Following Status and Percent Followers on Two-Lane Highways: Empirical Investigation

This paper presents an empirical investigation into car following interactions. Detector data from 15 study sites in Idaho, Montana and Oregon representing all three highway classes were used in this investigation. Investigation of speeds and headways suggests that the headway at which vehicles start the car-following interaction (referred to in this study as the critical headway) varied in a range that is site-specific, with the lower limit generally varying between 1 and 2 seconds, and the upper limit varying between 6 and 7 seconds. The study also found that vehicles traveling at a perceived minimum safe headway increase in number as headways get smaller and more vehicles enter the car following state. Further, the study introduced a new approach for estimating percent followers in the traffic stream and used field data in deriving headway cut-off values. Headway cut-off value for percent followers at class I sites varied between 1.8 and 2.8 seconds, while those for class II and class III sites are slightly above 3 seconds. This paper presents an empirical investigation into the car-following interaction and the estimation of percent followers on rural two-lane highways. Field data from 15 study sites in Idaho, Montana and Oregon were used in this investigation. The most important findings of this study are summarized as follows: