Class I two-lane highway is a highway on which motorists expect to travel at relatively high speeds. The level of service (LOS) on a Class I two-lane highway is defined by two performance measures: average travel speed and percent time-spent-following (PTSF). The LOS thresholds defined by these two parameters are given in the following table:

Two-lane highways that are major intercity routes, primary arterials connecting major traffic generators, daily commuter routes, or primary links in state or national highway networks generally are assigned to Class I. Class I facilities most often serve long-distance trips or provide connecting links between facilities that serve long-distance trips.

As an alternative to classification as a Class I facility, a two-lane highway could also be classified as a Class II facility.