or small.

3 responses to “Scaling up and scaling down

  1. Interesting point when applied to door zone bike lanes. If you’ve just been passed by 3 cyclists since you parallel parked, even a child would think to look before throwing open the door. On the other hand, if the door zone bike lane has been empty, you’re less likely to think about it. DZBLs at least scale well if there is a steady trickle of cyclists.

    One could “draft” another cyclist riding in the DZBL, with the leader 100 feet ahead, absorbing all the dooring risk. Or maybe not: in previous times, our Executive Director was doing just that when the cyclist in front of him got doored. He proceeded to run over her. Ooops.

  2. Dave wrote: “If you’ve just been passed by 3 cyclists since you parallel parked, even a child would think to look before throwing open the door.”

    Which means that DZBLs don’t scale up. If that many cyclists are passing, you won’t be able to open your door at all. Or if you did find a momentary gap in the bike traffic to open the door, the next batch of cyclists would flood the adjacent travel lane to get around your obstruction.

    • Case in point: the Dana Laird dooring fatality in Cambridge, Massachusetts occurred on an arterial street with a clearly-marked bike lane and a heavy bicycle traffic.

      Regardless of the effect of the number of other cyclists, or of driving culture, riding in the door zone at more than a very low speed renders the cyclist defenseless against dooring and also against a variety of other risks — primarily drive-outs and walk-outs. There’s a fundamental difference in cyclist mentality at work here, between a cyclists’ believing that he/she is defenseless in the first place, and so the responsibility for safety must rest entirely in the hands of other people, or driving the bicycle defensively/assertively, staying out of the way of risks against which there is no defense, testing the intentions of other road users and interacting actively with them.

      Relying on safety in numbers to reduce the hazards of the door zone is like saying that it is safer to play Russian roulette with a bullet in only one chamber or the revolver, rather than bullets in two chambers.

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