Anyone who's driven within an hour of Boston will tell you, the problem is never speed, its deltas, just like the article said.
Speeds average north of 80 or south of 20 depending on time of day... and the real hazard when most people are driving 80 is the clods in the left lane doing 65 or 70. Thats when you get panic jamming on the brakes, or dangerous passing on the left.
IMHO, the real problem in the US isn't speed limits... its dangerous situations caused by unpredictable reactions to speed differentials. Driving in Maine is a pleasure most of the time -- people obey the more important law, that being to drive on the right and pass on the left. Drive in the left lane, from what friends who live there tell me, you'll be pulled over and ticketed for it.
I rarely, if ever, cruise in the left lane, unless my speeds are high enough that I'm passing everyone (not that I'd ever do that, of course)
Things would be a lot safer if everyone did that.