Question: My sentence is:
"Cars kill people; a lot of people."

I'm keeping the language brutally simple for crude emphasis.

Do you guys think this is a good way to use the semicolon?

Answer: 
To me, using a semicolon fails on many levels. It fails grammatically (which is already understood), it fails conceptually (the two clauses are not tied together closely enough to warrant a semicolon), and it fails to deliver the desired level of emphasis.

For emphasis, use a separate sentence (or, as the case may be, fragment):

> 
> "Cars kill people. A lot of people."
> 
> 
>
If you mean how would I rephrase the clause you originally posited, it's simply a matter of profiler punctuation and economy of language, as you already employed. Although your subject doesn't make sense since Cars don't kill people, rather other people driving cars. 

My edit: Cars don't kill people; people driving cars kill other people. People, preoccupied with something beyond the steering wheel or in the blind spots among their rearview and side mirrors, kill people while driving with reckless negligence.