Aside from spare tube, tools, water, etc... my MAIN concern as an occasional bike rider, is be to BE SEEN.
As Leigh said, if you don't want to be just another squashed hedgehog, use everything you can to BE SEEN :
- reflective tape on satchels, on bike, on pack, on helmet,
- reflective vest (mine is an old vest, used by airport staff),
- lights : flashing red LED on bike's rear, + another LED flasher (Photon III, Pelican mini-flasher, etc..) attached by a safety pin on your back ; white LED flasher on the front of the bike, headlamp on your helmet... + Inova T2, just in case + spare batteries.

As a car driver, I have seen (at the very last moment ...!!) too many cyclists whitout any light, riding in the dark.. That's just suicidal...

If you are run over, despite all this signalling, then your widow will be able to sue, not for accident, but for deliberate murder !!