they make sure that nearly all vehicles travel in convoy at the same speed.
To agree with your argument, I would have to introduce one very big assumption: cars travel at a constant speed.
This is patently not true. We've all experienced the situation where you drive at dead-on 70, and find yourself to be slowly approaching the car in front doing 68mph. You pull out to overtake, but the car in front now speeds up very very gradually. You look down and see you are still doing dead-on 70. You drop back and pull in but the same thing happens half a mile later.
This is sometimes because the driver in front is an a-hole who doesn't think you should be allowed to overtake him. However, in most cases the driver isn't even aware he's doing it...
If you accelerate up to 80, the driver of the other car won't involuntarily increase their own speed so drastically and you'll find that they drop back to the 68 they were doing before.
Even cruise control is not infallible... You still get variation of 1-2mph up and down over a period of a few miles due to gradient, wind direction, change of road surface etc, and different cars will be affected to different degrees.
On top of all this, we have different 70mph's. a true 70mph might show as 75mph in my car, 73mph in your car, 68mph in someone else's. If you fit GPS speedos to all cars, you still only have an accuracy of 0.1mph, so if I'm going 69.9 whilst you go 70.1, you will still catch up!
Driving at an arbitrarily set speed limit introduces these problems.
The only thing we can say with any certainty is that anyone driving slower than you is an idiot, and anyone driving faster than you is a maniac.
Joe, any idea if she's being done for all her crimes yet, or will you be marching her down to the cop-shop to turn herself in?