The 2000 "Highway Capacity Manual" (HCM) uses percent time-spent-following (PTSF) as the primary level-of-service (LOS) measure for two-lane highways. On Class I highways, average travel speed is used as an auxiliary measure. The new HCM provides methods for both two-way and directional estimation of PTSF, although the methods are not totally compatible. On Finnish two-lane highways, PTSF is lower than that estimated by the HCM 2000 methodology. Also, the impact of directional distribution is greater, whereas the impact of no-passing zones is less. From the beginning of the motorized transportation era, traffic engineers have searched for methods to define the traffic capacity of highways. Capacity estimation is not enough, however; traffic engineers and road users are both interested in the quality of traffic flow. After almost a century, this search is not yet over. In fact, transportation telematics has increased the need for reliable descriptions of quality traffic flow by using performance measures and concepts that are easily understood by road users. Several performance and service measures (to assign a LOS) suggested for two-lane highways are discussed. The properties of PTSF are discussed in more detail because PTSF has been the most important service measure for two-lane highways since HCM 1985. The estimation of PTSF and its adjustment for prevailing conditions in HCM 2000 are compared by the use of data from Finland. Finally, the applicability of PTSF as a service measure is evaluated against a set of qualifications.