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.
Step 1: Model
Use the dimensions of your sign and some cereal-box cardboard to come up with variations on folding strategies, leg configurations, and attachment schemes. Make several models, put them next to one another, and choose the one that looks the best and uses the available material most efficiently.