The effected area after a disaster is generally pretty small. Even with nuclaire disaster, the exclusion zone is only 20km (Fukushima) or 30km (Chernobyl). The 2004 Tsunami only reached 2km inland. So cell service might not be that far away.

War, flooding, hurricans, typoons will have bigger effected areas, but you can see them coming. Having rapid information (be it early warning, traffic information or just satnav) might be the key to get out quicker, and make it far easier to figure things out outside the effected area. A smartphone is a pretty good tool for that.