ST. PAUL, Minn. - Inside his unmarked squad car, St. Paul, Minn., Police Sgt. Jeremy Ellison studied the faces of drivers as they waited at a traffic-jammed stoplight. Finally, he saw a subtle sign of trouble: A man behind the wheel of a minivan stared down at an electronic device for several seconds, oblivious to everything else on the road.
Was he dialing a phone, which is legal? Or was he checking e-mail -- which is not?
Ellison emerged from his squad car, sneaked up to the driver's window and peered inside: Facebook. Busted.
"I can see it, and there's nothing they can say," Ellison said.
It was an uncommon catch. As Americans become ever more glued to their mobile phones, the use of the devices behind the wheel is fast becoming one of the riskiest road hazards, involved in nearly a quarter of crashes.
But four years into Minnesota's anti-texting law, authorities say the measure is proving difficult to enforce.
With the law allowing drivers to dial and talk on phones, officers often have to take a driver's word about what they were doing on their mobile device before they issue a ticket, which costs around $120.
"The officer has to be nearly on top of the other driver to be able to make a determination," said Frank Rondoni, president of the Suburban Hennepin County Prosecutors Association.
Nearly half of American adults now own smartphones, according to the Pew Research Center. Texting among teens also continues to rise, Pew found, with older teen girls sending a median of 100 texts a day in 2011.
But in the car, those habits can be deadly. The risk of crashing jumps four times for talking and anywhere from eight to 23 times for texting, studies show.
Most people think they're different than other drivers; that they're capable of multi-tasking behind the wheel, researchers have found. But reading a text message for five seconds at highway speed means a driver will travel the distance of a football field without looking up.
"The problem is, you don't know that you're not engaged in the driving until the emergency happens and you don't respond to it fast enough, and the results can be catastrophic," said David Teater, of the National Safety Council. "People get away with it day after day after day, and they think 'Hey, I must be good at this.' "
Minnesota's texting law specifically prohibits drivers from using a "wireless communications device" to compose, read or send electronic messages -- including texts, emails, Web pages and other similar data -- while a vehicle is part of traffic, which includes being stopped at a light.
Drivers may talk on their phones, however, and dial them. The law also contains exceptions for "hands-free" communications, emergency situations and emergency vehicles.
"I think we'd like to see a statute that doesn't have so many exceptions," said Tim Richards, a supervising attorney in the Minneapolis city attorney's office. "If you're dialing a cellphone to make a telephone call or if you're texting on a key pad ... you're creating the same risk to public safety on the roadway."
So far, 38 states have passed texting bans for all drivers, including three since April, and 10 states have passed bans against using hand-held cellphones, according to the Governors Highway Safety Association. Some researchers point out that talking on a cellphone hands-free is not much different than using the phone itself, however; both are about as dangerous as driving drunk, a University of Utah study found.
While it takes creativity to enforce against texting, concentrated efforts yield results. Extra patrols in New York and Connecticut found it helpful to put spotters on overpasses and to use unmarked SUVs and trucks to get a better view of drivers.
Teater, of the National Safety Council, said he thinks the laws are enforceable the way they are, but that police need more time and the resources to do it.