When you look at the
Saskatchewan AADT map 39 seems like the likeliest candidate, not for the border traffic so much as it would be for Weyburn and Estevan. But I guess this decision could only be made in conjunction with the US.... it would make the most sense to 4-lane the highway that links up with a 4-lane on the American side, and it seems doubtful that anything is going to happen anytime soon on that front.
Looking at the AADT map, there is not really very much traffic headed to the US on Saskatchewan highways so the current 2-lane arrangement makes sense.