As others have mentioned, for various reasons I don't think requiring cash is practical or will even be an option used by many. I do think some provision should

As others have mentioned, for various reasons I don't think requiring cash is practical or will even be an option used by many. I do think some provision should be made for those who would otherwise be stranded if there are problems with payment. Maybe users in need could get a certain amount of emergency charge for free or on loan up to some limit per year and tied to your license plate or drivers' license number.