A classic example of the law of unintended consequences is how seat belt laws gave reasons to drive more dangerously, as car drivers feel more secure. This idea has been popularized by Sam Peltzman and several follow-up studies.
Yong-Kyun Bae puts some serious doubts in this results by pointing out that all these studies were based on aggregate data. Using individual data, which allows to exploit individual characteristics, as well as the circumstances of accidents. And once you control for these factors and exploit cross-state variations of how seat-belt laws became more or less stringent in the last decade, it appears more stringent laws make people drive more carefully. Indeed, pedestrians are getting safer. If this result stands, the challenge is to explain it: do tougher seat-belt laws signal stronger enforcement of other traffic laws? In particular, as Bae suggests, these laws may come in tandem with cell-phone and texting-while-driving laws.