Whether you Sign, PIN, or Nothing actually is a bit more complicated and is dependent on the card you have as well as the terminal at which you are making the transaction.
Many US based Chip Cards with PIN capability actually still have Signature as the primary CVM (Cardholder Verification Method), so if the Terminal allows Signature, that is what you will do. Since most tourist places will allow signature, it is not unexpected that most of the time you will have to sign, it really does not have much to do whether if there is a person there or not...except that if a person is not present, then PIN or None is the only option. Recently in London, it was about 50/50 for me whether I signed or used PIN for a manned terminal.
As for the "none" option, that will be mainly for low dollar transactions and the level is set by the terminal, As an example, in the US I have a card that sometimes requires a signature if over $25 in some places, $50 in others, and in some I always have to sign. Your card of course would need to allow a "No Verification" transaction. If your card is PIN capable, it is likely that you will have to enter your PIN, though others without a PIN will not need to sign or use a PIN.
Being prepared with both Cash and Credit Card is good advice, I would even go so far as to say to have a second card as an option. More than once a card has inexplicably not worked for a transaction, but did at another place shortly before and after.
