The Choose Best Facilities tool finds the set of facilities that will best serve demand from surrounding areas.
Set the appropriate goal
The Choose Best Facilities tool allows you to specify the type of problem you are solving, called the goal. The tool provides a solution to that particular goal. Regardless of the goal, essentially the same underlying method is used to reach a solution: the demand amount from a given demand location is allocated to the nearest facility. The goals vary in how the interplay between distance or time and demand is handled. The tool considers both travel distance or time and amount of demand and, in some cases, the capacity of the facilities. If a goal favors minimizing travel time, for example, the tool will also try to maximize demand allocated. Similarly, if the goal is to maximize demand allocated, the tool will do so by minimizing travel time by assigning demand locations to the closest facility rather than a random facility.
Depending on the data and the settings you use, different goals may produce the same or similar results. Regardless, specify the goal that is the most appropriate for your analysis. The following list gives an overview of the goals you can specify:
- Allocate to existing facilities—Assign demand to facilities based on the capacity at the facility and the amount of demand from the surrounding area.
- Minimize travel—Choose facilities so the overall travel time or distance from all demand locations to their assigned facility is minimized.
- Maximize coverage—Choose facilities so the maximum amount of demand is allocated, with all the demand from each demand location allocated to the single facility closest to it.
- Maximize coverage with capacity—Choose facilities so the maximum amount of demand is allocated without exceeding the capacity of any facility.
- Cover a percentage of demand—Choose the minimum number of facilities needed to reach the specified percentage of demand.
Allocate to existing facilities
This goal assigns demand to facilities based on the capacity at the facility and the amount of demand from the surrounding area. Demand is allocated to the closest facility until the available capacity is met. If you specify a maximum travel range, any demand locations beyond the range will not be assigned to a facility, even if all the demand has not been completely allocated.
Since you are not selecting locations for a new facility, you do not specify a candidate facilities layer.
The capacity can be unlimited, can be the same for all facilities, or can vary between facilities. If the capacity varies between facilities, specify a field in the facilities layer containing the capacity values.
If the capacity is unlimited, the goal assigns demand to facilities so the overall travel time or distance between the demand locations and the facilities is minimized. If a capacity is specified, the goal attempts to assign demand to facilities so the maximum amount of demand is allocated, even at the expense of increasing the overall travel time or distance.
Minimize travel
This goal selects facilities so the overall travel time or distance from all demand locations to their assigned facility is minimized. This goal is often applicable when there is travel between the facility and all demand locations on a regular basis.
The goal tends to equalize the distance that people have to travel to a facility and is often used for locating public facilities, such as libraries or health care centers, to ensure equal access from all parts of the community.
Note:
Other goal types, such as those that maximize overall demand allocated, may result in some people having to travel much farther to one facility than other people do to another facility.
Maximize coverage
This goal selects facilities so the maximum amount of demand is allocated, with all the demand from each demand location allocated to the single facility closest to it.
A finite maximum travel range is required for this goal; otherwise, all of the demand might be assigned to a single facility. Demand locations that are beyond the distance or time of any of the facilities will not be allocated. Demand locations that are not allocated show where demand is not being met and where additional facilities may be needed.
To determine where to add facilities, include the existing facilities in the required facilities layer and the potential locations as the candidate facilities layer. If you are locating new facilities in an area where none exist, include a candidate facilities layer only, with no required facilities layer.
Maximize coverage with capacity
This goal selects facilities so the maximum amount of demand is allocated without exceeding the capacity of any facility. All the demand from each demand location is allocated to the single facility closest to it that has available capacity (that is, if assigning a demand location to a facility would exceed the available capacity of the facility, the demand location will not be assigned, rather than assigning a portion of the demand).
A finite capacity for each facility is required for this goal. The capacity can be the same for all facilities or can vary between facilities. If the capacity varies between facilities, specify a field in the facilities layer containing the capacity values.
If you specify a limited maximum travel range, only demand locations within the time or distance of a facility will be allocated to the facility. Demand locations that are beyond the distance or time of any of the facilities will not be allocated, even if available capacity has not been met.
To determine where to add facilities, include the existing facilities in the required facilities layer and the potential locations as the candidate facilities layer. If you are locating new facilities in an area where none exist, include a candidate facilities layer only, with no required facilities layer.
Cover a percentage of demand
This goal selects the minimum number of facilities needed to reach the specified percentage of demand. A demand location that is within the maximum travel range of two or more facilities will have its demand split proportionally between the facilities based on its distance from each. This goal is often used when there is a choice of which facility to visit. All other things being equal, it is generally assumed that the closest facility will be selected.
Unlike the other goals that select facilities when you specify how many facilities to select, this goal selects the number of facilities based on the percentage of demand you want to meet. It is assumed that the facilities do not have a limited capacity.
To determine where to add facilities, include the existing facilities in the required facilities layer and the potential locations as the candidate facilities layer. If you are locating new facilities in an area where none exist, include a candidate facilities layer only, with no required facilities layer. You would also use a candidate layer alone if you have existing facilities and want to determine whether any of them could be closed without reducing the percentage of demand currently being met.
If you specify a limited maximum travel range, only demand locations within the time or distance of a facility will be allocated to the facility. Demand locations that are beyond the distance or time of any of the facilities will not be allocated, even if the specified percentage of demand has not been met.
Facilities
Facilities provide a service to the surrounding area in one form or another. They can be permanent structures, such as fire stations, schools, or health clinics, or they can be transitory, such as school bus stops or staging locations for emergency response crews.
The Choose Best Facilities tool allows you to specify both required facilities and candidate facilities. Required facilities are existing facility locations that must be used in the analysis. Candidate facilities represent locations where new facilities can be placed. If you are allocating demand to existing facilities (such as allocating students to existing schools or allocating households to existing fire stations), specify a required facilities layer so demand will be allocated among the existing facilities. If the facilities are yet to be built (such as elementary schools or bus stops in a new planned community), specify a candidate facilities layer and indicate the number of candidates to select from the set of candidates.
You can include a combination of required and candidate facilities in an analysis. For example, there may be several existing fire stations in a city, and the city needs to build two new stations at locations selected from a set of candidate locations, with a goal of servicing the most households. In this case, include the existing fire stations as required facilities, the potential locations as candidates, and specify 2 as the number of candidates to select. The Choose Best Facilities tool will identify the two new locations that, in conjunction with the existing stations, will cover the most households.
Facilities can have unlimited capacity, meaning they can meet all the demand assigned to them, or limited capacity. If all the facilities have the same limited capacity, such as the number of households that a fire station can serve, specify a constant value. If the capacity varies between facilities, such as the number of available slots for freshmen at each high school, specify a field containing the capacity value for each facility.
Demand locations
Demand represents the need for a service that the facilities can meet. The demand locations represent the locations of individual features from which there is either travel to or from a facility. For example, demand locations can be points such as the street addresses of houses, students, customers, or businesses. Students may be traveling from their house—a demand location—to a school—a facility. Alternatively, a fire truck may be traveling from the fire station—a facility—to a street address—a demand location.
Demand locations can also represent data aggregated by area, such as the number of households in each census block. Demographic data is often available only as aggregated data by census areas (blocks, tracts, or counties), so the demand location would be a point representing the entire area.
Note:
For aggregated data, the point location may not accurately reflect the location of most of the individuals in the area, especially if the area is relatively large. Where the point is located can determine whether the demand associated with the area is allocated and to which facility.
Demand locations have an associated amount of demand. If the demand location represents a single occurrence, such as a customer or a student, the amount of demand is one for all demand locations. However, the amount of demand can be a quantity associated with the demand location. For example, if you are allocating workers to bus stops, each demand location represents an individual business and the amount of demand is the number of employees at each. For demand locations representing aggregated data, the amount of demand always represents a quantity associated with each location, such as the number of households in each census block or the population of each census tract.
The amount of demand can represent values other than the number of people or things. It can also represent amounts, such as the monetary value of structures, the total miles of streets to be covered by a fire station, or the amount spent annually by each customer.