urbanSTL

Design speeds won't affect the average grade needed, only the length of the transition sections at the top and bottom of the slopes.

You could significantly decrease the grade by raising the lid's elevation by a few feet and decreasing its length. Using the same numbers as zun1026 while raising the lid by 5 feet and trimming 20 ft on either side: 3.28% between Walnut and Market, 3.92% between Chestnut and Pine.

It should be noted, however, that a landscaped lid/bridge will be much deeper than the bridges that cross I-70 today.

Design speeds do change by roadway designation. A city street can be steeper than either a highway or Interstate. The lid won't be raised as the issue is lining up an unobstructed view from Smith Square to the new museum entrance. I'm guessing even a foot would change this design element. The lid has shrunken several times and is currently less than a block wide.