What's a major city? If we're still in the coarse-estimate mode, I'd say a city most people have heard of.

Maps of internet backbones will help you locate the ideal places, from a connectivity point of view.

http://www.nthelp.com/images/InternetMCI.jpg

In many places, a 3G or 4G backup works amazingly well. Certainly here in Australia, and probably in certain places in Asia/Europe too.