We turn next to another extensive category of urban service system problems, those concerned with determining good locations for the stationing of service vehicles or the construction of major facilities. These problems arise in the context of both routine and emergency services, but the objectives are usually different in the two cases.

As one might readily suspect, the "goodness" of a location depends on the measure of effectiveness being used. To take an extreme example, the center of a town might be considered an ideal location for a post office but would certainly be a very poor choice for use as a garbage-incineration point. In the case of the post office a reeasonable objective might be to minimize the average walking distance to the building for town residents--hence the choice of the center of the town. For the garbage incinerator, a more appropriate objective would be to maximize the minimum distance between the chosen point and any home or building in the town.

In the following sections we shall use the measure of effectiveness at hand as our principal guide to the classification of facility location problems. We shall examine here problems in the following three categories:

1. Median problems. Here a prespecified number of facilities must be located so as to minimize the average distance (or the average travel time or the average travel cost) to or from the facilities for the population of their users. Median problems arise very often in the context of facility construction for delivery of nonemergency services (e.g., post offices, transportation terminals, telephone interchanges, "little town halls," offices for government agencies dealing extensively with the public, etc.).

2. Center problems. Here a prespecified number of facilities must be located so as to minimize the maximum distance (or time or cost), to or from the facilities, that any user will have to travel. Center problems (also sometimes referred to as minimax problems) are more applicable in the context of emergency urban services, notably emergency medical care, fire fighting, and emergency repair services.

3. Requirements problems. These are problems in which certain standards of performance have already been prespecified for a service system and one seeks the number of facilities required to meet these standards as well as the locations of these facilities. Obviously, this type of problem is a more general one than the median or center problems and is applicable to both emergency and nonemergency services.

In the course of discussing these three types of problems, we shall find opportunities to mention some variations, extensions, or combinations of the three themes described above, all of which lead to significant applications in the urban service field.