One of my staff sent me a link to this story on Treehugger about this innovative solution to the problem of not enough parks. In Istanbul, the Nezahat Gökyiğit Botanical Garden is a botanical garden inside the cloverleaf of a freeway exchange. This mostly privately funded 125-acre garden has over 1,800 plant species under cultivation, an herbarium with over 1,000 species, a children’s garden with programs for youth, and it features geophytes (plants that grow from bulbs), Quercus species (oaks), Turkish endemics, Turkish rare and threatened species, economic, medicinal and aromatic plants, and insectivorous plants.
Can we adapt this idea for greater Los Angeles? Sure there are lots of questions and issues to be explored – especially safe access for people, air quality inside the exchange, and potential impacts of plant roots and water infiltration – but even if we developed the spaces inside interchanges just for nature, we would all benefit from the air and sound filtering effects of all that vegetation.
A quick visit to the internet will show you how many of these spaces we have tucked into the Los Angeles transportation skeleton. Already some are used for construction project staging, and some have developed purposes (self-storage facilities, CHP stations, etc), indicating that at least in some places the issue of people access has been considered and approved
. It seems that some which have pedestrian access (by design or accident) might be a great place to tuck away a pocket park. And the ones that have no easy access might prove very useful for habitat restoration projects. Here’s one exchange in El Monte that I pass by frequently. What better could we do with this land?
Microclimate Forms 
I’m astonished at the vast massive freeway scale that puts 125 acres inside a cloverleaf… that, in and of itself concerns me. I hope it’s not an entire 125 acres (half the size of Taylor Yard) inside that one teardrop on the image. 125-acres is a small town!
I think, and have written about it at L.A. Creek Freak – see http://wp.me/pi4Y0-yd – that, if we’re really going to restore our watersheds to health then we need to drive less and narrow and remove these oversized roads and freeways to make way for more permeable areas.
Having said my obligatory anti-car screed, I agree that there’s certainly a lot of unused real estate along and encircled by L.A.’s freeways… and that much of it makes a lot of sense to use as habitat, native landscaping, bioswales, etc.
Some drawbacks include: pollution (you mention air and water – but noise pollution is also very intense along freeways) and the difficulty in getting permission for these sites. Caltrans is often very reluctant to permit projects on their property… because they nearly always have plans to expand freeway capacity… so they usually don’t want other uses to pin them into their current foorprint.
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