Can you actually have a policy in place for this ?
Can you have a policy that states, since you are on probation, you will be terminated in case of an accident that was not your fault, was an act of god or completely the fault of a jerk driving drunk on that day ?
Since the driver is on probation, there is nothing in law from terminating the driver during the probation period (we are not talking of denying compensation related to the accident, right ?)
However, if it was not his fault, is that fair ? More importantly perhaps, are you losing a good driver for no fault other than that he was in probation ?
How do you determine whose fault it was ? Mostly you will believe the passenger he was carrying.
Or if you have GPS devices fitted that tells you what speed he was driving will give you a good idea.
Take a different view now :
Suppose you make a policy saying that any driver having an accident during probation will be terminated.
So every new driver will drive VERY carefully during probation period. He will probably driver slower than necessary causing delays and probably causing damage to the car due to excessive driving in lower gear. Once he is out of probation, he will start speeding and acting reckless may be out of pent up frustration of previous periods.
I would say you do not need a policy.
Instead you need to take each case on its merits and decide.