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.