Leader/Follower Status

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If a vehicle is identified as being a follower, it means that the vehicle is close enough (either in distance or time) to the vehicle in front of it that the car-following model should be included in the process of determining the appropriate acceleration rate for a vehicle. While an acceleration rate per the car-following model is calculated, it should be noted that a different acceleration rate may ultimately govern. If a vehicle is not identified as being a follower (i.e., it is a leader), then the calculation of an acceleration rate per the car-following model is skipped for that time step. The variables and logic used to determine whether a vehicle is considered to be in a follower status are as follows:

Variable Definitions
 * DistanceToLeadVehicle = The distance between the front bumper of the subject vehicle and the rear bumper of the lead vehicle, in feet
 * SubjectVehicleSpeed = Velocity of the subject vehicle, in ft/s
 * Headway = time elapsed for subject vehicle to travel distance equal to DistanceToLeadVehicle, in seconds
 * HeadwayThreshold = Base time headway value to consider for follower status. Default = 2.5 s, can be revised in CarFollowModelParms.xml file.
 * FollowerHeadwayBuffer = Additional headway buffer for which to consider a vehicle for follower status, Fixed value of 3 s.
 * CarFollowDistance = Distance between subject vehicle front bumper to rear bumper of the lead vehicle, for which to consider a vehicle for follower status as a secondary measure. Default = 110 ft, can be revised in CarFollowModelParms.xml file.

Logic Conditions
 * Initially assign subject vehicle to Status = Leader
 * if DistanceToLeadVehicle (ft) is unassigned (i.e., no vehicle in front of subject vehicle in same lane), Status = Leader
 * else
 * if (SubjectVehicleSpeed > 0)
 * Headway = DistanceToLeadVehicle / SubjectVehicleSpeed
 * if (Headway < (HeadwayThreshold + FollowerHeadwayBuffer)
 * Status = Follower
 * else if (DistanceToLeadVehicle < CarFollowDistance)
 * Status = Follower
 * else
 * if (DistanceToLeadVehicle < CarFollowDistance)
 * Status = Follower

Note: In the vehicle properties dialog displayed in the simulation animation and the time step data output file, a value of -1 for 'Distance to Lead Vehicle' means that there is no vehicle ahead of the subject vehicle in the same lane. In this case, the subject vehicle will be of type 'Leader'.