A method of generating a set of solutions to a route-planning problem includes iteratively applying a Probability Collective (PC) method, each iteration including sampling from a probability distribution defined over a set of decision variables relating to a route-planning problem and using an archive set of decision variables and a set of the sampled decision variables to update the probability distribution for use in a subsequent PC method iteration. A set of solutions, each including at least one decision variable with an associated objective function result evaluated by the PC method iterations, is output.