Video solution:

Click to load, then click again to play

Written solution:

Let the rush-hour speed be $s$ miles per hour. The $20$-minute rush-hour trip takes $\dfrac13$ hour, so the distance is $\dfrac{s}{3}$.

Without traffic, the speed is $s+18$ miles per hour and the trip takes $12$ minutes, or $\dfrac15$ hour. The same distance is $\dfrac{s+18}{5}$.

Set the distances equal: $$\dfrac{s}{3}=\dfrac{s+18}{5}.$$ Then $5s=3s+54$, so $s=27$. The distance is $27/3=9$ miles.

Thus, D is the correct answer.