One of the most frustrating things when planning out a sidewalk or bike trail is the need for a bridge. You have the perfect route laid out to get people from here to there and everything is great, except for that fifty foot canal running through the middle of your project. Grrr. Sometimes you can bring people up onto the roadway, but that is never a good option. Sometimes it is a multi-lane roadway you cannot get people across. (Think the old school game of Frogger).
I thought the need for a bridge was the worst situation you could get until I stumbled upon Lafayette street in Tallahassee. The problem was not a chasm, but a wall. Specifically the earthen berm holding up the CSX railroad tracks running through town. The bridge over the road says it was built in 1929 for the Seaboard rail line. I am sure back then it was perfectly safe to slip past the bridge abutment while a Model-T or such puttered past you. Times have changed. Lafayette street is signed for 30 mph and cars and pickups are much wider now. However, those bridge abutments have not gotten any further apart.
Fortunately, someone came up with a better idea. Drill a tunnel! I wish I could claim credit for this answer. I am sure it was expensive! It is an elegant solution though. This bottleneck on Lafayette cut off an entire corridor from the new Cascades park in Tallahassee. Now people pass freely along this section of sidewalk without needing to risk their lives running past the bridge.
The actual tunnel is about nine feet in diameter. The shell of the tunnel is made of individual steel panels bolted together. The bottom is a level pad of concrete and bollards protect either end from vehicles passing through. There are blind spots at either end that an attacker could possibly hide in, but the entire area is fully exposed to all the traffic on Lafayette. “Community Eyes” are keeping watch fairly well.
This would be a good study location for how pedestrian improvements can improve a neighborhood. Tallahassee has made several upgrades to Lafayette and the surrounding area. Someone with the right GIS skills (and time) could lay out these improvements and how they changed the property values of the area. Without a doubt, it has made a difference.