According to a Marc Fisher article in the Washington Post
Incredibly, with the Capitol visible at lower left, this illustration prophesies an elevated highway across the Mall- something with which I would disagree.
If that is supposed to be the obelisk of the Washington Monument, to the right of the Capitol below the letter "O" in "TOLLWAY", that perspective places this viaduct directly across and over the Reflecting Pool!
Such a route was officially planned during a part of the 1960s and 1970s, as the South Leg of the Inner Loop, though sensibly as a tunnel. This "diagonal" route, one of several options officially considered, had broad official planning group support during the late 1960s, with the primary objection that it would require replacing two spots of trees along the Reflecting Pool where it would pass directly beneath.
For that reason, a southerly tunnel route along Independence Avenue was instead selected during the early 1970s; yet the trees that the diagonal tunnel would replace along the northwest corner of the Reflecting Pool were nonetheless replaced anyway- perhaps due to a storm? By 1971 though, groups that had supported the concept of a tunneled link between I-66 and the SW Freeway had turned against the selected option of of a part tunnel, part open depressed, designed to be later covered option, and the project was canceled by 1976, with all of that corridor's traffic left upon the surface.
So the apparent message is new lanes, not for the general driving public as a tunnel, but rather only for the wealthy, and without regard to the surrounding environment.