Do I really need to tell you that it's a bad idea to play Pokémon GO while you're driving? I really hope it's not necessary, but just in case it is, here you go: It's a bad idea to play Pokémon GO while you're driving.
First of all, cars are dangerous, and you should try to remain as focused as possible while operating one to limit the chance of death or injury to yourself or others. Suffice it to say that zooming along at high speeds while your eyes are glued to your phone's screen in hopes of finding Pokémon is a bad idea.
Secondly, a lot of the Pokémon you come across in the game won't be right there in the middle of the road for you to pick up. While I'm sure there is a more rational subset of Pokémon GO drivers that would take the time to pull over and get out of the car before hunting down these Pokémon, there's always the possibility you could get caught up in the moment and pull your car off-road to investigate some rustling leaves. Playing Pokémon GO while driving is a bad idea, and taking your car off-road while playing Pokémon GO is a really bad idea.
Thirdly, anything over 15 mph doesn't count for egg hatching, so if you're looking to hatch a new Pokémon, you're better of hoofing it anyway, or paying a non-distracted driver to go super slow for you.
While you can technically play GO and drive, there are many other reasons not to actually do so, like the possibility of getting ticketed, arrested, doing damage to your car or someone else's, getting shot at, or worst of all, hurting you or someone else. But do we really need to get into those? Just put your phone down and do something else while you drive.
One last time, it's a bad idea to play Pokémon GO while you're driving.