Because I apparently didn't. At least not all the way. It's not like I was going to run out and buy a "baby on board" sign for the rear window, but I think my driving has become a bit more conscientious and a bit less aggressive since the kid arrived. After all, driving alone in the car--or with one other adult--is a different proposition from driving with a kid.
For example, on the cusp of the evening rush hour today, I was getting back onto the Merritt Parkway in Connecticut from one of those roadside Mobil stations with zero merge lane. Looking, looking, I saw a gap in traffic, so I punched it, laid down a big cloud of black smoke, and took off. It's the kind of thing an adult could anticipate, but not a kid. Becasue if she had, she would not have chosen that very moment to bring a bottle of sparkling water to her little lips. The bottle's contents leapt out at her face so fast, it was like a re-enactment of that scene from Alien, only with water.
[in less amusing news, the kid also hurled--twice--in the car. If it hadn't happened between New Haven and Cambridge, I'd have sworn it was Greek Week.]