Trying to understand routing decisions (VRP)

Discussion created by platinumdesign on May 17, 2012
Latest reply on May 30, 2012 by nan-esristaff
Our client is in the people transportation business and we have been working with them to assist in scheduling.  This example will no doubt seem odd when considering the overall intention of ArcGIS and Network Analyst, but please bear with me.  Often times our client will start their scheduling by assigning some (usually one to five) trips [Order] to a driver [Route].  These trips [Orders] consist of both a pickup location and a drop off location [Ordered Pair].  It is not uncommon for some of these pre-assigned trips to be impossible to accomplish with hard window time frames.  Here is an example:

Trip A has a 4:45 pickup time, Trip B has a 4:46 pickup time and is 5 miles away.  Both trips are going to the same destination address.

Our Client will assign these two trips to the same driver as they are in the same general proximity.  Their passengers typically know that they are not going to be picked up exactly at the times listed, so the actual times they have listed are more to help our client visually see them together.  The problem however is trying to get Network Analyst to accept these pre-assigned trips with the route.  So far, the only way I've been able to avoid a violated constraints message is by setting the service time to zero for both pickup locations, opening the TimeWindowStart1/TimeWindowEnd1 by 20 minutes and setting the MaxViolationTime1 to 20 minutes as well.  Each [Order] has its [Route] pre-assigned and uses AssignmentRule 2 and is given a Sequence that increments by one.

Now on to the confusing part.  With the above conditions in place, the VRP will finally accept the pre-assigned orders and provide result data.  What is confusing however is that it lists Trip B (4:46 pickup) as first in the result sequence, followed by Trip A (4:45 one).  Initially I thought that this was because Trip B was closer to the client's main office [Distribution Center], however if we switch the times, the results are similar with the solution assigning Trip A (now at 4:46 and further away) first and Trip B (now 4:45) second.  This has become very confusing and frustrating.  Anyone have any insight into why the VRP wants to stop at a later appointment first?

Thank you for your assistance.