The Association of British Drivers has this interesting page on the subject of speeds and limits. I think it is fully relevant to the US situation as well.

That is a GREAT link. . . everyone discussing this should read it... it's got stats for the US as well.

"A comparison of international motorway speed limits and fatality rates is shown on the chart below. It is quite apparent that there is no correlation between the two. The United States, with one of the lowest motorway speed limits, has an almost identical fatality rate to Germany, with no limit. There are clearly far more significant factors than speed that must explain the large differences in fatality rates between countries."