Thanks, that does make sense. I was aware of the nearby truck crossing, but there are plenty of similar examples of two close crossings, so that doesn't seem like a reason to ban trucks. I was also aware of the Peace Arch park, but hadn't considered the impact of the trucks or a potential expansion on the park itself.
Well, even if they wanted to have trucks, there are no commercial vehicle inspection facilities at Peace Arch / Douglas.
Edit: Which, to be clear, has been a conscious decision with trucks not being allowed. But this allows for more "regular" or NEXUS lanes too, which makes sense since most cars heading to that crossing are coming up I-5 or from BC-99, both freeways. Better to give as much capacity to regular cars as possible, and let trucks detour.
Yes, that all makes sense - just interesting that trucks were seemingly allowed prior to 1970, so there must have been a clear decision to reroute all truck traffic. No doubt the standards for commercial vehicle inspection have changed since then and there's probably lot more of it now too.
The Wikipedia Article for WA 543/BC 15, and its sources, seem to imply that trucks were at least discouraged at Peace Arch even before 1970, as they state that before the current routing of SR543 was constructed, the truck route of US 99 "ran east–west along D Street from US 99 to the Pacific Highway border crossing" (the crossing a mile east of Peace Arch that trucks now must use). Said current routing was opened in 1970, so that may have been part of the reason for the truck ban at Peace Arch.
The main reasons as far as I can tell are the lack of space at Peace Arch to accommodate proper truck lanes and inspection areas, and the desire to separate trucks out into their own facility. Even at the Pacific Highway Crossing, the truck lanes split off and take a different route through the customs area.
Incidentally, the other end of I-5 actually has basically the same setup, where trucks are banned from the San Ysidro crossing at the end of I-5 and must use the Otay Mesa crossing to the east.