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Estimate of the Area Affected Ecologically by the Road System in the United States

RICHARD T. T. FORMAN

Harvard University, Graduate School of Design, Cambridge, MA 02138, U.S.A.

(Abstract of paper)

Introduction

The 6.2 million-km system of public roads in the United States, used by 200 million vehicles, permeates and links essentially every local area. These roads and roadsides cover about 1% of the land, equivalent in area to Austria or South Carolina. A suite of ecological effects of roads involving species, soil, and water has been identified, with effects varying in distance outward from meters to kilometers. The outer limits of these significant ecological effects along a road thus describe a "road-effect zone".

A limited amount of evidence for the width of the road-effect zone is available from Europe and North America. Therefore, my objective was to estimate the total area or proportion of the United States ecologically affected by the road system.

The ecological factors determining the road-effect zone are related to species, soil, and water. Effects extend outward and can be linked directly to a specific road. I excluded from my analysis the indirect ecological effects of transportation, such as air pollutants in vehicle emissions that accumulate in the atmosphere and cause ecological effects often far from the source road .

The total area of the road-effect zone for a nation was first estimated for The Netherlands by R. Reijnen et al. (1995) and was based on avian community composition in forest and agricultural grassland at varying distance from main roads. Evidence indicates that traffic noise is the major cause of the degradation of avian communities near busy roads.  Traffic volume on Dutch main roads apparently ranges from 10,000 to >100,000 vehicles per day, presumably a higher average than in the United States. 

R. Reijnen et al. (1995) estimate that 10% of the land area of The Netherlands is significantly disturbed ecologically by traffic noise. This estimate is based on the average distance that a significant effect extends from a road-for all bird species combined. In contrast, based on the most sensitive bird species, which are of primary conservation importance, the authors estimate that >17% of the land is disturbed. Avian diversity is reduced and total bird density is one-third lower in the areas affected by roads and vehicles. These estimates are based on 1986 data, and, due to traffic increases, the figures are projected to be 15-20% higher in 1999.

A recent study measured and estimated nine ecological variables along a 25-km, divided four-lane highway in Massachusetts and incorporated the Dutch results in the analysis.  Some effects such as traffic noise appeared along much of the road length. whereas a few effects including stream channelization occurred only at points along the road.  Thus the Massachusetts and Dutch studies provide the basis for a preliminary estimate of the total area ecologically affected by a road system.

Estimate for the United States

To make an estimate of the total ecological effect of the U.S. system of public roads, I made the following assumptions:

(1) Total area of United States is 9,372,610 km2.

(2) Total road length includes 3,836,381 km of secondary roads in rural areas (county, town, township, federal [U.S. Forest Service, U.S. National Park Service, and reservations that are not part of the state or local highway systems], and other local public roads), 1,237,198 km of primary roads (under state control) in rural areas, 927,122 km of secondary roads in urban areas, and 178,394 km of primary roads in urban areas.

(3) All road length in rural areas and 25% of road length in urban areas are roughly estimated to be near natural ecosystems (for both primary and secondary roads). Natural ecosystems include agricultural land (which, for example, contains grassland birds of conservation interest) but exclude built areas (thus ignoring ecological effects there). Near natural ecosystems means adjoining or close enough to produce road effects.

(4) Traffic volume (24 hour total, Monday through Friday, half the vehicles going each direction, roughly estimated averages) for primary roads in rural areas (highways in towns, villages, and more remote areas) is 10,000 vehicles/day and in urban areas is 50,000 vehicles/day.

(5) Road-effect zones for primary roads (in both rural and urban areas) are calculated as two times the effect distances for the most sensitive bird species highlighted in the Dutch studies (R. Reijnen et al. 1995, 1996). The effect distance is 305 m for 10,000 vehicles/day in woodland, 365 m for 10,000 vehicles/day in grassland, and 810 m for 50,000 vehicles/day in natural ecosystems in urban areas (average of results for forest birds and grassland birds; Forman & Deblinger 1999).

(6) Road-effect zone for secondary roads is 200 m wide, a rough estimate for a highly variable zone. Lower traffic volume may be associated with localized short-distance effects, such as most roadkills, or road dust and road salt effects on vegetation, whereas higher traffic volume produces long-distance effects, including blocking wildlife corridors and subdividing populations into smaller, less stable subpopulations.

Estimated Area

Calculations based on the preceding assumptions indicate that 19% of the total area of the United States is directly affected ecologically by roads and associated vehicular traffic (Fig. 2b). A breakdown of this total by road type and location shows that roads in rural areas have by far the greatest total ecological effect (16.7% vs. 2.5% in urban areas). Primary and secondary roads have the same total ecological effect, even though primary roads are only one quarter of the total road-network length. Excluding Alaska (which has few roads in a large area) and Hawaii means that 22% of the contiguous United States is estimated to be ecologically altered by the road network .

Conclusion and Policy Implications

My calculations suggest some policy approaches that could have a significant effect on reducing the huge area affected directly by roads and vehicles. The following five actions could make a difference. 

(1) Perforate the road as a barrier to animal movement by using tunnels, underpasses. overpasses. and other mitigation technology, thus reducing the effects of blocking wildlife corridors and of subdividing populations into smaller, less stable subpopulations. 

(2) Close, and where ecologically unimportant, remove logging and other roads in remote areas to reduce the disturbance effects of human access. 

(3) Increase the use of soil berms, plantings, depressed roads, and other construction techniques to reduce the distance and area ecologically affected by traffic noise. 

(4) Concentrate traffic on primary roads, especially in rural landscapes, and minimize the conversion of secondary roads from light to medium-traffic usage. 

(5) Reduce traffic noise by changing tire design, vehicle aerodynamics, road surface, proportion of truck traffic, and total amount of vehicle miles (kilometers) traveled. 

Such policy changes would reverberate through society, yet I suspect that the demand for appropriate solutions to the ecological road-system effects that permeate the land is closer than we think.

Literature Cited

Forman, R. T. T. 1999. Horizontal processes, roads, suburbs. societal objectives, and landscape ecology. Pages 35-53 in landscape ecological analysis: issues and applications. Springer-Verlag. New York.

Forman, R. T. T., and L. E. Alexander. 1998. Roads and their major ecological effects. Annual Review of Ecology and Systematics 29:207-231.

Forman, R. T. T., and R. D. Deblinger. 1998. The ecological road-effect zone for transportation planning. and a Massachusetts highway example. Pages 78-96 in G. L. Evink, P. Garrett, D. Zeigler, and J. Berry, editors. Proceedings of the international conference on wildlife ecology and transportation. Publication FL-ER-69-98. Florida Department of Transportation, Tallahassee.

Forman. R. T. T., and R. D. Deblinger. 1999. The ecological road-effect zone of a Massachusetts (USA) suburban highway. Conservation Biology 14:36-46.

Forman. R. T. T., and A. M. Hersperger. 1996. Road ecology and road density in different landscapes. with international planning and mitigation solutions. Pages 1-22 in G. L. Evink, P. Garrett, D. Zeigler and J. Berry, editors. Trends in addressing transportation related wildlife mortalitv. Publication FL-ER-58-96. Florida Department of Transportation, Tallahassee.

Reijnen, M. J. S. M., G. Veenbaas, and R. P. B. Foppen. 1995. Predicting the effects of motorway traffic on breeding bird populations. Ministry of Transport and Public Works. Delft, The Netherlands.

Reijnen, R., R. Foppen, C. ter Braak, and J. Thissen. 1995. The effects of car traffic on breeding bird populations in Woodland. Ill. Reduction of density in relation to the proximity of main roads. Journal of Applied Ecology 32:187-202.

Reijnen, R., R. Foppen. and H. Meeuwsen. 1996. The effects of car traffic on the density of breeding birds in Dutch agricultural grass lands. Biological Conservation 75:255-260.