Thank you so much!!! The English I've learnt in China only concerns being grammatically correct. However, as I read through a lot more materials, I have so many doubts about how I make sentences.

I'm going to comment instead of answer, because I can't explain why I would make these changes, only assure you that they're valid. You're really close. Firstly, for #1 I'd change it to: "Individuals owning cars have raised concerns about the environment." You wouldn't need the possessive apostrophe on the end of "Individuals". It's legible, but not technically correct. As for "has" you need to replace it with "have" in this context. For #2: "The crime rate of terrorists is rather low nowadays, thanks to police efforts." Adding "The" at the beginning of the sentence would be proper English, it sounds abrupt or sudden without it. Now for the end of the sentence, while the way you wrote it would be easily understood, I wrote an ending that sounds more like natural English would.

Individuals' cars have raised concerns about the environment. second one is correct 我的朋友