Last May, I wrote a post about the need to account for air quality issues when designing bike infrastructure. In it, I argued:
With all this in mind, the concept of “vulnerable road users” takes on a new meaning. Cyclists are not only at a greater risk of being injured or killed in a collision, we are also at a heightened risk of suffering the ill effects of [transportation-related air pollution].
Planners must start taking this into account. Bike infrastructure that may make sense from a safety standpoint may not hold up when we account for air pollution. And don’t get me started on vehicular cycling advocates. Cleveland’s decision to design bike lanes that buffer the curb already made no sense from a safety perspective. When you add air quality to the equation?…
Ultimately, it’s time to broaden our horizons on bike infrastructure. Just as we shouldn’t expect individual cyclists to bear the risk of being run over to improve road safety, so too shouldn’t we expect cyclists to inhale poison so the rest of us can breathe cleaner air. Let’s start accounting for air pollution exposure and intake when planning bike lanes.
But never fear – I have come back to rectify this shortcoming.
In the past few months, two new studies have come out that shed light upon how planners can actually operationalize my vague statement of principles, allowing them to consciously protect cyclists from the harmful effects of air pollution. The first adds further insight into how to best craft vegetative buffers to intercept pollution, while the second investigates how to design streets that facilitate active transportation while mitigating pollution concentrations.
Properly designing vegetative buffers
In a study published in May’s issue of Atmospheric Environment, K.V. Abhijith and colleagues from the University of Surrey in the United Kingdom dig into the existing literature on the relationship between street side vegetation and air quality in urban settings.
While previous research had investigated how trees, hedges, green walls, and green roofs affected pollution levels in cities, no previous study had further broken down these effects based upon built environment characteristics or developed a comprehensive set of recommendations on how best to deploy greening interventions in urban areas to mitigate pollution levels.
Abhijith et al. categorized the impact of vegetation on neighborhood-level air quality based on different urban forms, particularly street canyons (streets flanked on both sides by rows of densely packed tall buildings) and open road settings (less densely developed areas).
Based upon a meta-analysis of the existing literature, the authors concluded that “there is a consensus that an increase in pollutant concentrations in street canyons occur with the presence of trees.” Because trees can interfere with the movement of wind in street canyons, they can actually elevate pollution levels by 26-96%.
Hedges, on the other hand, do not limit air flow in street canyons. As a result, they can reduce pollutant exposure in these settings by 24-61%, depending on their height. Based upon model simulations, the authors concluded that the optimum height for hedges along the roadside in street canyons is 1 to 2 meters.
In open road settings, such as residential streets or broader boulevards, on the other hand, trees do not appear to affect air quality adversely. In contrast, the authors recommend creating a vegetated buffer of both trees and hedges roughly 5 to 10 meters in depth along open roads; these buffers can halve particulate matter (PM) concentrations.
But Abhijith and colleagues actually went further than this by examining how the presence of passive objects – specifically on-street parked cars – affect street-level pollution.
Interesting, the authors found that combining trees with on-street parking actually generated a greater air quality benefit than a vegetated buffer alone. This effect was greatest when using smaller trees that are properly spaced from one another.
The combination of vegetation and solid passive air pollution control measures has the potential to maximize the reduction in pollutant concentrations and improve personal exposure conditions, more than that achieved [by] any individual intervention in both street canyon and open road conditions.
Identifying the right corridors to target
The second study, which appeared in April’s issue of Environmental Health Perspectives, considered estimates of bike/pedestrian traffic volumes and PM concentrations in Minneapolis.
Researchers Steve Hankey, Greg Lindsey, and Julian Marshall hoped their work could inform further active transportation planning by identifying particular characteristics of the road system associated with low-exposure bike and pedestrian infrastructure networks.
The authors found that roughly 20-44% of all active travel occurs in so-called “active and exposed” area – those high levels of both active travel and pollution; in turn, just 3-9% occurs on “sweet spot” areas – those with high levels of active travel and low levels of pollution.
According to their analysis, shifting bike/pedestrian traffic over just one block from major roads can reduce individual exposure to PM by 3-19%.
The authors call for identifying those roads at least 200 meters away from major corridors. Doing so can help planners better inform their decisions, allowing them to create networks that limit exposure to both vehicular traffic and vehicle pollution. This step can reduce an individual’s pollution exposure during transportation by roughly 9%.
Based on their findings, the authors argue,
Minor shifts to the bicycle and pedestrian network may reduce overall exposure, for example, by moving cyclists away from pollution by strategic location of bicycle infrastructure on low-traffic roads and/or moving pollution away from pedestrians by shifting the location of emission sources (e.g., bus routes or stops).
Bringing air quality into the bike planning discussion
Taken together, these two studies constitute the beginning of an effort to actually operationalize air quality considerations in bike infrastructure planning.
When possible, planners should listen to Hankey, Lindsey, and Marshall and identify streets adjacent to major transportation corridors and place bike infrastructure improvements there first.
But when this is not possible, or when planners have identified major corridors where bike infrastructure is absolutely necessary, they should learn from Abhijith et al. The best bike infrastructure for such settings would likely be a lane separated from traffic by both on-street parking and a vegetated buffer, most likely of mid-sized hedges.
This sort of design could go a long way to reducing cyclists’ exposure to both cars and the pollution they create. This, in turn, would truly demonstrate a city’s commitment to sustainability and protecting its vulnerable road users.