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.
Step 2: Legs!
The geometry is straightforward: divide the piece lengthwise into three 4" x 18" strips. Run diagonals from two outside corners to the two middle lines. I laid it out in chalk; you can draw on the back with a Sharpie as well.
Use an 1/8" drill bit to put pilot holes on one inch centers. Go back and drill every other one with a 1/4" bit; get the alternating ones with a 3/8" bit. To bend, screw the sign down to a piece of wood through the smaller diameter holes, and hit with a mallet or small sledgehammer to achieve the bends.
Once the two "wings" overlap, hammer them flat and clamp them together. Drill through with a 3/16" drill bit and pin the metal together with #8 or #10 machine bolts and washers.
As you can see, these legs are asking a lot of the material. All four of them had at least some minor cracking. This can be avoided with newer, thinner signs; this table would also turn out quite well if executed in thinner sheet steel, as it isn't as tough to bend.
Be careful not to cut yourself on the sharp edges of the aluminum; the shavings from the drilling make for nasty little splinters that are hard to see and hard to get out but hurt a lot.
Step 3: Toppin'
To bend, I put the line of holes on the edge of a curb and stood on the sign, then beat it with the hammer until it was 90 degrees. If the curb by your house is chamfered at an oblique angle, bend it as far as you can with that curb, then lay it down flat with the edges curled up. Lay a 2" x 4" along the seam and stand on that, then swing the mallet towards your legs to bend the sides upward.
Clamp a block to the corners and curl the triangular flap in at each corner. The joinery and folding of the top is basically like wrapping a Christmas present.
As you can see in the photos, the two short end pieces broke off from the stress of the bends. One of the main reasons for that is the lever arm of the bend is only three inches; if the rim of the table was ten or twelve inches, there would be enough material to act as effective lever and peel up the bend.
If they had stayed attached, the table would stronger and more stable. However, it is not a strict requirement that they be attached. The table is plenty strong with them being additive pieces; that said, try to avoid them breaking off if possible.
As I said in the introduction, this is an experiment, and it is up to you to improve and expand this basic process.
Step 4: Assembly
It's pretty straightforward; three bolts through the short side of the top into the side of the legs, and two bolts on the long side. Each bolt does double duty, securing the legs to the body of the table as well as securing the corner bends of the top to itself.
Measure out a slope on the side pieces and mark with a Sharpie as shown in the photo. Clamp in place, drill through, and tighten up. Run a level (or in this case, a 2" x 4" with a tiny combo square, because I don't have a long level) across the bottom of the legs to make sure the thing will sit flat when you're done.
Repeat for all four legs. Flip it over and straight chill.