More Cars

[jack]

Are these correct? If not, why? What is the subject and verb here?
1. More cars means more traffic. (Is 'means' not 'mean' because 'more cars' is one idea?)

Or should it be:
2. More cars mean more traffic.

If #1 and #2 are correct, what do they mean?

What about these ones?
3. When they are more cars, it means more traffic.
4. When they are more cars, that mean more traffic.
5. When they are more cars, this mean more traffic.
6. When they are more cars, it mean more traffic.

Thanks.

[Casiopea]

If there are more cars on the road, that means there will be more traffic on the roads.