For DC:
With DC's height limit, there isn't much of a history here of notable canceled skyscrapers,
except one which I wouldn't put on my "to save" list. The most popular answer for DC would probably be the proposed
eastern extension of the National Mall, but I'm honestly happy that was never built (the Mall is already too big). So here's my actual list of things I wish had been built:
1. The streetcar/light rail network. As of 2014 there was 60 miles of surface rail transit planned in the DC area. As of 2018 there's been some progress but much of it has been effectively canceled. The 16-mile Purple Line is under construction and DC opened 2 miles of streetcar, with a further 5 or 6 still on the books (much in dedicated lanes), but 2/3 of this system has either been canceled outright or indefinitely postponed.
2. The more monumental Memorial Bridge. Memorial Bridge is generally considered DC's most beautiful large bridge, but even it is pretty understated. Original designs were more grand, and would've been more of a landmark.
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3. National Mall university. In 1816 Congress proposed putting a university on the National Mall, at the location where the Lincoln Memorial is today. The memorial is nice, but a university here would make the Mall so much more of a living space. I think it would have been cool.
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4. The Pennsylvania Avenue triumphant arch. Various temporary arches have been built on Pennsylvania Avenue near the White House for presidential inaugurations. It would've been cool if we had a real one.
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5. The grand Silver Spring station. Silver Spring, MD is DC's most prominent uptown. One thing it lacks is a prominent civic landmark. It has large commercial and residential buildings, but its civic buildings are those of a small 19th century suburb. It doesn't have public buildings appropriate to such a large place. Meanwhile, Silver Spring is also the most important transit hub on the north side of the metro area, with Metro, MARC, light rail, and Maryland's biggest bus terminal all coming together at one spot. Ever since the 1970s planners have been hoping to knock out two birds with one stone by building a grand civic transit station. There were several proposals over the years, ranging from
ugly to pretty nice. I've pictured the 1990s version, my favorite of the lot. Unfortunately, what was ultimately built just a few years ago was, um,
not grand.
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