No doubt, one must mind their 3-year-old, but have you had to watch a 3-year-old? Luckily, ours isn't a crazy adventurous one. Still, we are just now getting to the point where we can watch her approach a street several paces ahead of us, only ready to spring into action if she forgets about the 'danger of cars' thing. In contrast, her same-age cousin is still apt to play real life Frogger if he isn't physically prevented from doing so.
If you knew that guillotine elevators were an environmental hazard, you'd have to adjust. I'm just glad I don't have to.