As more states ban texting behind the wheel in a fight against deadly driver distractions, police departments around the country have found enforcing those laws difficult, if not impossible. Now a new federal grant will pay for experimenting with the only technique shown to work so far -- spying on motorists while they drive.

The $550,000 grant announced this week by the National Highway Traffic Safety Administration will let police departments in Connecticut and Massachusetts test a variety of anti-texting moves over the next two years, from ad campaigns to roving patrols. The aim: To find "real-world protocols and practices to better detect if a person is texting while driving," said NHTSA chief David Strickland.

While 38 states ban texting behind the wheel, proving that someone is using their phone to type text rather than look at a map or some other permitted use has become a roadblock for law enforcement agencies. Only 10 states ban all hand-held cellphone use behind the wheel, so in most states with a texting ban, simply holding a phone in your hand isn't enough for a ticket; officers have to see a driver thumb type before they can pull them over. In Minnesota, police wrote 1,200 tickets for texting in 2011, compared to 200,000 for speeding, according to the Minneapolis Star-Tribune. In Scranton, Penn., police issued 10 tickets in six months after that state's ban went into effect -- and one of those was to a driver who admitted texting after a crash.

That's why the NHTSA grant will pay for "spotters on overpasses" and other roadways who could identify drivers while they type, and there's already evidence for how such a program can work. In Bismarck, N.D., police wrote 31 distracted driving tickets in two days during a crackdown earlier this month where they used unmarked, high-riding trucks or SUVs to peer down into cars and catch texters in the act. Since North Dakota bars not just texting but Internet browsing behind the wheel, officers had to see what specific apps drivers were using, with one officer telling The Bismarck Tribune that they could have written twice as many tickets, but couldn't get enough evidence.

While some safety groups have called for an outright ban of cellphone use behind the wheel, such proposals haven't gained much support in Congress or legislatures around the nation. No federal agency has the power to control what people do with smartphones while driving, and there's still an open debate about how serious a risk texting or other electronic distractions pose compared with better-known dangers such as drunk driving. Thousands of Americans have already been the victim of a motorist who should have been steering instead of typing, and the U.S. Supreme Court has said there's no expectation of privacy when driving on public roads. But if the only way to enforce texting bans involves undercover police reading cellphone screens over driver's shoulders, the debate over how to make roads safer will take a different route.