There's no evidence that data motivates people to start doing anything. There's plenty that shows data helps you continue once you've started (if you're succeeding, otherwise data works as a negative reinforcer), but if you haven't started yet, additional data won't help.
There's evidence that the threat of punishment for not slowing down makes you slow down. The signs act as a reminder of that. If it weren't for the law, would many people who are already speeding pay attention to the signs? That would be an interesting thing to test.