Instead, if you have a certain amount of alcohol in your blood, or marijuana , illegal drugs, or prescription drugs in your system, you can be charged.
The state prosecutes DUI even when the driver is not driving erratically because it believes that certain drugs, including prescriptions, can make a driver unsafe on the road due to increased reaction time, drowsiness, weakness, or reduced coordination. One or all of these can make a driver have poor judgment in traffic.