South Capitol is passable (between Firth Sterling & Malcolm X and/or Overlook) , but you gotta have steady nerves. And can really only do it southbound. As mentioned before else where on this forum, the very best way between the Fredrick Douglas (South Capitol) bridge and Blue Plains (and onto Oxon run trail up to National Harbor & Wilson Bridge) is through Bolling, but I’m not sure if one can pass through there with just a picture ID the way one can pass through Ft Meyer.
The other alternative to what TwoWheels says above is to take MLK all the way to Blue Plains Drive. In any case, when traveling from central DC to the Beltway Potomac river crossing, traveling down the DC/Maryland side of the Potomac adds at least one, and on most routes at least two, substantial ridges to the ride.
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