The issue has always been lack of space due to the Lafarge cement plant. From a traffic volume perspective, a NB --> WB loop would probably be sufficient, but there's no room so they would have to squeeze in a flyover instead.
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Considering that around the time that Deerfoot Trail was constructed, the CoC was studying an E-W expressway along 50 Ave S (Peigan Trail is the only constructed portion), one could speculate the the original plan never was to have Glenmore Trail as the main E-W crosstown route. The plans are at the Central Library, and indicated the E-W route would have continued due east from the present-day Glenmore Trail/37 St SW interchange along 50 Ave alignment to Deerfoot Trail near the CP underpass (just south of the Calf Robe Bridge). If you ever grab an ice cream in Britannia, visit Sandy Beach, or the newer developments at MRU, imagine that's the freeway site. Good thing that never materialized, but we're left with trying to upgrade Glenmore Trail.