Street Greenery is Better than Thought

As it turns out, street greenery is even better at reducing pollution than thought. The research that I had found for last week’s post showed greenery can filter out some of the worst particles but only up to 8% of the total pollutant plume. New research just released shows that green walls, when employed in an “urban canyon” environment, can filter out up to 60% of the particulate matter in diesel pollution. That, coincidentally, will include the SMART train.

The urban canyon environment is when buildings go up on either side of the street and form single walls interrupted only by cross-streets. San Rafael’s Fourth Street is a good example of this in Marin, and San Francisco’s Market Street is a superb example in any context. These canyons create their own wind environments, circulating air up and then down and then up again in a vertical circulatory pattern.

Green walls are either plants growing out of the wall, like vertical gardens, or plants growing down walls, like ivy from planter boxes or wisteria from the ground. If these walls line a part of the urban canyon, the wind patterns will run polluted air through leaves multiple times, allowing the air to be filtered again and again.

Cities should actively encourage green walls to capture this effect, and SMART should plan for it where trains will run through residential areas. In places where buildings may rise above the freeway, as in the Downtown San Rafael Station Area Plan, green walls could be especially helpful in filtering the worst part of the air. Any investment in greenery for health reasons will be best put here. Similarly, where the freeway runs at ground level, ivy should be encouraged to grow on the sound walls.

As an added bonus, greenery cuts down on urban noise. Given how loud both the freeway is and the SMART train will be, encouraging leafy walls will be able to make our city streets that much more livable.

Investing in greenery is the single most cost-effective way to reduce pollutants and keep our cities healthy. With the new construction and higher-density zoning slated for areas up and down the 101 corridor, city councils and planning agencies need to take it seriously as more than just environmental greenwashing.

About David Edmondson
A native Marinite working in Washington, DC, I am fascinated by how one might apply smart-growth and urbanist thinking to the low-density towns of my home.

4 Responses to Street Greenery is Better than Thought

  1. Richard Hall says:

    Have to agree with you on canyons of green. Few Americans drive diesel cars though, wish more did – the mileage efficiencies could dramatically improve US mpg figures. I see my parents driving diesel cars for over 10 years that do over 60mpg yet they seem taboo here in the US. Cars are taxed in my home country based on their CO2 emissions – not so here in the US. Couldn’t encouragement via cheaper diesel and car tax based on CO2 emissions eclipse radically expensive initiatives ($695m and I think I’m understanding) such as the SMART train?

    If these innovations could be embraced by just a fraction of the US population then we wouldn’t need to spend $1/2 billion or more by inflicting the SMART train at substantial taxpayer cost.

    Can you identify the greenhouse gas emissions savings of the SMART train and identify what the equivalent cost would be to achieve via:
    – an improved bus system
    – continuing adoption of more fuel efficient cars (which seem to be occurring in any case without imposing unpopular and uneconomic transit systems).

    You might take a trip to Europe – there are some lessons to be learned from my homeland (England). I love trains when they make reasonable returns and used London Transport and London Underground religiously for over 6 years. European rail systems are true transit networks that connect to employment centers (E.g. SF, Oakland) and airports – these are the factors that ensure adoption and achievement of realistic farebox recovery rates.

    SMART does seem a little ambitious with farebox recovery projected to be at a meagre 20% (who is going to pay the other 80% pray tell?). Granted that the ridership numbers are for 2035 for the full length of the track (Cloverdale – Larkspur) but this is not exactly what’s being built.

    • No, I don’t have time for a full EIR with alternatives. But here’s what my gut says:

      Do all three, along with SMART, to address CO2 emissions. But CO2 is only part of the story.

      It’s more about moving beyond the automobile, and giving people a viable option to live without one in the more built parts of the county. At the moment, the options for Marinites are car dependency and bike dependency, with either supplemented by bus service. The car option is expensive for the towns and for the users, while the bike option can’t be used by everyone. So, how do we improve transit?

      This post addresses a different question, How do we improve air quality? It’s a follow up to last week.

      Off-topic, SMART will pay for the extra 80% through its existing sales tax. It’s rather worse than London Underground’s 50%, but is comparable with GGT.

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