A ban on teenagers in North Carolina using mobile phones while driving appears to have backfired, a study found this week.

Incidences of teenagers driving while on the phone actually increased after the state ban was enacted.

Two months prior to the ban in December 2006, 11 per cent of teen drivers were observed using cellphones as they left school in the afternoon. About five months after the ban took effect, this had risen to 12 per cent.

Most drivers were using handheld phones. Nine per cent were holding the handsets to their ears, while fewer than one per cent were using hands-free devices. About two per cent were observed dialling or texting.

Cellphone use remained steady at about 13 per cent at comparison sites in South Carolina, where teen driver cellphone use is not restricted.

While young drivers and their parents strongly support the restrictions, they believe that the ban is not being enforced.

"Most young drivers comply with graduated licensing restrictions, such as limits on night time driving and passengers, even when enforcement is low," said Anne McCartt, senior vice president of research at the Insurance Institute for Highway Safety.

"The hope in North Carolina was that the same would hold true for cellphone use, but this was not the case. Teen drivers' cellphone use actually increased a little.

"Parents play a big role in compliance with graduated licensing rules. Limiting phone use may be tougher for them since many want their teens to carry phones."

Most parents and teen drivers agreed that police officers were not looking for cellphone violators. Some 70 per cent of teens and 60 per cent of parents indicated that enforcement was 'rare' or 'nonexistent'.

Only 22 per cent of teenagers and 13 per cent of parents surveyed believed that the law was being enforced 'fairly often' or 'a lot'.

"Cellphone bans for teen drivers are difficult to enforce," added McCartt. " Drivers with phones to their ears are not hard to spot, but it is nearly impossible for police officers to see hands-free devices or correctly guess how old drivers are."