How Far to the Nearest Road?
Kurt Riitters1 and James Wickham2
Forest Service, Southern Research Station, Research Triangle Park, NC
2US Environmental Protection Agency, National Exposure Research Laboratory, Research Triangle Park, NC
1US
Ecological impacts from roads may be the rule rather than the exception in most of the
conterminous United States. We measured the proportion of land area that was located
within nine distances from the nearest road of any type, and mapped the results for 164
ecoregions and 2108 watersheds nationwide. Overall, 20% of the total land area was
within 127 m of a road, and the proportion increased rapidly with distance, so that 83%
was within 1 km of a road, and only 3% was more than 5 km away. For forest land area
only, the proportions differed by less than 2% for all distances.
1.0
Proportion of total area
The proportion of
total area located
within a certain
distance from the
nearest road
increases rapidly
with distance.
0.8
Window size
(grid cells)
1x1
3x3
5x5
9x9
17 x 17
27 x 27
49 x 49
81 x 81
243 x 243
0.6
0.4
0.2
Distance
(meters)
42.4
84.9
127.3
212.1
381.8
594.0
1060.7
1739.5
5176.0
All land area
Forestland only
0.0
0
1000
2000
3000
4000
Distance to nearest road (meters)
5000
6000
How dense are roads in your watershed? The figure below shows that 78% of the 2,108
Hydrologic Accounting Units in the conterminous United States have at least 40 percent of
their land area within 382 meters of a road. Regions with more than 60% of their total land
area within 382 m of a road include nearly all coastal zones, as well as substantial portions
of the southeast US and the basins of the Ohio, Brazos, Colorado, Sacramento, and San
Joaquin Rivers. Three hundred eighty-two (382) meters is about equivalent to one lap
around an athletic field track.
The proportion of total
area in a watershed that
is within 382 m of the
nearest road exhibits
substantial geographic
variation. Inset:
Watersheds where roads
are closer to forest land,
in comparison to all
land-cover types.
Imagine that the conterminous US has been subdivided
into 8.6 billion parcels the size of a baseball diamond
infield, and then consider standing on home plate in
each one. According to our data model, in one out of
22 cases you will find a road no farther away than
second base (~43 m). In one out of every five cases, the
road is no further away than the center-field fence in
Yankee Stadium (~125 m). This alone is a compelling
reason to consider the possible systemic impacts of
roads on ecosystems.
Publications
Riitters KH, Wickham JD. 2003. How far to the
nearest road? Frontiers in Ecology and the
Environment, 1:125-129.
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Riitters KH, Wickham JD, O’Neill RV, et al. 2002.
Fragmentation of continental United States forests.
Ecosystems, 5:815-822.
Wickham JD, O’Neill RV, Riitters KH, et al.
Geographic targeting of increases in nutrient export
due to future urbanization. Ecological Applications,
12: 93-106.