For a new land use being surveyed, one or more appropriate independent variables need to be identified, measured, and analyzed. When identifying a potential independent variable, the following points should be considered:

• The data for the independent variable should be readily available.
• Variables for similar sites should be provided directly and not be merely estimated from a different variable. For example, the number of employees at a site may appear to be a valid independent variable, but it should not be used if the value is typically derived by factoring in another independent variable, such as gross square footage of the development site.

Employees: Unless otherwise stated in the land use description, the number of employees refers to the total number of persons employed at the survey site, including all shift workers.

Gross Floor Area (GFA)[i]: The sum (in square feet) of the area of each floor level in the building, including cellars, basements, mezzanines, penthouses, corridors, lobbies, stores and offices, that are within the principal outside faces of exterior walls, not including architectural setbacks or projections. Included are all areas that have floor surfaces with clear standing head room (6 feet, 6 inches minimum) regardless of their use. If a ground-level area, or part thereof, within the principal outside faces of the exterior walls is not enclosed, this GFA is considered part of the overall square footage of the building. However, unroofed areas and unenclosed roofed-over spaces, except those contained within the principle outside faces of exterior walls, should be excluded from the area calculations. For purposes of trip generation and parking generation calculations, the GFA of any parking garages within the building should not be included within the GFA of the entire building. The unit of measurement for office buildings is currently GFA; however, it may be desirable to also obtain data related to gross rentable area and net rentable area. With the exception of buildings containing enclosed malls or atriums, gross floor area is equal to gross leasable area and gross rentable area.

Gross Leasable Area (GLA)[ii] : The total floor area designed for tenant occupancy and exclusive use, including any basements, mezzanines, or upper floors, expressed in square feet and measured from the centerline of joint partitions and from outside wall faces. For purposes of trip generation and parking generation calculations, the floor area of any parking garages within the building should not be included within the GLA of the entire building. GLA is the area for which tenants pay rent; it is the area that produces income. In the retail business, GLA lends itself readily to measurement and comparison; thus, it has been adopted by the shopping center industry as its standard for statistical comparison. Accordingly, GLA is used in this report for shopping centers. For strip centers, discount stores and freestanding retail facilities, GLA usually equals GFA. 

Gross Rentable Area (GRA)[iii]: The area computed in square feet by measuring the inside finish of permanent outer building walls or from the glass line where at least 50 percent of the outer building wall is glass. GRA includes all the area within outside building walls excluding stairs, elevator shafts, flues, pipe shafts, vertical ducts, balconies, and air conditioning rooms. 

Independent Variable: A physical, measurable, or predictable unit describing the study site or generator that can be used to predict the value of the dependent variable. Some examples of independent variables used in this book are GFA, employees, seats, and dwelling units.

Net Rentable Area (NRA)[iv]: Computed in square feet by measuring inside the finish of permanent outer building walls or from the glass line where at least 50 percent of the outer building wall is glass. NRA includes all the area within outside building walls excluding stairs, elevator shafts, flues, pipe shafts, vertical ducts, balconies, air-conditioning rooms, janitorial closets, electrical closets, washrooms, public corridors, and other such rooms not actually available to tenants for their furnishings or to personnel and their enclosing walls. No deductions should be made for columns and projections necessary to the building. Typically, the NRA for office buildings is approximately equal to 85 to 90 percent of the GFA.

Servicing Position: The number of vehicles that can be serviced simultaneously at a quick lubrication vehicle shop or other vehicle repair shop. That is, if a quick lubrication vehicle shop has one service bay that can service two vehicles at the same time, the number of serving positions would be two.

Vehicle Fueling Position (VFP): The number of vehicles that can be fueled simultaneously at a service station. For example, if a service station has two product dispensers with three hoses and grades of gasoline on each side, where only one vehicle can be serviced at a time on each side, the number of vehicle fueling positions would be four.