You can get a poor man's "funnel of doom" with barricades. I've seen them on choke points from the AI - that's about it, but then, the AI rarely has enough workers anyway.
Definitely a poor man's funnel of doom if used correctly! Judging by this thread I may be one of the few players who uses them regularly.
In a peaceful game playing an industrial civ, I usually have some spare worker turns; using them on forts/barricades can really some in handy in a defensive war. I you put one every other (or all but one) square on your border with one of your best defenders in each the AI will mostly ignore them unless they have
way better attackers. When they move past you get a free shot, and this works like a mini-funnel to knock lot of hits off of units.
If you have, say a lux or strategic you wish to guard, sometimes just a unit fortifed on it is not enough to deter AI (esp on higher levels where their tech lead may be significant) And you need at least a fort, if not a barricade. These situation are rare but the barricade does a lot since they cannot
move onto it and pillage it in one turn! Even with no defensive unit!
If you have railed all that you can and have too many workers left over for what you need for pollution control (in the later game obviously) you can barricade pretty much wherever you want to limit the movement of the AI's armies if they attack.
I usually end up building at least one fort-and-barricade per game.
Edit: In some games I have even had enough spare workers to make Maginot-style defensive line along a short border. No matter what Patton may say, they work wonders against an AI too stupid to use artillery. +100%defensive bonus is like defending a metro!