An 18-wheeler reaches a speed of 70 mph, then slows to 60 mph. At which speed does it have the least kinetic energy?

At 60 mph. The 18-wheeler is moving slower, so its molecules have less kinetic energy.

An object that has motion - whether it is vertical or horizontal motion - has kinetic energy ... This equation reveals that the kinetic energy of an object is directly proportional to the square of its speed. That means that for a twofold increase in speed, the kinetic energy will increase by a factor of four.