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."
> 
> 
>
While there are multitudinous proper uses
Of a semi-colon, its primary function is circumscribed by the type of punctuation the writer is assigning to the semi-colon. 1) the semi colon is a substitute for a period meaning that independent clauses must appear before and after the semi-colon. Hence your example may be acceptable and effective for prose, but it is grammatical incorrect as the second clause is dependent (a lot of people is the subject but a predicate (Verb) is lacking ... so no dice. 

The other most common use of a semi-colon is as a surrogate comma, separating items in a list (following a colon). Typically the items in such a list are more complex than what can be expressed in a few words, hence they are more detailed and part of a program, process, components of an initiative or plan. The semicolon is a fancy comma or a learnéd period. Subtle, demure, and not common.