A Sydney motorist has been fined for exploding in rage after a woman twiddled her little finger at him a small penis gesture featured in a road safety advertisement.

Simon Jardak, who received the hand signal while driving over the Anzac Bridge, responded by throwing a bottle out his car window.

In the television campaign, people flash the little finger signal at reckless drivers, with the slogan reading "Speeding: no one thinks big of you".

"She started doing that hand gesture, you know, the RTA one," Jardak reportedly told the magistrate.

"And it offended me … because of, you know … she implied I had a small penis."

Jarkak pleaded guilty to maliciously damaging property, admitting his behaviour was stupid. He was fined $400.