> If cars match speeds across lanes, it impedes faster traffic.
I think this undersells it a little. It does not just impede faster traffic, when the lanes are pacing each other it makes navigating harder -- simply switching lanes is more difficult. The highway moves so much more efficiently with a small but steady difference in speed between each lane.
> Results: A 5-mph increase in the maximum state speed limit was associated with an 8.5% increase in fatality rates on interstates/freeways and a 2.8% increase on other roads. In total during the 25-year study period, there were an estimated 36,760 more traffic fatalities than would have been expected if maximum speed limits had not increased—13,638 on interstates/freeways and 23,122 on other roads.
That doesn't make sense to me. If you want to change lanes, and worst case scenario you're right next to someone, go 2mph slower for 20 seconds and they'll be shifted by 60 feet. I'm sure you can plan your lane changes on a freeway 20 seconds in advance.
It is the dynamics. When lanes are pacing each other the gaps all tighten up. So sure, you can slow down a bit to find the gap behind you, except that gap is not big enough to fit in. So you turn on your signal and wait for someone polite enough to let you in, meanwhile the guy behind you is riding you like a pony because you are no longer keeping up with traffic.
When traffic isn't balled up so tight, you can plan for a lane change in advance and accomplish it without having to slow down traffic. Everything flows better.
I think this undersells it a little. It does not just impede faster traffic, when the lanes are pacing each other it makes navigating harder -- simply switching lanes is more difficult. The highway moves so much more efficiently with a small but steady difference in speed between each lane.