At the risk of going wildly off topic, this raises a very interesting point about the way people perceive risk.
Just grabbing statistics off the net here: in 2005 there were 6.4million auto accidents in the US. In 2004 there were 1.3million assaults (murder, rape, violent assault, robberies, etc, source DOJ, roughly equal to the preceding year). It strikes me that the objective risk of crime against the person is less than the risk of accident involving an automobile. How you quantify risk is, of course, difficult because the risk is highly variable depending on circumstances. However, overloaded cars would naturally be at a higher risk than suitably loaded ones.
Anyway… regarding the incident itself, if it was such a short ride at such a low speed, why couldn’t the driver make two trips? It would hardly have been any hardship.
I think recklessness is sinful because it displays inappropriate lack of care for oneself and one’s companions. I would say it’s a grave matter because of the serious consequences had anything gone wrong and because, in this case, nobody of college age can honestly claim to be ignorant of the potential for injury in such cases. To not consider such a thing is to wilfully disregard what should by understood as wrong by simple human reason alone - that is to say it is clearly and objectively sinful.
I think it certainly merits mentioning in Confession.