Even if you forgo the fact that the timing of this thing is EXTREMELY comfortable, it's physically impossible for Batman to actually go out & buy the insane amounts of fuel, create the batsymbol in fuel, create a trail to where apparently Gordon will be for ome magical reason, have Gordon light it by accident and show the city the Bats has returned.
It would've been faster, easier, cheaper and more reliable to go buy a spotlight, mount it on a random building and shine a makeshift batsignal in the sky.
He is batman.
He had the fire batsignal prepared beforehand.
Never mind that it was a serious fire hazard, justice must be served.