You will get as many different answers as there are people on this forum with that question. Heck, some people (me included) will give you three or four different (and contradictory) answers, depending on your particular firearm, holster, and comfort/training level...
I carry a full-size Para Ordnance S-14.45. I carry it openly and concealed (depending on clothing). I almost ALWAYS carry it in "Condition 3" (sometimes called "Israeli Carry"). I am perfectly comfortable carrying it in Condition 1, and sometimes I do. The reason I carry in Condition 3 is not because I'm not comfortable with C1, and not because I don't trust my firearms mechanical safeties. It's because it the REMOTE chance that someone should take it from my holster (I'm more worried about LEO's than BG's doing this), it would be much safer in the hands of someone who is not familiar with the 1911 if it's in "C3".
I also own (and have owned in the past) a few DAO pistols. I always carried them with a "round in the pipe" and the manual safety on. (sort of "C1", but technically it's not, because they are DAO).
When I carried a SA/DA revolver (Taurus 689--WAY back in the day), I carried it with all 6 chambers loaded and the hammer down. I'm not sure what "condition" that would qualify as...
So it depends on your particular firearm, and what YOU are comfortable with, and how well-trained you are in deploying your firearm from a holster and from a concealed position.
From concealed, I can have my Para out, chambered, up, and an accurate round downrange in WELL under 5 seconds. From OC, I can do that in well under 3seconds, and that is from Condition 3 carry.
Carrying Condition 1 shaves about 1 second off my time from OC and about 1.5 secs off CC deployment. But I practice a lot, and have owned and been carrying various 1911-type pistols (and revolvers, and other semiautos) for over 15 years...
I'm sure that answer is as clear as mud to you. :P
As for vehicle carry, if you have a CHP, it is perfectly legal to carry it concealed while in the vehicle. Putting it in the trunk does you no good at all--you might as well leave it at home. And handling it (putting it on, taking it off) in public areas is likely to draw unwanted attention. The less you have to handle your firearm out in public, the better.
Putting it on your seat is not that great either, IMO, because you run the risk of it slipping off the seat if you stop or swerve suddenly.
The SAFEST and most effective place for a firearm is in a holster, on your hip, especially if you are in a car. This is one of the MAIN reasons I got my CHP. Plus if you have the CHP, it often helps "smooth things out" if you ever have to deal with LEO's in a traffic stop.
Let us know a little more about your situation:
1) What sort of firearm are you carrying?
2) what sort of holster do you use (does it have a thumb snap or other retention?)
3) how fast and accurate is your deployment? What is your experience level? Have you had any sort of "formal" training in defensive handgun use/tactics?
4) do you travel out of state often, or do you need to travel onto or through school property regularly? These things can GREATLY influence how you carry in a car...
We want to help, but it's difficult if we don't know the particulars of your situation...