"We found images with trash in it, and took the trash out, and we noticed a 30 percent increase in perception of safety. It's surprising that something that easy had that large an effect." So Phil Salesses, one of the authors of a PLOS ONE paper studying the perception of safety, class and uniqueness in city streets tells The Atlantic Cities. By comparing images from Google Street View of Boston and New York in the US and Linz and Salzburg in Austria, and photoshopping out individual elements (such as trash or graffiti) to fine tune the results, Salesses and his collaborators have gathered quantitative evidence to answer an often subjective question: what makes citizens feel safe? Learn more about Salesses' research at The Atlantic Cities and read the paper here.