The chemical warmers are localized warmth - but heavenly for cold hands and feet. Well worth having in the car all the time but for a road trip I'd pack them in conjunction with a 120-hour candle or candlelier (and Reflectix).

FWIW -- I recently opened up some chemical hand and feet warmers that I found in a box and whose sell-by date was 2008. They worked great and for quite a long time (I don't recall exactly how long)! (Grabber brand).

If you are at Costco soon, look for a deal on a box of them that they had going a couple weeks ago.

http://www.rei.com/product/405012