People who like to drive rarely enjoy being driven and are often tempted to second-guess the driving of others. Usually, it's not a good idea.
Here's a Backseat Driver's Companion, a short list of do's and don'ts to follow when someone else is behind the wheel:
Hold Your Tongue
You may not like the way someone else drives. Maybe he doesn't notice when the light turns green as fast you might. Maybe he's more hesitant than you might be pulling into traffic or merging. However, unless there's an immediate danger you're pretty sure the driver hasn't noticed, proper etiquette is to remain silent even if it makes you stew inside. When you're back in your own car and behind the wheel again, you can drive how you like. Hectoring those who don't drive like you won't change their ways. It's just going to create stress and may even make them drive unsafely.
Maybe you do know a better way to get across town or know the car will fit in that parking spot up ahead. Ultimately, it's not your call. It's fine to give advice if it's sought. Just don't nag imperiously. You may in fact be a much better driver, know the quickest way to get across town and could easily parallel park the car in that tight space. But since you're not behind the wheel, it really doesn't matter. Grin and bear it. Everyone will be the happier for it.
Abide by "House Rules"
That means doing what the driver asks, within reason of course. It's his car; he gets to lay down the law. If the driver asks you to wear your seatbelt, for example, it's proper to do so without complaining, even if you prefer not to wear a seatbelt. The same goes for smoking, eating and drinking. If it's not your car and you're not driving, deference is the order of the day.
Don't Create Distractions
It's unsafe to yak on a cell phone while driving because your attention isn't fully focused on the task of driving the car. For the same reason, passengers who distract the driver can be just as dangerous, or more so. The driver has little or no control over how passengers behave. You can turn off a cell phone, but it's virtually impossible to "turn off" a distracting passenger. This especially is a problem for teenagers and young drivers. Put a bunch of kids in the backseat and a kid in the driver's seat, and the odds of a distraction-induced accident go up several notches. If animals are onboard, keep them under control. A dog running amok inside a car is another great way to set up a tragic accident-via-distraction.
Offer to Share the Driving
On longer trips, it's courteous to make it known you're willing to help with the driving if the driver wants a break. This should be done in a non-confrontational way. Don't say, "I'll take over now." Say something along the lines of, "Whenever you feel like taking a break, let me know. I'd be happy to drive some of the way." So long as you're not offering a critique of the other person's driving and implying you could do a better job, the offer will usually be taken in the right spirit.