Abstract: Percent Time-Spent-Following (Percent Time Delay) is an important index to estimate Level-of-Service of tow-lane highways. Vehicles traveling at headways of less than 6 seconds may be taken to be followers on two-lane highways in China. Through the analysis using data of headway, speed, and other characteristics from interrupting traffic flow on a highway on mountainous areas, the relationship between delay headway and speed dispersion differs from that of on level areas. The paper advises not to adopt the Percent Time-Spend-Following as the index of measuring the LOS. And the result can provides some useful data to the study of highway capacity about tow-line highways on mountainous areas.