First time I've been through the new pedestrian/bike tunnel underneath the Diagonal, which connects Airport Road with the LoBo trail.
Heading toward Longmont on the Diagonal near Airport Road, you'll see the tunnel sign to your right. There is a bus stop there too, with a bike rack:
The tunnel itself is not very long and only goes under one direction of the Diagonal.
The tunnel goes under the eastbound (bottom yellow line) direction of the Diagonal. To cross the westbound section, you use crosswalks on Airport and then over the Diagonal. My route is in red.
Next up for construction is another pedestrian tunnel just up the road where the Diagonal intersects with Hover Road.