A study by the Indiana Public Policy Institute attempted to gauge the success of their new laws. They found that the number of accidents involving teenagers with passengers has remained constant, as has the number of late-night accidents. But the ban on cellphone use did have an effect.
The percentage of all wrecks involving teenage drivers on cellphones dropped from 0.71% to 0.56%, which represents a 21% reduction in teenage cellphone-related accidents. We should also consider that officers were probably more likely to report the involvement of a cellphone now that it's illegal. So the percentage before the new law might be even higher.
While we agree with the motives of the state of Indiana, we're not sure about their measurements of success. For example, they're claiming a victory against cellphone-related accidents, but teenage accidents might have remained constant overall. It doesn't matter that John Doe Teenager wasn't talking on the phone when he ran over a mailbox.
source: Indiana University via Gizmodo