Time cut-offs are useful because they are slightly easier to calculate. Some tool is needed to figure out how far one can go on a system in the specified amount of time, and then all points corresponding to a destination within that area are counted up. They also tend to be easier to communicate to the public or to decision makers. 10,000 jobs accessible in 30 minutes from a given area, or a new line or faster service increasing the number of jobs available in 30 minutes of travel is a relatively easy idea to understand.
Time cut-offs suffer a few disadvantages however. A large employment center just outside of the time range is completely disregarded. So if there are 1,000 jobs available in 30 minutes but 2,000 jobs available in 31 minutes, the analysis will ignore the jobs just outside the cut-off and yield misleading results. Cut-offs also fail to discriminate between where destinations are located within a region. For example, this type of analysis will provide the same results if all of the jobs are five minutes away or thirty minutes way, although being five minutes away clearly provides better access. Some of these disadvantages can be overcome by conducting multiple analyses with different cut-offs, such that instead of having one number for a certain distance, the change as
one gets farther away can be observed.