This study attempts to comprehensively and objectively measure subjective qualities of the urban street environment. Using ratings from an expert panel, it was possible to measure five urban design qualities in terms of physical characteristics of streets and their edges: imageability, enclosure, human scale, transparency and complexity. The operational definitions do not always comport with the qualitative definitions, and provide new insights into the nature of these urban design qualities. The immediate purpose of this study is to arm researchers with operational definitions they can use to measure the street environment and test for significant associations with walking behaviour. A validation study is currently underway in New York City. Depending on the outcome of this and other follow-up research, the ultimate purpose would be to inform urban design practice.