I am looking for references to the variant of the vehicle routing problem over Manhattan distance metric where the aim is to optimize the number of tours starting at the depot.Is the following problem NP-hardProblem :The distance constrained version of the vehicle routing problems over Manhattan distance metric considering an complete $N\times N$  grid or a grid where some edges are missing  ?The grid graph is a weighted one.
Vehicle routing problem over Manhattan distances
cc.complexity theory;reference request;graph theory;np hardness;optimization