A Case of Amnesia?
There are some cases where it is obvious. I've seen autopsy photos where the deceased driver was still gripping their cell phone.
There are other situations where the driver readily admits using a cell phone at the time of the crash.
However, in many cases a driver is involved in a crash and they tell the responding police officer that they were NOT using their cell phone when they crashed. Upon further inspection (often as a result of a subpoena), the billing records indicate that the cell phone was in use at the time of the crash. In many instances, these billing records are substantiated by eyewitness accounts (in fact, the eyewitness accounts are often the reason that the billing records are requested in the first place).
What is striking is how adamant some drivers are that they were not using their cell phone. This begs the question: Why do so many drivers claim that they were not using their cell phone after a crash? Here are some possibilities (the list is neither exhaustive nor mutually exclusive - feel free to add your own suggestions).
It is possible that the billing records are inaccurate. This possibility is difficult to reconcile with the eyewitness accounts...
The driver may deliberately misrepresent the facts. Simply put, they may be lying.
The crash may cause minor head trauma that impairs the driver's memory for events immediately before and after the event. This sort of amnesia (both retrograde and anterograde amnesia) could be caused if an airbag deployed or if the driver received a jolt to the head. In this case, the driver may not remember that they were on the phone. If you've ever seen someone who has had his or her "bell rung", then this possibility should make sense to you. Here, the driver is telling the truth - they really have no memory of using the phone at the time of the crash.
It is possible that there is some motivated misremembering (or motivated forgetting). The driver may feel guilty and "forget" that they were on the phone. This could result in some form of memory blocking - the memory of the cell phone might be inaccessible. This is one defense mechanism for dealing with traumatic events.
It is possible that the failure to remember may be due to a false memory. This sort of problem is very common in the memory literature. Repeatedly recalling the events surrounding a crash can change the details permanently. In this case, the memory is "false", but the driver would be "telling the truth" in the sense that their memory for events has been rewritten. This could explain the discrepancy between memory several months after a crash and cell phone billing records, but has difficulty explaining a false statement at the time of the crash.
Many of these cases are not counted as crashes associated with the use of a cell phone. This leads to unreliable estimates of the total crash risk for drivers using a cell phone. Even so, the National Safety Council estimates that 1 out of 4 crashes are caused by some form of cell phone usage.
If you would like to read more about memory-related issues, I highly recommend Daniel Schacter's book "The Seven Sins of Memory".