1. On a wireless device capable of communicating over a first radio-access network (“RAN”) and a second RAN, a method for selecting a RAN to be used as the second RAN for offloading traffic from the first RAN to the second RAN, the method comprising: receiving, from a first network element, a first set of parameters for evaluating available RANs; receiving, from a second network element, a RAN-selection policy, wherein the RAN-selection policy includes a second set of parameters for evaluating available RANs and a preference indicator to indicate whether the first set or the second set is a preferred source of parameters for evaluating available RANs; determining, based at least in part on the preference indicator, whether a corresponding parameter in the first set should be used in place of a parameter in the second set when applying the RAN-selection policy; and applying the RAN-selection policy to select the RAN, including if it is determined that the corresponding parameter in the first set should be used in place of the parameter in the second set, then applying the RAN-selection policy using the corresponding parameter in the first set in place of the parameter in the second set.