I have a typewritten card in my wallet with contact details for my nearest relation.
I realise that this is helpful if I am hurt, but of no use whatsoever, if I forget my bag somewhere or my wallet is stolen.
I do have a key-safe with an extra front door key in it, so I can get back into the house. I bought it, after I had lost my keys one evening, the friend that had my spare key was not at home, and I had not at that point realised that being widowed means among other things that there is no-one in the house who can let you in. On that particular evening I had to break a window to get in!
I always have cash in the house, and here a taxi driver will usually be willing to drive an elderly woman home, if she can assure him that she will pay for the trip as soon as they reach her address. I also have an extra credit card which I never take out of the house, as ATMs often "swallow" cards and either you cannot ring the bank when that happens, or it takes them up to a week to return your card.
I suppose we should all have a card with personal details and phone numbers for emergency contacts on us at all times. Unfortunately, most of us would probably forget to move this card from one coat pocket to another when we decide to wear a different coat or jacket than usual.
We could write a memo to ourselves and put it up on the inside of the door we go out of, when we go out.