Road Density This EnviroAtlas national map estimates density for each 12-digit hydrologic unit (HUC) in the conterminous for 2011 as the total length of within the HUC per HUC area in square kilometers (km/km2). Why is road density important? Roads are obviously critical for transporting people and goods, but they can also have a negative impact on the environment and on human health and living conditions. Roads affect ecosystems and human society by altering the availability of necessary resources (e.g., habitat, water, and nutrients), changing the dynamics of plant and populations, accelerating the introduction of non-native species, and exposing humans and other organisms to harmful pollutants.1 Roads and road traffic contribute sediment, contaminated runoff from (e.g., oil, , and has shown that an estimated road density of 0.5 km/km2 or ), air pollutants (e.g., nitrogen and sulfur dioxides, less in watersheds is necessary for the long-term persistence methane, ozone, and ) and herbicides to roadside of wolves.5 Roaded areas may be avoided entirely by 1 communities and landscapes. Pollutants from aerial and organisms requiring vegetative cover and undisturbed terrestrial sources accumulate on road surfaces until runoff habitats. Conversely, roads can create a conduit or corridor for from a precipitation event carries them into nearby terrestrial the movement of more disturbance-tolerant as well as areas and through ditches and storm drains into waterbodies. the spread of invasive fauna and flora.1,6 Research in road As impervious road surfaces increase, runoff and road design seek to mitigate some of the negative 2 increases in quantity, speed, temperature, and pollutant load. effects of roads on wildlife populations, for example, through the use of strategic fencing and road over- and underpasses. Harmful air pollutants such as airborne , nitrogen dioxide, and are found in high The amount of in watersheds is one of the concentrations along busy roadways. Elevated levels of these best indicators of the effects of roads and development on pollutants, relative to community averages, can persist as aquatic communities. Recent studies have found that 10–15% 3 much as 300 meters or more from the road edge. When imperviousness represented a transition zone where fish and compared with the rest of the population, people who live, aquatic insect species numbers and measures of fish and work, and go to school near roads are at a greater risk for aquatic insect community quality began to decline adverse health effects associated with near-road air , significantly.7 Aquatic organisms have evolved with a natural such as respiratory and cardiovascular symptoms, asthma, stream disturbance pattern of flooding and debris flows that 4 poor birth outcomes, cancer, and premature mortality. rearrange gravel bars, backwater channels, and instream woody debris. However, research has shown that road Outside of developed areas, the largest overall effect of roads networks in watersheds have increased the occurrence of peak on ecosystems is , where continuous flows and flooding as well as landslides and debris flows.8 habitat is divided into smaller patches. Habitat fragmentation can result in increased disturbance, , and isolation of How can I use this information? wildlife species, leading to reduced wildlife populations and This EnviroAtlas national map estimates road density for each subsequent declines or local of sensitive species, 12-digit HUC in the conterminous U.S. for 2011 as the total such as neotropical migrant songbirds, small mammals, and length of roads in kilometers per square kilometer. This layer 5 amphibians. Wildlife numbers are significantly reduced by can be compared with other EnviroAtlas map layers showing road-related mortality as animals cross roads to forage, the proximity of roads to streams and road crossing count and 1 migrate, or reproduce. Road density also negatively density in each HUC to assess the relative impact of roads as influences sensitive species with large territories. Research

CONTINUED ON BACK 1 a landscape stressor. For conservation efforts, this map may What are the limitations of these data? be overlaid with supplemental EnviroAtlas data such as Calculations based on these data are estimations. NAVTEQ ecoregions or protected lands (PADUS). Lower road density roads data are regularly updated and subject to automated HUCs may indicate the existence of larger parcels that may validations tests to check accuracy. NAVTEQ field teams serve as candidates for restoration or preservation. HUCs with drive roads and streets to collect new data and help verify the high road densities may be assessed further to prioritize road- accuracy of the database. related problems. How can I access these data? An EnviroAtlas user may overlay the road density map on an EnviroAtlas data can be viewed in the interactive map, aerial imagery basemap and zoom in to see the character of accessed through web services, or downloaded. The datasets the landscape and the pattern within specific HUCs. used to calculate the streams and roads metrics can be For more information, one can explore other road-related map downloaded from the websites hyperlinked in the fact sheet. categories in the EnviroAtlas table of contents such as near- road environments, roads near streams, and impervious Where can I get more information? surfaces. There are numerous resources on road density; a selection of these resources is listed below. To ask specific questions How were the data for this map created? about this data layer, please contact the EnviroAtlas Team. Total length of roads per HUC came from the 2011 NAVTEQ roads dataset. Road density was calculated as the total length Acknowledgments of roads within the HUC per HUC area in square kilometers 2 Don Ebert, EPA, created the data layers for the road density, (km/km ). The road density metric was generated using the roads near streams, and roads crossing streams maps. Sandra Analytical Tools Interface for Landscape Assessment Bryce, Woolpert, Inc., wrote the data fact sheets. (ATtILA) toolbox, an Esri ArcGIS toolbox created by EPA, that calculates many commonly used landscape metrics. For more detailed information on how the metrics were created, see the metadata and the ATtILA user's manual.

Selected Publications 1. Coffin, A.W. 2007. From to road ecology: A review of the ecological effects of roads. Journal of Transport Geography 15:396–406.

2. Arnold, C.L., and C.J. Gibbons. 1996. Impervious surface coverage: the emergence of a key environmental indicator. Journal of the American Planning Association 62:243–258.

3. Karner A.A., D.S. Eisinger, and D.A. Niemeier. 2010. Near-roadway air quality: Synthesizing the findings from real-world data. and Technology 44 (14): 5334–5344.

4. USEPA. 2016. How mobile source pollution affects your health. Accessed April 2020.

5. Heilman, G.E., J.R. Strittholt, N.C. Slosser, and D.A. Dellasala. 2002. Forest fragmentation of the conterminous United States: Assessing forest intactness through road density and spatial characteristics. BioScience 52(5): 411–422.

6. Parendes, L.A., and J.A. Jones. 2000. Role of light availability and dispersal in exotic plant invasion along roads and streams in the H. J. Andrews Experimental Forest, Oregon. Conservation 14(1):64–75.

7. Schueler, T.R. 2003. Impacts of impervious cover on aquatic systems. Watershed Protection Research Monograph No. 1. Center for Watershed Protection, Ellicott City, Maryland.

8. Jones, J.A., F.J. Swanson, B.C. Wemple, and K.U. Snyder. 2000. Effects of roads on , geomorphology, and disturbance patches in stream networks. Conservation Biology 14(1):76–85.

EnviroAtlas: Led by the U.S. Environmental Protection Agency April 2020