I saw this project among other impressive new design / inventions at the Brit Insurance design awards at The Design Museum.
"Love Letter is a project by Stephen Powers with the City of Philadelphia Mural Arts Program and is sponsored by the Pew Center for Arts & Heritage through the Philadelphia Exhibitions Initiative."
Stephen Powers used to be a prolific tagger and graffiti artist (under the name ESPO) in Philadelphia and New York in the 90s, his work blurring the lines between legal and illegal. In 2000 he gave up to become a proper artist, and later in the decade he began Love letter, for which ESPO and his crew painted more than 50 murals along the elevated train along Market Street in West Philadelphia.
The book and a slideshow of the images was on display in the museum. Here are some of the murals.