De Niro's taxi cab license, on IMDb trivia about Taxi Driver: De Niro spent a month driving cabs as preparation for this role

http://www.imdb.com/title/tt0075314/?ref_=fn_al_tt_1 shows that the movie was released in 1976.. so I'll go with the notion it was needed for filming the movie scenes

Found on http://hispotion.tumblr.com

Thanks to Fabio for finding the trivia!