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 want to keep the original wording, I think an [em dash](https://en.wikipedia.org/wiki/Dash#Em_dash) is the most natural choice:

> 
> **Cars kill people—a lot of people.**
> 
> 
> 

Em dashes are a good general-purpose way to introduce a pause (like a colon) or offset a phrase (like parentheses) where no other punctuation mark is quite right. An em dash tends to indicate more *emphasis* than a colon, a semicolon, or parentheses, which is a good fit for this case.

Your example isn’t a standard use of a semicolon, because the second part isn’t a complete phrase. Another punctuation mark such as a comma or colon would also work here, but may require rephrasing:

* Restating the noun for emphasis

> 
> Cars kill a lot of people.
> 
> 
> ↕︎
> 
> 
> Cars kill people, a lot of people.
> 
> 
>
* Stating the topic, then explaining with more detail

> 
> Cars kill over 30,000 Americans per year.
> 
> 
> ↕︎
> 
> 
> Cars kill people: over 30,000 Americans per year.
> 
> 
> 

(“A lot of” doesn’t fit in this case because it doesn’t add clarification, just emphasis.)