VRP: Different solutions to the same problem

Question

I'm calculating  several options to pickup the urban waste to get the least number of truck routes.

In the first option, I use 22 trucks and the solver notes me that it use 14 trucks to pick up all containers, but in a second option I indicate 14 trucks, the solver notes me there isn't a complete solution and the 14 truck can't pick up all containers. Why does it happen?

LuisDel_Vigo · Answer

Thanks Heather!

As you tell me I have a lot of orders, approximately 2000 orders. My doubt is beacause I uuse, in the firts option with 22 trucks, and the solver offers me one solution with 14 trucks. UIn a asecond option with 16 trucks, the solver offers me the same solution, with identical routes, but in the third option with 14 trucks the solver offers me a partial solution. The most curious is if in a new option (fourth option) with 14 truck too, I take the orders from the previous options configuring all orders Asigment rule in "Preseve route and sequence" I get the same solution that 22 and 16 truck with only 14 trucks. In the third option 14 trucks, Asigment Rule to the orders are in "Overrride".

I will try introducing a fixedcost, is a great idea!

Thanks!!

HeatherMoe · Answer

The VRP Solver is a heuristic meaning that it follows a set of steps to produce an answer.  It, therefore, can be sensitive to the initial set up of the problem when starting the solver. In your case that seems to be the case and is why when you  changed the number of routes that are available the solution is different.

For trying to minimize the number of routes used, the best way to do this is to add a high value to the FixedCost.  This will indicate to the solver that it should try not to open an additional route because that is more costly then taking an existing route and extending the travel time and distance.  You might need to experiment with the FixedCost value to determine what is large enough to get the desired results for number of routes.

You mention that you are doing urban waste pickup, I usually associate this with a high density of order locations (several on the same street right next to each other).  Is that true for you?