I think you would want to go over, not under. It seems easier to build a 30' bridge that can support foot traffic than to dig a 30' tunnel that needs to suppor

I think you would want to go over, not under. It seems easier to build a 30' bridge that can support foot traffic than to dig a 30' tunnel that needs to support semis driving over it. One thought, if there isn't a high volume of foot traffic, is to just the fact that these are typically paired (one to swap lanes, another to get them back to the "right" side). Put a pedestrian crossing in the middle, and you can stop traffic from both directions at once. Bonus points that cars are supposed to stop pretty far away, so pedestrians could see someone running the light from much farther away rather than having to wait til they're 15 feet away to tell they aren't going to stop.