About Backtrack Occuring near the Drop-Off
The following factors are taken into consideration when Schedule Server checks for backtracking near drop-off events:
- Destination Retrenchment Threshold (DRT) - Like the Origin Retrenchment Threshold (also known as the Distance Threshold), it defines the radius of a circle which, once entered, triggers Schedule Server to check whether the vehicle has subsequently left that circle and then flag a BT violation if applicable. A destination retrenchment ratio (DRR) check does not occur if the vehicle enters a circle around the destination within the length of the radius, as defined by this the DRR switch. If travel then leaves this circle, a Backtracking (BT) violation might be incurred. Note that for the origin, the Distance Threshold switch setting applies.
- Destination Retrenchment Ratio (DRR) - A value expressed as a percentage of the DRT that is intended to draw a boundary for leaving the drop-off area (destination) compared to the closest point driven to the destination along the way.
To define the boundaries for backtracking to a destination event, the distance is measured from the destination to the first event that takes place within the Dest Retrenchment Thresh (DRT) to determine the Destination Reference Distance (DRD). The length of the DRD is multiplied by the value defined by the Dest Retrenchment Ratio switch to determine the radius of the Destination Backtracking Circle (DBC), which indicates whether a trip is considered to be backtracking. Any events that occur outside this circle does not generate a Backtracking violation (BT).
Assume that the DRT, represented by the orange circle, is set to 600m, and the DRR is 150%. The distance from Drop-Off 1 (DO1) to Drop-Off 4 (DO4) is 500m, which determines the radius of the DRD, represented by the black circle, because DO4 is the first event that occurs within the DRT on the way to DO1. Multiply the DRD by the DRR (500m x 150% = 750m) to get the radius of the Destination Backtracking Circle (DBC), represented by the red circle, outside of which backtracking is flagged. After DO4 is performed, no other event can be performed outside of the DBC without generating a BT violation.
In this example, events such as Drop-Off 3 (DO3) generates a BT violation because they occur outside of the DRD. However, events such as Drop-Off 2 (DO2) does not generate a BT violation because they occur within the DRD.