@Boris the Cockroach
In your contrived example, if someone had been hit and killed by a driver who was texting:
a) The driver would likely be found guilty of causing death by dangerous driving, or at least of manslaughter*. The 'texting' bit would surely be superfluous.
b) If the police wanted to show that the driver had been sending a text message at the time of the accident, surely they would ask the mobile operator to provide timings of text messages sent/received at around the time of the accident.
c) If access to the driver's phone were required, and it were locked, what is to stop the police getting a warrant to require the driver to unlock said phone? IIRC, it is an offence for someone to not do so.
d) What evidence would you expect to ACTUALLY find on a locked phone to PROVE that a driver was texting at the time of an accident.
If you ask me (which, of course you didn't, but I am somewhat opinionated), the police should not have de facto access to any mobile phone they should happen to pick up. They should, however, have some means of accessing data on a mobile device which the courts have deemed may be pertinent to their needs to investigate a given case. In such a case, the police officers in question should apply to a judge for a warrant. If there is nobody who can actually access the data on the device, if for example, it has been locked by someone who has subsequently become a murder victim, then the phone manufacturer should aid them where possible. If the phone uses any decent sort of encryption for its data (as it should in this day and age), then tough shit; go and do some proper police work. See how it works? The last thing we should be doing is giving those with little or no oversight more freedom to observe and control the private data of those who have committed no crime.
*IANAL of course.