PROPOSAL FOR THE INEVITABLE INTERCHANGE FOR THE ROMEO LEBLANC INTERNATIONAL AIRPORT
The only way this will work (given the existing railway crossing across Highway 11/15 at the airport exit) is if you elevate both the northbound and southbound lanes of Highway 11/15.
If you accept the premise that you have to elevate the highway rather than build an overpass over the highway, then everything else falls into place. Construction should actually be quite straight forward. In particular, the current on/off ramps (for the airport) from 11/15 northbound will never have to be closed during the construction period and the rail crossing also will probably never have to be closed during construction as well.
First, close the southbound 11/15 roadway (at the construction site) and divert traffic to 11/15 northbound (making it two-way traffic), allowing for elevation of this portion of the highway. Once the new elevated segment is complete, then reverse the process, closing northbound 11/15, diverting traffic to the southbound roadway (making it two-way traffic) to allow construction of the northbound elevated segment.
Construction would take several years, but I'm convinced that if you did the process in a stepwise manner such as this, alternately closing the north and southbound roadways, that traffic disruption would be relatively minimal and that the current airport on/off ramps could remain operational during the entire construction period and the rail line would also be unaffected.
By building an interchange here, this would also open up new land along the SW corner of the TCH-11/15 cloverleaf for commercial or industrial development. I included a possible internal road layout for this potential development.
This would work.