A road running north to south crosses a road going east to west at a point P. One car is driving north along the first road and a second car is driving east along the second road. Assuming they avoid a collision, both cars leave point P at the same time. If the first car is driving at a speed of 45 mph and the second car is driving at a speed of 60 mph, at what rate (in mph) is the distance between the cars increasing two hours later?