My problem was the conclusion: "As a nation, drivers in the United States travel slightly too fast and could improve overall life expectancy by decreasing their average speed slightly."
As a nation, drivers in the United States travel at pretty much the average speed. (See: http://en.wikipedia.org/wiki/Speed_limits_by_country) The national comparison seems like a rhetorical device, suggesting American's do something inferior to other countries, therefore we should change to keep up. Instead, you should only conclude that a decrease in speed could increase life expectancy.
As a nation, drivers in the United States travel at pretty much the average speed. (See: http://en.wikipedia.org/wiki/Speed_limits_by_country) The national comparison seems like a rhetorical device, suggesting American's do something inferior to other countries, therefore we should change to keep up. Instead, you should only conclude that a decrease in speed could increase life expectancy.