Transportation systems exist to improve individual accessibility. However, emerging applications of GIS in transportation (GIS-T) and intelligent transportation system (ITS) focus on throughput (the amount of system flow) rather than accessibility. Throughput is related but not equivalent to accessibility. Sensitive transportation planning requires rigorous, realistic and tractable accessibility measures. This paper reports on the development of GIS software that implement space-time accessibility measures (STAMs). The STAMs reflect the benefits that individuals receive from the transportation system. They are easily interpreted, particularly with respect to changes in accessibility. The STAMs also consider the locations and travel velocities dictated by the transportation system as well as individuals daily activity schedules. Tractable computational procedures allow calculation of the STAMs for detailed, urban-scale transportation networks. A prototype ArcInfo^®-linked software system that runs on Windows NT^® or Sun Solaris^® platform implements the STAMs with user-friendly interfaces and project management tools.