By looking at estimates of gasoline consumption from a statistical model developed by NAHB’s Housing Policy Department, based on data from the most recent (2001) National Household Travel Survey (NHTS), we’ve found that multifamily has an advantage when it comes to gasoline consumption.

The results of the model show that, controlling for factors available in the NHTS data set, gasoline consumption and the associated CO2 emissions are lower for developments that consist of multifamily units, are lower still if the development consists of rental apartments rather than multifamily condominiums, and (holding structure type constant) decline as the compactness of development increases. 

The model also analyzes other factors related to gasoline consumption: Vehicle Miles Traveled (VMT), the efficiency of the vehicles owned, and the efficiency of the speed at which they’re driven. The results show that, compared to single family home owners, multifamily renters tend to own vehicles that get about a mile and a half more to the gallon. The results also show a clear “congestion” effect: all else equal, as the compactness of subdivisions increases, VMT declines, but the vehicles tend to be driven at less efficient speeds. However, this congestion effect is not strong enough to totally offset the effect of reduced VMT, so that gasoline consumption and the associated CO2 emissions still tend to be lower in more compact development.

Background Information

Vehicle use and CO2 emissions have attracted substantial attention in recent years. According to the Energy Information Administration, CO2 has the largest impact on global warming of any of the monitored greenhouse gases. About 33% of total U.S. greenhouse gas emissions are generated from the transportation sector, and about 95% of these have to do with CO2 emissions (Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004, U.S. EPA, 2006).  

Concerns about these numbers and their possible implications for climate change issues have prompted states such as California, Massachusetts and Washington to require that developers quantify greenhouse gas emissions from vehicle use in large residential projects for which they are seeking approval. However, these states typically do not provide any guidance on how to perform the calculations, and there is currently no well-established, verifiable method for estimating vehicle use and CO2 emissions for a particular development.

Nevertheless, the NHTS provides data can be used to develop such a model.
The NHTS is conducted by the Federal Highway Administration (FHWA) within the U.S. Department of Transportation. It is designed to provide comprehensive data on travel patterns in the United States for local transportation planners, and it is often used for that purpose. In addition to information on vehicle use, the NHTS data contain estimates of gasoline consumption per household. They also include substantial information on household characteristics and a number of geographic variables, so it’s possible to develop a statistical model that controls for these factors.

Households and Geography

The household and housing unit characteristics used in the NAHB model are
the same ones that the U.S. Department of Transportation, the Environmental Protection Agency (EPA), and the Department of Energy use in their NHTS-based travel forecasting models. By incorporating these characteristics, the NAHB model is able to estimate that gasoline consumption tends to be higher for households that

• are larger
• contain more workers
• have higher incomes
• are younger
• are less educated
• have a male householder
• have a white householder
• have a Hispanic householder

For location characteristics, the NAHB model essentially uses all the geographic information available in the data. The NHTS data don’t identify individual states, but do indicate the four principal Census regions. The data also indicate whether a household is in an urban area, or a Metropolitan Statistical Area (MSA), and provides some information about the metro area’s population and whether or not it has a rail transportation system (without identifying individual metropolitan areas specifically). The NHTS data also indicate whether the home is located in an MSA with a rail transportation system. There is no information about how close the transportation system comes to a particular home.

One of the strongest results in the NAHB gasoline consumption model is that households in urban areas consume less gasoline than households living in rural areas. For example, the model estimates that a household in an urban area in an MSA with population size less than 1 million consumes about 276 fewer gallons of gas than a household living in a rural area not in an MSA, all else being equal. If the household is in an MSA with rail transportation, the model estimates that it would, on average, use about 70 fewer gallons of gas. Among the four regions, the model finds that gasoline consumption tends to be lowest for households in the Northeast and highest for households in the South.   

Structure Type and Subdivision Compactness

Characteristics on the scale of a metro area, such as its population or whether it has a rail system, generally are beyond the control of an individual developer. However, the data also indicate if home is single-family or multifamily, whether it’s owned or rented, and contain some information about what is in the surrounding neighborhood or “block group.” Block groups are defined by the Census Bureau to capture approximately 500 housing units, about the size of a residential subdivision. We convert housing units per square mile to housing units per acre and call it “compactness of subdivision” to avoid possible confusion over the term block group. The NHTS data don’t show the compactness of the subdivision precisely, but group it into six categories—ranging from fewer than 0.08 units per acre, to more than 7.81 units per acre.

The results show that, controlling for all the factors mentioned in previous sections, households consume less gas if they live in multifamily units, if they are renters rather than owners, and if they live in a subdivision that is more compact. 

To give some idea of the magnitude of these effects, Table 1 shows the estimated annual gasoline consumption and CO2 emissions for an urban subdivision containing 100 households (with average household characteristics for the entire sample) in a Northeastern metropolitan area with a population greater than 3 million and a rail transportation system. Gasoline consumption is the variable contained in the NHTS data and estimated by the NAHB model.  It is converted into CO2 emissions by applying a simple formula obtained from the Energy Information Administration, which is based on the number of carbon atoms in a gallon of gasoline and assumes complete combustion.

Data source: 2001 National Household Travel Survey, Federal Highway Administration.

*The data set contains information about housing units and land area for the block group in which the housing units are located.  Block groups on average contain approximately 500 housing units which is about the size of a residential subdivision. The term "subdivision" is used as a proxy in the table to avoid confusion over the term block group, with which most readers are likely to be unfamiliar. Note: characteristics of the occupants are set to be national averages. Estimated for a urban subdivision in a Northeastern metropolitan area, and the metro area has population 3 million and up and rail transportation system.

Table 1 shows results for 100 owner-occupied single-family homes, 100 condos, and 100 rental apartments. Within each category, the table shows what happens as compactness of the subdivision changes. For completeness, all subdivision categories are show, even though some of them would occur only rarely in practice. For example, 7.81 units per acre would be unusually dense for a subdivision composed entirely of single-family detached homes. And for a multifamily subdivision to average less than one-tenth of a unit to an acre, the subdivision would have to contain very large tracts of open space or non-residential structures around the apartment buildings.

Holding subdivision compactness within the bounds of 1.56 to 4.69 units per acre, the model estimates that residents would drive about 2.1 million miles, use 96 million gallons of gasoline, and generate 1.9 million pounds of CO2 emissions if the subdivision were composed entirely of owner-occupied single family units. If the units were rental apartments, the estimates would be noticeably lower: 1.8 million miles, 79 million gallons, and 1.5 million pounds of CO2. Multifamily condos fall somewhere in between.

The table also shows that estimated gasoline consumption decreases as the subdivisions become more compact. Because in practice a change from single- family to multifamily development is likely to imply greater density, a more realistic comparison may be between a less dense single-family and a more dense multifamily development. In this case, the differences in miles driven, gas consumed, and CO2 emitted between the single-family and multifamily alternatives would be even greater. 

Estimated efficiency measures also are shown in Table1.  Again, these estimates control for a variety of household and other characteristics. The table shows little relationship between efficiency of vehicles owned and subdivision compactness, except that residents in the least dense subdivisions tend to own less efficient vehicles. But it does show that households in multifamily structures tend to own more efficient vehicles. In the single family detached subdivision, as long as the density is above 0.08 units per acre, vehicles owned average right around 21 miles per gallon. In the rental multifamily subdivision, the averages are close to 22.5. Again, multifamily condos fall somewhere in between.

On the other hand, if density is held constant, the type of structure built doesn’t greatly affect the speed at which vehicles are driven. However, the table does show a relationship between average speeds and subdivision compactness.  As the subdivision becomes more compact, the estimated results show that vehicles are driven fewer miles, but they tend to be driven at less efficient speeds.  However, the congestion effect (less efficient driving speeds) is not strong enough to completely offset the effect of reduced VMT. So on balance, households in more compact development still tend to use less gasoline and thus generate fewer CO2 emissions. 

More complete results—including estimates for other regions, and categories of metropolitan areas, estimates for households with different characteristics, as well as the structure of the statistical model and its estimated coefficients—are available from the NAHB Housing Policy Department. Interested readers may contact Helen Fei Liu (800-368-5242 x8488, fliu@nahb.com) for more information. [ return to top ]