I would probably use a zippo hand warmer before a heat pack because you can get more time and heat. Also, I have been experimenting with cold weather gear because it is always a good thing to have something to keep you warm if the car broke down. For the past week while traveling up north, I had bought a Canada Goose expedition parka (not a Chinese copy)which took me awhile to save up and buy one but I tested it out and wore it every day on the trip and even up to ski lodges with my friends. The jacket will keep you very warm to (tested) 18 degrees F with just a tee shirt on with no layering and 30 MPH wind as well. It has tons of pockets and built in hand warmers for the pockets if you have no gloves. (basically gloves in pockets sewn in)I was able to carry tons of gear in the pockets with plenty of extra to spare and was thinking if you had that jacket alone with a pair of wool long johns or equivalent in your vehicle when traveling, it could take you down to 0 degrees with no problem if you had to stay in the vehicle for extended amounts of time. Anyway I think the military parka may do the same for cheaper but have not tried it. The idea would be throw a jacket in the car, the jacket has a lot of gear already in the pockets, it stays with you and in emergency's deploy it. When you get home, take it in the house. Utilizes hardly any room. The jacket is packed with down and you get hot in it above 35 degrees with a tee shirt only.