About "stepping out of the car", wouldn't a human body in the car effect the response? If we can live with it (the tone), wouldn't it be better to stay in the car?

Theoretically, probably. But bear in mind that background noise will affect the level that is read - to counter this, we probably want to measure at a level that wouldn't be pleasant to live with.

Also bear in mind that most modern cars have speaker placements designed to counter the human body's effect to start with. Bass frequencies don't get absorbed as much as treble frequencies, hence why the bass speakers are usually low in the door panels and why the tweeters are often high. (In '99+ VW Golfs front tweeters are on the inside of where the side mirrors sit, and rear tweeters are high on the door panel)