https://www.quora.com/How-much-does-aerodynamics-affect-fuel...
It would probably require electrically driven thrust carriages rather than pneumatic - you would need one for every car using the system. Trucks might need 2 or 3 or more to provide enough thrust. If a magnetic coupling really works, it would be possible to attach and detach from the thrust carriage at motorway intersections without slowing down any more than cars do at the moment.
Not necessarily one per car. You'd probably have a chain of thrust carriages, with maybe some spacing in between each individual carriage. One or several cars would clump onto a thrust carriage -- there's nothing to prevent them from being quite long.
When it's time to get off the highway there might be a hole in the clump for a bit until someone getting onto the highway fills it in.
The one issue I see is that, in order to disconnect from the thrust carriage, you have to have a way to do so. The logical method is to use an electromagnet that can be powered on/off to connect/disconnect but then the car has to power that -- and that might sap it's range a bit.