If speaking to a driver doesn’t increase the risk of an accident, then talking on the phone couldn’t possibly be more dangerous than talking in-person — because they both wouldn’t be dangerous at all.

The stimulus is comparing talking with a passenger to talking over the phone. It doesn’t matter whether talking with a passenger is more dangerous than not talking at all, as long as it’s safer than talking over the phone.

For the purposes of this passage, it doesn’t matter what people believe regarding their cell phone usage — the stimulus is dealing with what is more dangerous, not what people believe is more dangerous.

This would actually undermine the stimulus. If warnings don’t help, and they are the main difference between talking with a passenger and talking over the phone, then why is talking with a passenger safer?

This being false wouldn’t make the stimulus untrue. If conversing on a cell phone was more dangerous than talking to a passenger who continues speaking during difficult driving situations, then this would actually support the stimulus even more.