It will lead to huge traffic jams, because all vehicles WON'T be travelling at exactly the same speed, there will be slight differences and the faster vehicle will want to overtake the slower one. See how long it takes one truck to pass another since they were limited to 85 kph. And see the queue of cars all trying to get into the outside lane to overtake them.
It'll just take a whole lot longer to pass, and that can be very dangerous on some roads - I give you the single carriageway sections of the A9 as an example.
I'd suggest it's kite flying and will be quietly forgotten.