I used to process accident claims for a major insurance company. I sometimes had to determine if DUI was a factor.
https://www.dmv.org/ca-california/automotive-law/dui.php
Per California's driving under the influence (DUI) laws, it's illegal to operate a motor vehicle with any of the following blood alcohol concentration (BAC) percentages:
0.08% or higher― 21 years old or older operating a regular passenger vehicle.
0.04% or higher―operating a commercial vehicle.
0.01% or higher―younger than 21 years old.
Also, size matters:
http://www.businessinsider.com/drinks-before-driving-if-bac-...
If the GP is a big guy and has one or two drinks over the course of the evening with friends and then drives home, he may not ever be stopped. He may be less impaired than, say, someone driving while on cold medicine or short of sleep, which is typically not illegal, but can be significantly impairing.