Answer
Let's assume that the car is traveling uphill. As $\theta$ increases, the distance $D$ decreases.
This agrees with our driving experience. The required distance to slow down on a steep hill is less than the required distance to slow down on a gentle hill.
Work Step by Step
$D = \frac{1.05~(V_1^2-V_2^2)}{64.4~(K_1+K_2+sin~\theta)}$
Let's assume that $\theta \gt 0$ which means that the car is traveling uphill. As $\theta$ increases, $sin~\theta$ also increases. Then the denominator increases, and it follows that the distance $D$ decreases.
This agrees with our driving experience. Let's suppose we want to slow down while going up a hill. If we are braking while going up a hill, the required distance to slow down on a steep hill is less than the required distance to slow down on a gentle hill.
