1) I think what made famous urban legend is that the legends are spreading quickly and by oral tradition but I don’t think necessary to be credible, is more, I don’t think the legend has to be true.
2) I think the urban myths can be variations from traditional folklore tales and for that reason there are many stories and legends that are true but I think the vast majority are false, I even think the best for a successful story is that it was false.
3) The modern legends actually are emerging on the internet and years ago the legends had success with word of mouth. In the past I think the legends usually were created by just one person, but in the age of technology and internet is more complicated and there are modern legends created in groups and others created by one person.
4) The protagonist who have more success are the ghosts, In Spain we have two great myths legends. One of them is the Sack Man, the people use this legend when want to frighten some little child misbehaves, basically the legend says exists a man searching of missing children to take them tucked into a large sack to an unknown location, and the other great legend is the famous Girl in the curve, very similar to the Vanishing Hitchhiker.
5) The reality is that now is easier to create urban legends, as I said before with the rise of the Internet is easy to create legends or hoaxes hiding in the vastness of the network with the intention to manipulate people although it’s not necessary to be silly stories.