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Reclassifying the wildland–urban interface using fire occurrences for the United States

Date: July 31, 2020

A new WUI classification shows more communities and homeowners at high risk.

The wildland–urban interface (WUI) occurs at the intersection of houses and undeveloped wildlands, where fire is a safety concern for communities. Defining and classifying this area can help prioritize investment in planning, protection, and risk mitigation.  Previous definitions of the WUI do not explicitly account for differences in fire risk. However, data are now available to use objective measures of fire occurrence to refine the definition.

Analysis of fire occurrences in the conterminous United States with several classifiers indicated housing density was the preferable basis for definitions. Fires overall were predicted to occur at housing densities of less than 100 houses/km2, as long as percent vegetation cover was at least 10%.

A map of the United States divided into Western, Mountain, Interior, Northeast, and Southeast regions, with fire occurrences shown in black.
Regions with occurrences of fire greater than 0.4 ha during 2008-2012.
The updated classifications based on this method support continued use of existing definitions for wildlands (<6.17 houses/km2 ) and the low-density intermix  (6.17 to 50 houses/km2). Departing from other definitions, this method defines the medium-density intermix as 50 to 100 houses/km2 and the high-density intermix as100 to 200 houses/km2. Interface, or suburban, communities are 200 to 400 houses/km2.

The resulting revised WUI classification includes a larger area classified as greater fire risk (low and medium-density intermix), meaning more communities and homeowners are included in this high-risk status. The low-density class has the greatest risk of fire exposure, but the medium-density class contains a greater concentration of houses.

Classification of the WUI based on actual fire occurrences provides an objective foundation for identifying residential densities at risk of fire exposure, which permits disclosure of risk, prioritization of resources to communities and homeowners with greater wildfire exposure, development of strategies for communities to coexist with fire, and responses to reduce vulnerability.

Key Findings

  • Housing density had a stronger relationship than human density with fire occurrences.
  • Fires overall were classified to occur at lower housing densities below 100/km2, as long as percent vegetation cover was at least 10%.
  • These revised classifications result in a larger critical area (855,000 km2) classified as greater fire risk (low and medium-density WUI below 100 houses/km2) during 2010.


Principal Investigators: