Gov. Bill Ritter has signed a bill making it illegal to text while driving.

The law will be much stricter for drivers under 18 because they will also be prohibited from talking on handheld cell phones while driving. Hands-free devices are allowed, and there are exceptions for emergencies.

Drivers older than 18 can still talk on handheld cell phones.

The text message ban takes effect in December. Drivers will be fined $25 on their first offense for texting while they drive and $50 for any subsequent offenses.

Ritter signed the bill Monday in Fort Collins, where 9-year-old Erica Forney died on thanksgiving, two days after being struck by a driver who was talking on her cell phone. The driver, Michelle Smith, pleaded guilty in March to misdemeanor careless driving resulting in death.

Smith’s plea deal called for two years of probation, a $300 fine and 150 hours of community service. She was also ordered to write a letter of apology to Forney’s family.

At least a dozen states and the District of Columbia have laws that ban the practice or that take effect this year. Text message bans also await governors' signatures in Illinois and Tennessee.

Now that the governor signed the bill, law enforcement agencies across the state will undergo training to understand the law and how it will be enforced.

“We will enforce the law, but we will enforce the law reasonably,” said Sheriff Grayson Robinson, of the Arapahoe County Sheriff’s Office.

Deputies and officers across the state said enforcing this law will not be black and white.

“I think it is going to be a little bit of a challenge,” said Robinson.

“That is going to come up to the experience and discretion of the officer,” said Trooper John Sparks, of the Colorado State Patrol. “Can (the trooper) articulate the driver was punching keys on the cell phone for 5, 6, 7 seconds or were they engaged in a cell phone conversation.”

Drivers will legally be allowed to talk on their cell phone, which means they can dial a number. So officers said they will have to watch for other indicators.

“We are not going to walk up and ask someone to see the text,” said Robinson. “That is just not appropriate.”

At this time, it’s unclear if asking to do so without a search warrant is even legal.