There are the airbag sensors, of course, but the crash has already started at that point. So let's look at how aircraft recorders work. The digital flight records store parameters for the entire flight, so that not only can it be used to assist crash investigators, but it can also used by maintenance depots to detect faults before, during, and after a flight so that a failure cascade can't progress to the point where the aircraft falls out of the sky.
The cockpit voice recorders also record constantly; the fact is the system saves only the last 30 minutes or so of audio, erasing the tape or memory as it records over the older material. They record multiple audio channels in parallel, too.
So for a "consumer-grade" road "flight recorder", I see the main car computer recording everything all the time, but reusing memory space so that only the last "n" minutes of data is retained. This precludes using flash memory, because flash has a limited number of write cycles it can do before the storage cells fail. So that means another form of NVRAM needs to be used, or working RAM has to be dumped at the end of a crash event to flash. The engineering is not that difficult, particularly if the amount of data is held to a minimum.
The engineers might then say "why limit the memory? If we use 4 or 16 GB flash, we can record information directly in real time, and not wear out the flash memory for a considerably longer time." Not exactly cheap, but an easier design, because you don't have to worry about "staying alive" long enough to dump temporary memory. I can see it now: "at 30,000 miles, change the compact flash card." It becomes just another maintenance item.
And a long-term window into your driving habits.
Flo already knows....