If you're driving on black ice, keep in mind that you may lose control of your vehicle at any time. Your brakes and tires may have no control in this driving environment. So consider your surroundings and ask yourself how fast you want to be going if you were to lose control and veer off the road, or into a semi-truck in the next lane. Standard practice on most highways in stormy weather is to stay below 45 mph, but check in with your comfort level, since this is a cap and not a floor.