In the instance of a witness who just walked into the room, a good defense attorney can get an acquittal if the smoking gun turns out to have not been fired by you.
But people have a tendency to attribute capabilities to a machine humans do not posess, and although the data may be as stated (the wheels were turning at a rate indicating 58 mph, the engine RPM were conformably high, the brakes were not applied, etc.), the data must be correctly interpreted in order to reach the correct attribution of fault or error.
Not only is 5 seconds of data possibly quite insuficient, the scope of that data is insufficient.
The box is not recording everything going on out there, it may record the data input flawlessly, but it will neither record nor process the full spectrum of data that the human operator does.
How many years has it taken some of the best minds in the country to come up with a vehicle which could pass the DARPA Challenge at a rate the average farm kid could muster on a slow day?
Somewhere between black box and jury you must find people and attorneys who are both knowledgeable enough about what their automobile does as they are driving it, (or who can be educated to be on the spot), and who can interpret the data within the framework of the circumstances involved.
If there has been another vehicle overtaking in the rearview mirror, sliding down the hill behind the vehicle, evasive maneuvers called for would have included speed, not attempting to slow down.
For this reason, we humans, with all our failings, continue to operate vehicles, to land the plane, etc. We have not built machines capable of a sufficient spectrum of data input, nor of interpreting that data well enough.
With that in mind, if automobile "accidents" were investigated to the standards the aviation industry has set, "operator error" would be high on the list of causative factors, just as pilot error is with aircraft crashes.
Seldom in states which provide for citations for not using "care required" to avoid an accident does the driver get a pass. I knocked over a very old street sign one fine winter evening, moving at approximately 10 mph, on clear ice which the sand had melted through and the ice frozen over.
My speedometer barely registered motion as I attempted to turn my vehicle and the front wheels lost traction. The impact did not dent the front of my old van which is all metal. The investigating officer pulled up at a low rate of speed, (slid to a stop), got out of his vehicle, and promptly slipped and fell on his a$$.
The street appeared to have been recently sanded, but ice had formed over the sand, which had melted into the surface earlier in the day.
Despite the obvious facts, I was written up anyway. Oh well, you broke it, you bought it.
Would the 'box' have gotten me out of the ticket? No, because the accident was prima facie evidence that I was going too fast for the (deceptive) conditions.
Data are always subject to interpretation. Our society relies on juries for the ultimate determination. It may not be the best system, but it is the best we can do right now.