Accessibility is increasingly identified in the academic literature and in planning practice as a key criterion to assess transport policies and urban land use development. This paper contributes in two respects to the growing body of literature on accessibility and accessibility measurement. First, it proposes a set of accessibility measures that directly relates transit-based and car-based accessibility to each other. Second, it presents a tool based on a geographic information system (GIS) that measures accessibility at a high level of resolution. The tool, called Urban.Access, has been developed as an ArcGIS extension and can be used in urban regions worldwide, provided high-resolution GIS data are available. Urban.Access enables a detailed representation of travel times by transit and car and thus makes it possible to adequately compare accessibility levels by transport mode. The first application of Urban.Access to the Tel Aviv, Israel, region shows substantial gaps between car-based and transit-based accessibility throughout the metropolitan area. The paper ends with a brief discussion highlighting the policy consequences of this finding.