Here is a copy/paste from the (San Carlos, Sonora, Mexico Forums > Topical Forums > San Carlos Forum > Great day has arrived (I think)) from the 14th of May.
There are huge signs south of the toll booth but the general idea is that will turn off the main highway at blvd Morales to head west. Then after 1/2 mile or so, you will make a U-turn and then head back towards the highway. It is all very will marked. Eventually you will cross over the highway, loop around and will be back on the highway heading back NORTH. Almost immediately afterwards you will come to the entrance to the new bypass road on your right. Take that. Eventually you will reenter the main highway about 5 miles south of Hermosillo.
The new bypass makes a loop well to the EAST of Hermosillo.
Eventually there will be a new overpass that will allow you to enter the bypass directly but for now you will need to to the Morales U-turn route. Immediately south of the toll booth, there is a road running of to the west but that is not it. It is the 2nd road to the right after the toll booth, maybe 1 1/2-2 miles south of the toll booth. There are many LARGE signs so it shouldn't be a problem.