... but why are they tailgating?
Frequently, because there is a person oblivious to their surroundings puttering along in the center lane in the belief that this means they don't have to think about merging traffic and never have to check their mirrors.
No problem if someone wants to stay in the center lane, but keep up the pace, and if the pace is too fast, go to the right!
For heavy trucks, the center lane is their passing lane, although the speed limiters messed that up and now they can't overtake.
In Toronto, frequently the traffic volume is such that there is more than a vehicle every two seconds, which makes maintaining that distance impossible. If someone tries to, it is only by slowing down ... and then the ripple effect starts, because everyone behind them now has to slow down.
It would be nice if trucks would stay to the right but I can understand why they often don't ... because it means they would have to change lanes to allow someone else to properly merge at a junction, and when the drivers in the next lane don't let them do the lane change, then there's a problem, and if the drivers merging don't get up to speed then the truck would have to slow down to avoid a collision, and it then takes forever for them to speed up again (and the ripple effect starts again).
Stupidity/obliviousness is at the root of this, but fixing that ... is a challenge.