Basically, I drilled holes in the sign to weaken it, then hit it with a hammer while holding it against an edge to effect the folds. Again, because these signs were quite old, and not in the best condition, some cracking and breaking occurred. I also was pushing the bending radius and tightness of the folds, which obviously was pushing it a bit too far in some cases. Lastly, these older signs are considerably thicker than newer ones, which I suspect also contributed to the deterioration of the seams.
However, I still think the Instructable is valuable, in that this table is an experiment that enabled me to learn a lot about the material and the aesthetic possibilities of road signs.
I got the signs for free from some friends who inherited them with their apartment; other possible sources are junkyards, recycling centers, and eBay. Don't steal signs. Road signs are in place to protect people on the road; removing them illegally could have serious consequences, whether or not you get caught.
Since the signs were free, the only costs were drill bits and #8 machine bolts. I estimate the whole project was between $10-15. However, this method is pretty time-consuming. Fortunately, it is the sort of thing that can be broken up over a series of weekends or nights. I took me a month of working in my spare time to make it, roughly 25 hours.
Dimensions are all approximate, as I understand you may be trying to replicate this with different-dimensioned signs. The important thing to keep in mind is the over ergonomics and human scale; a coffee table should sit no more than 17-18" off the ground, and be at least 18" wide by whatever length the couch is.