Ποιος/ποια/ποιο are used for both 'who' and 'which one'. Τι is used to literally ask 'What is this?'
Saying "Τι είναι αυτός ο δρόμος;" would prompted someone to reply "What do you mean 'what is it'? Do you mean is it a street, a boulevard, an avenue..?" :)