People drink coffee in the car all of the time. For many workers during that early morning commute, it is just a staple of their everyday routine. Others buy it late at night when they need to keep driving but they know they're getting tired. Even just getting a cup of gas station coffee, though it may not be delicious, helps them keep going.
That said, there is a downside. In some places, the police classify drinking coffee in the car as distracted driving, just like sending a text message. They claim that it takes your full attention off of the road. There can be visual issues as you tip the cup up in front of your face, and needing to hold that cup with one hand means that you're only using one on the wheel. There is also, of course, the risk of spilling hot coffee on yourself while you drive and causing a car accident when you lose control of the vehicle.
On the other hand, drowsy driving is quite dangerous and can cause accidents much the same way drunk driving does. In some instances, drivers fall asleep behind the wheel. In others, they just drift on the edge of consciousness and are not ready to react quickly when needed -- like when they come around a bend and there is a stop sign close at hand.
So, should you drink coffee and drive? The best option is to avoid the distraction and only drive when you're naturally awake. As you know, though, a lot of drivers don't do this. If one of them causes an accident and injures you, make sure you know what options you have.