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

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

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

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

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.