A road like Deerfoot in a major American city would be an eight or ten lane road - or larger. The problem is simply congestion. Too many cars for the road to handle. At that point, all it takes is one idiot driving too slowly to mess everyone up.
The City of Calgary website has traffic volume maps dating back to 1963. It's amazing to compare how the city has grown and changed in the past 40 years:
Transportation maps