This study describes the development and application of a set of accessibility measures for the Twin Cities region that measure accessibility by the automobile mode over the period from 1995 to 2005. In contrast to previous attempts to measure accessibility this study uses travel time estimates derived, to the extent possible, from actual observations of network performance by time of day. A set of cumulative opportunity measures are computed with transportation analysis zones (TAZs) as the unit of analysis for the years 1995, 2000 and 2005. Analysis of the changes in accessibility by location over the period of study reveals that, for the majority of locations in the region, accessibility increased between 1995 and 2005, though the increases were not uniform. A "flattening'' or convergence of levels of accessibility across locations was observed over time, with faster-growing suburban locations gaining the most in terms of employment accessibility. An effort to decompose the causes of changes in accessibility into components related to transportation network structure and land use (opportunity location) reveal that both causes make a contribution to increasing accessibility, though the effects of changes to the transportation network tend to be more location-specific. Overall, the results of the study demonstrate the feasibility and relevance of using accessibility as a key performance measure to describe the regional transportation system.