Given that we drove 18 miles in 20 minutes at a constant speed. Let’s first find the speed:

$$Time=20min\times 3=60min=1hour$$

$$Distance=18\times 3=54miles$$

$$\therefore Speed=\frac{54}{1}=54mph$$

So the constant speed is 54 mph. As 54 is less than 55 so this could have been the lowest speed. Therefore, Choice H is correct answer.