A critical headway threshold value of 2.5 s is used to determine whether a vehicle is in a follower state. Thus, any vehicle following another vehicle at a headway of 2.5 s or less is considered to be in a follower state and percent followers is simply the percentage of all vehicles with a headway of 2.5 s or less at a given roadway location. The general relationship between percent followers and flow rate.
Note that even at a capacity flow rate, 100% followers will not be reached. This is because there will always be some gaps between platoons within the traffic stream as a result of different platoon lead vehicle desired speeds.
Follower Density Follower density is defined as the number of vehicles in a follower state per mile per lane. Mathematically, it is the percent followers multiplied by density. This measure is used as the service measure for two-lane highways because of its sensitivity to both traffic demand and geometric alignment variability. On two-lane highways, it is possible for two roadways to have similar values of density but very different levels of percent followers. Likewise, it is possible for two roadways to have similar percent followers but very different values of density. Therefore, service quality is better represented by considering the combination of follower percentage and density.