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 "The Green Streets" program:

CONCERNS:

• Asphalt is a toxic material and not "green" in any meaningful sense - http://www.nature.nps.gov/hazardssafety/toxic/asphalt.pdf
• Asphalt also presents many of the same problems presented by concrete - see above
• The Percy Street model creates a cistern under the streets, using clean stones and lined on either side of the road with plastic, that they claim will protect nearby basements from water damage.  However, we are concern that it is a fatally flawed design concept.  Typically plastic liners, even landfill liners, are only 1/10 of an inch thick and are vulnerable to cracking and breaking due to heat and cold, wear and tear. Our preliminary research shows that buried "impermeable" plastic liners only last from 15-20 years. Not good news for homeowners. http://www.gfredlee.com/Landfills/SubtitleDFlawedTechnPap.pdf 
• According to the video above, the Percy Street model seems to discourage use of trees on small streets
• Pete Riley, Design Unit, Philadelphia Water Department may be a good person to contact for addition information

LINKS:

• https://beta.phila.gov/media/20160504172218/Green-Streets-Design-Manual-2014.pdf
• http://www.phillywatersheds.org/what_were_doing/green_infrastructure/programs/green_streets  Also see video http://www.phillywatersheds.org/new-video-philadelphias-first-porous-street. 
• We found important details about the Percy Street project in this article,  http://www.southphillyreview.com/news/Percy-Street-goes-porous-122194513.html?printView=y, which states,  "their (new) block contains eight inches of porous asphalt, a material that will allow rain to seep through the pavement to soak into a nearly 30-inch layer of stone... (and then) line the curb with an impermeable plastic liner to prevent water from migrating back toward houses uses...".