The purpose of the study summarized in this article was to explore which physical characteristics influence people's perception of density on urban residential streets. Three streets in San Francisco, California, were evaluated and compared in terms of visual characteris tics, such as variety and distinctness of form, colors, materials, and patterns. These measurements were compared to the perceived density of these streets as indicated by both residents and non-residents. The study found three physical characteristics to be very strongly associated with perceptions of lower density: (1) greater building articulation; (2) less facade area or smaller buildings; (3) a greater number of "house"-like dwellings. These findings have significant implications for urban policy and design practice.