45-70: The volume of water we are talking about would not gain enough heat to make it worthwhile.
You can not use hot water to clear the roads. That would make a very nice skating rink. Hot water freezes faster than cold water.
My wife has fun with newbies the first time the temp drops below 50 below. She takes them outside and has them toss a glass of cold water into the air. The cold water will string out into icicles then freeze. That will make hundreds of little icicles as it falls to the ground. Then they throw a class of hot water up into the air, the hot water flash freezes into a cloud of frozen mist, and floats away as ice fog, never reaching the ground.
Other little tidbits: The sidewalls of tires freeze and if you hit something like rail road tracks or a big pot hole hard it will pop the bead, and the frozen tire will roll right off the wheel. Springs become brittle, and will break if you hit a pot hole too hard, so will shocks. Do not use your parking break after 30 below, it will freeze on and the vehicle will not be able to move till it is taken into a garage and thawed out. Do not remove a vehicle from a warm building, such as a garage and park it outside. It must be kept moving till the tires get cold, otherwise the tires will freeze down and the vehicle can not be moved till the temp warms up. That could be spring. Saw a guy try and force his Chevy Blazer to move after the tires had frozen down. He had removed his truck from a warm building and parked it out side, leaving it running. The tires froze to the ice on the ground. He came outside and put the truck in low range. Gunning the engine he dumped the clutch. Ripped the protion of the tire that was in contact with the ground right off the wheel. He ended up driving home on four rims.
Another deadly thing. After my wedding in November 1972, one of the guest at the reception hid a bottle of booze outside. It was 65 below that night, and he had been drinking all afternoon and was already drunk. He went outside and took a big swig. Instantly froze his throat and stomach. He was found dead in the driveway, unable to make it back into the house. The cold can be rough on drunks, and fools.