TTF is indeed the more useful metric for many applications. However, the TTF has a probabilistic aspect that must be understood before its use in, for example, setting an integrity reassessment interval.
The TTF calculation uses segment lengths since degradation mechanisms are usually related to surface areas. A probabilistic TTF per mile is an intermediate calculation for time dependent failure mechanisms even though differences in length do not necessarily imply a change in TTF.
It is NOT appropriate to simply choose the shortest TTF in a collection of segments (eg, trap to trap) to defend a TTF for the whole collection. The length implications of the segments’ TTF’s must be considered when determining the integrity reassessment interval for the collection. This is sometimes done by translations of TTF into PoF.
Furthermore, the TTF for each time dependent mechanism is only a part of the understanding of overall PoF. We need a PoF from each failure mechanism in order to compare them with other threats and get an overall PoF.