Is it possible to make that the units going one way oftener than any other way?

Either create a bigger connection with bigger hubs (not sure if this works) and a smaller connection with smaller hubs on the other side OR create more hubs and connections on one side than the other.

Useful resource: http://www.gametoast.com/viewtopic.php?f=27&t=20144

Change weight (planning tutorial on this forum)

Thank you two I will try