California Vehicle Code 22350:
No person shall drive a vehicle upon a highway at a speed greater than is reasonable or prudent having due regard for weather, visibility, the traffic on, and the surface and width of, the highway, and in no event at a speed which endangers the safety of persons or property.
This code is often referred to the Basic Speed Law. Note that it doesn’t have an actual number. It’s a subjective law. Were you driving safely? That’s the question.
When traffic engineers set the posted limit, they always set it at 10mph over the safe legal driving speed, to account for people’s nature to speed just a little. Think of how you might have a friend who sets his watch five minutes ahead so he is less inclined to run late. It’s the same principle. So, driving less than 10mph over the limit is, by definition, safe and reasonable, given the conditions of the road. If it was raining or icy or heavy traffic, then going even 1mph over the speed limit might not be safe, and you’d deserve the citation.
If you get a ticket for going, say, 9mph over the speed limit, and the traffic was light, and the weather was good, and the conditions were clear, then that’s going to be an easier ticket to beat.
Here’s the link to California Vehicle Code 22350 on the California Legislative Information website.