[MRM]

Almost all recent GM models will show basic information on engine speed and braking for the 30 seconds or so before an accident. Depending on the GM model and year (and most vehicles manufactured in the last couple of years) complete black data is recorded. The technology is so well developed that the software to read the black box is widely available. Almost any accident reconstructionist in the US can download the data directly without bothering with GM. I'm not up to date on other manufactuers but I would expect the same is true for almost all manufacturers.