The Donald is bringing it up......]
Without an arrest, search requires probable cause—the officer must have some reasonable basis for believing that a crime has been committed, and that a particular search will turn up evidence relevant to that crime. It’s hard to see how cellphone data could be relevant to a traffic stop. Instead, searching cellphones looks more like a fishing expedition: Having gotten access to you with a traffic stop, officers are just looking around to see what they find. That’s explicitly forbidden by the Constitution, and with good reason. Letting government officials snoop on anyone they choose, for no particular reason, is a bad idea.
If you consent to a search, however, all bets are off. It’s hard to see why anyone would do so: If you’re a criminal, you’ve got something to hide; and if you’re not a criminal, why would you want to let the police paw through your email? And remember that when you consent to have your smartphone searched, you’re also giving up data on all your contacts, who haven’t consented. The legal ramifications to that have yet to be worked out. [Be bold! Refuse to do so, respectfully, without a warrant.]