The existing tight diamond interchange at South Capitol St/Malcolm X Ave in Washington, DC dates from the 1950s, with I-295 being added in the late 50s/early 60s.  It features tight weaves on southbound S. Capitol St and northbound I-295, outdated ramps and loops, and very narrow signal spacing along Malcolm X Ave.

This idea is mainly a consolidation of existing access, with the net results being both improved operations and a reduction in the interchange footprint.

The background imagery is 2007 aerial imagery from NAIP.  Click on the image for a larger graphic (warning, larger graphic is 1.5MB).

Some features of my proposal:

• The I-295 mainline is shifted slightly to the west, centered on the open space between existing South Capitol St and the existing southbound I-295 lanes.
• South Capitol St is reconstructed to split and be on the outside of the I-295 lanes, with access ramps to/from I-295 and at-grade signalized intersections at Malcolm X Ave.  Landscaping, traffic calming techniques, and synchronization with the signals to the north and south (Firth Stirling Ave and Martin Luther King Junior Ave, respectively) will reduce the excessive speeding that currently exists along South Capitol St.  A speed limit of 35 or 40 MPH is suggested.
• This design also allows for dual left turn lanes at the South Capitol/Malcolm X intersections, which would likely be needed on 3 of the 4 approaches (excluding westbound Malcolm X).
• Connecting ramps on each end provide access between I-295 and both South Capitol St and Malcolm X Ave.
• An additional set of ramps serves the heavy traffic movement between I-295 to/from the north and South Capitol St to/from the south.
• The existing loop ramps on northbound I-295, and the associated weaving section, are eliminated.
• Signal spacing along Malcolm X Ave is increased from the existing 100ft to approximately 250ft.  This provides additional storage space for left turning vehicles and better flexibility for signal synchronization.
• Proper channelization with the ramps south of Malcolm X Ave would eliminate the potential weaves along South Capitol St there.  The remaining weaves along South Capitol St, between Malcolm X Ave and the ramps to/from the north, would both be longer than 1,200ft, an acceptable distance.
• Although the bike/ped trail idea is not directly related to this interchange proposal, the proposal supports my idea for a "South Capitol trail" connecting the Anacostia bridges and trails with the Woodrow Wilson Bridge.
• This design also supports a trail or sidewalk along the northbound side of South Capitol St, south of Malcolm X Ave.

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Page last modified 11 September, 2009