The debate on whether to use underscores or dashes in URLs has been a pretty old one. Google finally cleared up the difference on their interpretation between the two. Their answer is simple: Dashes are considered are separators while underscores are not.
What this means is that for our URL
it is clear to Google that each word is a separate one helping them determine keywords. If the URL was written as
instead it would be possible that Google could make a mistake in parsing the URL. However, for those who currently use underscores to separate words in their URL Google’s advice is to simply leave the existing URLs as they are since they are constantly improving and say that in time there will be less distinction between the two punctuations.