==Accessibility vs. Mobility==
Transit systems have been increasingly focused on accessibility when evaluating their systems and new projects. Although accessibility has always been a chief goal of transit, past measures have tended to focus on mobility, looking at number of people who can access the transit system, system capacity, and line speeds. While improvements in service strengthen the transit system, these measures do not evaluate whether people have access to important destinations. Fast transit lines reaching many may nevertheless fail to connect people to job centers, grocery stores, or schools. Far reaching transit systems may force multiple transfers to reach a destination, such that the trip becomes unreasonable long. Accessibility directly addresses these questions by measuring people's ability to reach desired destinations.
==Decay vs. Cut-offs==
One major distinction in measuring accessibility is how parameters are set for determining if a destination is accessible. One method is to create a time cut-off. For example, one can ask how many jobs can be reached in 30 minutes using transit from a given location? Or, how many household/job combinations are there within 30 minutes of each other along a transit line or in a transit system? The alternative is to use a decay function. A decay function weights destinations based on how far they are, so that a farther locations are given less weight than closer ones.