What can be asserted without evidence can be denied without evidence. In other words, I'm sorry but I don't have any study to provide you. Probably such a study

What can be asserted without evidence can be denied without evidence. In other words, I'm sorry but I don't have any study to provide you. Probably such a study cannot exist, since it would be unethical to perform the sort of double blind study (needed to separate correlation from causation ) involving humans, transportation and employment. Observationally, people are moving to bigger cities, I think this is common enough knowledge to not require a citation.