Ridesharing is safer than solo driving (unless you're a male teenager)

Handsfree phone calls still cause accidents

Keen to contribute towards your state government's treasury? Just drive past a police car with your mobile phone held to your head and you'll get the opportunity to donate around $200 to the government. Road safety authorities aren't too fond of people talking on the phone.

And for good reason: talking on the phone dramatically increases your risk of having an accident up to 600% (Violanti and Marshall 1996). It slows your reaction time down by 0.25 seconds, (equivalent to a blood alcohol reading of 0.08) which is apparently enough to cause an accident (Caird et al. 2008). It doesn't matter whether you're hands free or using a handset, talking on the phone while driving, is a very bad idea.

Having passengers in the car reduces accidents
What I've been curious about is whether having passengers in the car has a similar effect. Sometimes when I'm chatting in the car with someone and having an in depth conversation, I notice that they often start making driving errors (e.g. not looking before changing lanes) that could lead to a major incident. I get particularly concerned when I'm in the car with very visual conversationalists who insist on maintaining eye contact while driving. "Keep your eyes on the road", I want to scream!

I needn't worry. The research shows that having passengers in the car actually reduces the risk of an accident and the more passengers you have, the safer everyone is (Lee and Abdel-Aty 2007). It might be because the driver feels a greater sense of responsibility for safety when they've got other people in the car, it might be because 'back seat drivers' notice hazards that the driver didn't pick up or it might be because in our sleepless modern society, passengers help keep the driver awake.

Visual cues are vital for driving safety
The reason why having conversations with someone in the car is safety enhancing, whilst talking on your bluetooth headset with someone will make you more likely to have a crash, is apparently because of 'conversation modulation'. That's basically when you stop talking to the driver because you can see and feel a dangerous situation coming up on the road.

Maciej et al. (2010) put people in a driving simulator and found that when the conversational partner had visual cues, they usually slowed down the conversation or stopped talking altogether to give the driver a chance to concentrate on driving. The person on the other end of a mobile phone can't see what's going on and so they keep blabbering on regardless. The hapless driver is distracted to death!
 
Stay safe: share a ride
The takeaway message is: leave your bluetooth headset in your bag, keep your phone switched off and have a conversation with the person next to you instead.

...Unless you're a teenage male driver
In which case, you should kick your passengers out as well because the research shows they'll make you more likely to crash. You can imagine why: having four testosterone charged males encourage you to 'drag' other drivers is hardly a recipe for safety.