...the jump in call volume just after 9 p.m. on weekdays should have translated into a jump in the number of crashes, too.

Why? We used to have a cell phone plan with free off-peak calls and, yes, we took advantage of it, but almost all those calls were made from home. Most people with a frugal enough nature to care about waiting for off-peak rates aren't the type to be burning up $3.00 per gallon gasoline running the roads after 9 p.m. They're making those free calls from the sofa, and even the most reckless of cell phone users can't cause an accident unless he's actually in a car.