Marianna argues that, although public education and stricter laws have helped alleviate the problem of drunk driving, additional measures are still necessary. She supports this claim by pointing out that people still drive after drinking, which greatly increases the probability that they will cause an accident "involving death or serious injury."

David responds by suggesting that Marianna is exaggerating the risks of drunk driving. He points out that a driver in an automobile accident is slightly less likely to be seriously injured if that driver is drunk, compared to if they are sober.

Marianna's justification for why additional measures are needed to prevent drunk driving was that drunk driving increases the chances of an causing an accident "involving death or serious injury", a category that presumably includes not just the risk of serious injury to the driver or drivers in the accident, but to pedestrians and anyone else involved. David claims Marianna is exaggerating the risks of drunk driving, but only provides evidence that a driver who happens to be in an accident is less likely to be injured if drunk than sober — he doesn't make it clear if these drivers have caused these accidents, or about the risk of serious injury or death to others.

So a major flaw in David's argument is that he seems to be talking only about a subset, if not an entirely different set, of risks than Marianna is talking about. Marianna is talking about the risk of causing an accident involving serious injury or death to anyone; David is talking about the risk of serious injury to a driver who is already in an accident (not necessarily one they've caused) when they are drunk versus when they are sober.