Where are your statistics? This sounds like something just pulled out of no where.
Here's some facts. - Vehicles travelling the same speed and same direction cannot cause can accident. It's called physics. We can all agree on this. If the car in front of you is going 120 KM/H, and you are going 120 KM/H, neither of you change directions, it's physically impossible to collide.
If the average driver is going 120 KM/H, and you are going the speed limit of 110 KM/H, you are now an object in the way of every other drive on the road. Now, as road safety is for the masses of the road, not the individual, who is flawed? Clearly the single person causing a disruption in the flow of traffic.
Also at higher speeds, the average driver tends to leave significantly more space between them and other drives. That's from my personal experience on the road, I've seen plenty of examples at higher than usual speeds where traffic is quite spread out.
Also if vehicles are moving at a faster rate, it's more likely that vehicles will remove themselves from that road faster, preventing congestion. This will also increase the distance between vehicles.
The slower traffic is going, the more moves drivers make to try and get ahead, changing lanes, having to change speeds because traffic is slowing down in front of them. This leads to accidents.
Statsitically, the more variance in speed in direction, each time you make a change, you increase your odds of getting into an accident, as mentioned above, same speed, same direction, no accident. So that means 1 of out X changes in speeds or direction changes, there will be an accident.
Reducing the variance in speed and direction changes is the only true solution. If 80% of people are driving faster than the speed limit (Seriously, the highways out here, everyone always goes 10-20 KM faster than the speed limit, entire flow of traffic except the odd driver) then the speed limit should be based on the 80%. If more drives increase their speeds to the new speed limit, 85%, or even 90% of drivers going the same speed will reduce risks.
Trying to make people slow down in places that don't make any sense isn't working. Why don't we try speeding up some of the other drives in places that are safe to do so?