Foregone benefits of the open space that is sacrificed through urban sprawl are hard to quantify. We obtain a simple benchmark measure by introducing a demand for trips beyond the urban boundary into the monocentric city model. The externality arises from the increase in travel costs that expansion of the city imposes on its prior inhabitants. An empirical application illustrates the moderate informational requirements. It indicates that open space externalities warrant rather mild restrictions on urban expansion. © 2013 Wiley Periodicals, Inc.