From my experience with the lens is that you have to be more prepared and prefocus on your subject. I have found that using servo has helped improve my success rate. I am generally tracking my son with one focus point on his eye and wait patiently for the right moment. I will typically shoot a small burst of images that generally helps get at least one sharply focused image. A toddler will challenge the focusing capabilities of any lens and camera.
In your situation of shooting underneath an overpass, even at noon, it can be extremely dark for the camera. This will make it difficult for any lens to focus. If you were trying to focus on graffiti and were trying to focus on a solid color area, then the camera will have difficulty due to a lack of contrast. Trying to focus on an area with multiple colors and with the camera's center focus point. This may help to improve your focusing next time.
I just purchased this lens over the holidays and I can't get enough of it. The auto focus is not as bad as people say if you use it correctly. I have about the same success rate with the 85mm 1.2 as my 35mm 1.4 for sharp pictues of my two year old son who hardly ever stops moving.
I have this lens as well and, admittedly, while I love it, I have the autofocus issues that you referenced. In one case, I was underneath an overpass at noon taking pictures of the graffiti, and the lens simply refused to focus. How would you suggest one utilize the autofocus "correctly"?