Researchers walked over 3,700 miles (6,000 kilometers) in five countries to conduct the survey, during which they used the elephants' dung to estimate their numbers.
Poaching activity was gauged by the number of carcasses found with gunshot holes in their skulls, missing tusks, or other evidence of illegal killing.
The likelihood of finding slaughtered elephants decreased with distance from the nearest road, the study found, with none discovered farther than 28 miles (45 kilometers) away.
The influence of roads on elephant numbers was reflected in the different protected areas surveyed, the team said.
Salonga National Park in the Democratic Republic of Congo was three times larger than any other wilderness reserve studied, yet the team put its forest elephant population at just 1,900.
Salonga is crisscrossed with old roads and navigable rivers that provide human access to nearly half its 14,000 square miles (36,000 square kilometers).
In contrast, two smaller national parks in the region had elephant densities more than ten times higher than Salonga. Both parks have remote areas at least 37 miles (60 kilometers) from the nearest roads.
The situation may be far worse outside of protected areas, where wildlife conservation measures such as anti-poaching patrols are scarce, the team added.
The team found that even in national park areas with road access, incidents of poaching were lower and elephant abundance higher than just beyond their boundaries.
Blake, of the WCS, said that while logging is the main spur for new road construction, old highway networks are also being rehabilitated in countries including DRC, which has recently emerged from civil war.
"Such roads obviously bring important social benefits for people, but biodiversity conservation is often last on the list of considerations behind economic factors," he said.
Blake and his team are calling for better planning and development of roads to minimize impacts on wildlife.
The WCS-led survey was carried out as part of a Convention on International Trade in Endangered Species (CITES) program aimed at countering poaching of forest elephants.
The poaching is driven by a booming illegal ivory trade to nations such as China.
(Read related story: "Illegal Ivory Trade Boosted by Angola Craft Markets, Conservationists Say" [October 27, 2006].)
Conservationists fear that legalizing even limited trade in ivory from savanna elephants in southern African countries where populations are no longer threatened would put the forest elephant in even graver danger.
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