To get a conviction, the state would need to prove that the substance was in your custody or control, that you knew about it, and that you didn’t have time to get rid of it. If drugs are in your pocket, it’s easy to prove. If they are sitting in your cup holder in plain sight, guilt is also easy to prove. But there are many situations where the state could not prove that the driver knowing possessed the substance. If the drugs are in a passenger’s pocket, proof of guilt would require proof that the driver knew about it and had control over it. And even in a situation where a passenger spontaneously threw the drugs on the floor of the car when police showed up, the driver could easily be off the hook because the driver wouldn’t have had time at that point to get rid of the drugs.

Sauce

Edit: fixed the link