Certainly an unlikely place to find a sunflower.
Not sure how it get there. Perhaps a bird dropped a seed? Someone was eating a snack and dropped a few kernels in the sewer grate?
Or maybe someone wanted to amuse those passing by. Bring a smile to the faces of the thousands that travel this road each day. Provide an improbable bit of color for the daily commute.
Unfortunately most people drive by at 50 mph and do not notice, and undoubtedly do not stop to smell the roses (OK, sunflower).