The Forty-Ninth Parallel
At this point the 49th parallel of latitude north of the equator crosses the highway. This line forms the southern boundary of the western provinces of Manitoba, Saskatchewan, Alberta and most of British Columbia. In 1818 a treaty between Great Britain and the United States designated it as the international boundary from the Lake of the Woods to the Rocky Mountains. Further west in the Oregon Territory a bitter dispute arose which almost precipitated war between these two nations before they agreed in 1846 to extend the boundary to the shore of the Pacific along this parallel.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.