"The research is clear that when speed limits go up, fatalities go up," said Russ Rader, a spokesman for the nonprofit Insurance Institute for Highway Safety.
Higher speed limits get people to their destinations faster, Rader said, "but the trade-off is more crashes and more highway deaths."
A 2009 report in the American Journal of Public Health studied traffic fatalities in the United States from 1995 to 2005 and found that more than 12,500 deaths were attributable to increases in speed limits on all kinds of roads.
The study also said that rural highways showed a 9.1 percent increase in fatalities on roads where speed limits were raised, but did not cite specific numbers in those instances.
Most highways in the United States top out at 75 m.p.h., and there are no longer any roads in the United States with no speed limit like Germany's autobahn. Some highways in rural West Texas and Utah have 80-m.p.h. speed limits.
The Texas legislature last year approved 85-m.p.h. limits for some new stretches of road. The strip of toll road running from Austin to Seguin, about 35 miles northeast of San Antonio, will be the first to allow that speed when it opens in November.
The Texas Transportation Commission, which is appointed by Gov. Rick Perry, approved the 85-m.p.h. speed limit at a public meeting Aug. 30. A transportation department spokesman said commissioners would not comment on their decision.
Agency officials had previously said they would study the toll road's topography, the speeds that most drivers were reaching, and the safety of access points and cross sections before approving the 85-m.p.h. speed limit.