You don't really notice 1000ppm, and unless you have some ventilation going you can probably reach that quite quickly in your car. I am not saying that it didn't work out or that it was reckless. Just make sure you have adequate ventilation.

Just as an indicator, you can get up to 5,000 ppm with two passengers in a car after an hour's worth of driving.

Exactly: 5000ppm means significantly worse decision making and crisis response. How much more co2 does sublimating dry ice put out? Well packaged probably not that much, but certainly more than another person breathing beside you.