Percent Time Spent Following (PTSF) is the average percent of total travel time that vehicles must travel in platoons behind slower vehicles due to inability to pass on a two-lane highway. It therefore represents the freedom to maneuver and convenience of travel.

Percent time-spent-following is difficult to measure in the field. The HCM, however, suggests that the percentage of vehicles traveling with headways of less than 3 seconds at a representative location be used as a surrogate measure.

LOS criteria for two-lane highways are based on both PTSF and average travel speed.