Keep identity documents, car registration and insurance card, and any credit and bank cards in a card wallet separate from your cash wallet or purse. This hardly needs explanation. It's a bad idea to lose both cash and identity at the same time. Carry the card wallet in a front pocket in your jeans or slacks. Front pockets are difficult to pick, not impossible, but difficult.
If you haven't joined AAA (the automobile club), do so. They will get you out of a lot of jams. They'll give you a jump. They'll open your door if you lock your keys in. They'll tow you off the freeway. Good stuff. Oh yes, they will also copy your car key for you in plastic and embed the fake key in a credit card frame. Perfect for tucking into that card wallet.