Unless you also have a boat, all of us should carry at PLB (personal locator beacon). They are about the same size and weight of a modern over sized mobile phone and freely available. I bought mine from BFC - made here in Aust by GME and cost $349 ( if you belong to BFCs club, you then get a $20 rebate off your next purchase).

It takes about 5-10 mins to register it on-line and about 2 seconds to make sure its working ok. Factory re-placeable battery lasts 7 years. Fitted with strobe light on top and has a soft carry case made to attach to belt or back-pack.

Carry it when ever we go walking in the country (although should carry it in the cities - more risks there!!).

An EPIRB is larger, heavier and more expensive and made specifically for boating. PLB's have to meet various ratings and that means they most also be water proof although not meant to splash around in the open ocean for hours