In this paper we address the issue of locating hierarchical facilities in the presence of congestion. Two hierarchical models are presented, where requests are attended first by lower level servers, and then some of the served customers are referred to higher level servers. In the first model,...

Previous emergency service covering models consider all the calls to be of the same importance and impose the same waiting time constraints independently of the service's priority. This type of constraint is clearly inappropriate in many contexts. For example, in urban medical emergency...