SUMO - How to generate routes without using shortest path algorithms?

When running a simulation in SUMO with routes generated by duarouter on a grid-like network, after a certain amount of time, the vehicles start to concentrate on the middle of the grid and in junctions due to the fact that the routes are generated using a shortest path algorithm (dijkstra, astar, CH or CHWrapper). How to generate random routes that are not created with a shortest path algorithm and do not make vehicles behave in that manner?