Does cellphone use cause car accidents?

Two economists exploited a clever natural experiment by comparing auto accidents in California just before 9pm and just after 9pm.  What makes 9pm special is that cell phone rates go way down at 9pm.  Using data from cellular towers, the economists saw an increase in cellular calls, but no increase in auto accidents.  They offer three explanations:

1. People who start talking while driving become more cautious;
2. Behavior does not change; or
3. Although cellphones clearly distract some drivers, they may also help other drivers stay alert.