How do people come up with statistics like that texting is as dangerous as drunk driving?

They use that statistic on the news a lot, but I can't really think of a good way for how they could have tested it. With drunk driving I believe they use random traffic stops to develop a base line for how often people drive drunk, and then can compare that to how often drunk people and non-drunk people get into accidents, but I don't see how that would work for texting.