But you still have the whole "great for those on the road with the crossing, awful for everyone else" situation.
Is it really so awful? Instead of crossing big wide lanes of traffic directly, you cross a bridge over the road, because the drivers on that road and the people crossing over or under it don't interact with one another, so less risk of an accident. You mentioned the creation of dead ends for local streets in the previous post, but grade eliminations on railroad lines do the same things as the construction of freeways and other limited-access highways. I was going to bring up something about this when I was doing a GSV shot of the Metro-North New Haven Line in Mount Vernon, New York a day or two ago, but I think I'd rather bring that up on the Mass Transit board.
Definitely consider the reply above mine. The comfort of using a crossing is measured by more than just how many "big wide lanes" you have to cross. Much of the time, being alone in a narrow place or having to go through somewhere dark just isn't an enjoyable experience. Then yeah, you have to consider the detour time, which can sometimes add a significant amount of time to someone's foot-based journey over just using a crossing. This is especially true for those on bikes or in wheelchairs, where ramps can add a lot of time to a journey over just crossing at-grade.
To those worried about traffic: totally fair! There is no way an at-grade US-1 through Crystal City could possible improve over grade-separation (but then, the freeway is pretty short already). And for pedestrians, we're talking about increased crossing time at those roads with underpasses (I'm not aware of any overpasses). But Crystal City and Pentagon City, especially the northern areas near the two metro stations and the Fashion Centre, is really meant to be more of an urban, walkable area, and US-1 creates this wall that segregates traffic to just a few crossing points (not ideal for a walkable area). Pentagon City has always felt nicer to me, and that might be because of how crammed Crystal City is, having a bit on either side of US-1, but with the eastern side sandwiched between US-1 and the railway; bringing the two neighborhoods together by eliminating the raised freeway and using some creative urban design could really stand to help Crystal City by physically opening it up a bit. And speaking frankly, Crystal City needs help...it's like a 70s dream frozen in time.
One thing I'd love to consider is a one-way couplet with a wide median. The US-1 right-of-way is wide enough to
potentially separate the two carriageways enough that signalization could actually be improved even at the current at-grade intersections, and there'd be this large open area that could be used for many different purposes. I'm imagining it like Texas frontage roads but without the overpasses.