Just for reference, here’s an elevation profile for the corridor through downtown Durham, from Campus Drive on the west to Alston Avenue on the east. Drawn in are my suggested uses for the spaces under the viaduct.
The three dips, from left to right, are the existing underpasses at Gregson, Chapel Hill, and Roxboro.
The fact that Downtown Durham is actually at a low point within the corridor works tremendously to our advantage when it comes to grade-separating the entire corridor with a long viaduct. Rise to an elevation of 420’ on an embankment from Buchanan to Gregson, and maintain that elevation - flat as a pancake - on a viaduct, 1.1 miles long, all the way through downtown - with the viaduct ending at Fayetteville, and descending back to ground level on an embankment between Fayetteville and Alston.
The only thing left to figure out is the uses for the space under the viaduct. An 80 foot wide viaduct, times 1.1 miles, minus the space taken by the street rights-of-way underneath, is around 400,000 square feet - which is a lot of space. But it’s the literal center of Downtown Durham, the intersection between Main Street and the Amercan Tobacco area - so it should be possible to find uses for it all.
My first suggestion would be to actually abandon the existing Durham Station bus facility, except maybe the headhouse and the Greyhound station, and put the GoDurham station under the tracks between Duke St and Chapel Hill Street, directly beneath the Amtrak and Commuter Rail platforms. Then sell off the existing platform area for redevelopment (including, probably, a parking deck for the train station.)
Next would be to fill the space between Duke and Gregson, and between Chapel Hill and Mangum, with retail. Put a new pedestrian passageway at the midpoint between Chapel Hill and Blackwell. This could serve as both a connection from Five Points to the American Tobacco area, and also part of a connection from the Tobacco Trail to the Belt Line Trail.
East of Mangum, you’re getting out of the “core” area of downtown Durham, so maybe the viability of retail would be reduced. Some retail would probably work, but not everywhere else. Fill the rest with office space (for both public agencies and private companies) and, perhaps (distastefully but necessarily) the parking to go with it.