I am by no means a snake expert, but.......
There is only one kind of venomous snake in California (other than a very rare sea snake that I don't think we have to worry about in this context). And that is the rattlesnake. Rattlesnakes are very easy to identify. So if you see a snake and it isn't a rattlesnake....Don't Panic!!
I hike in the Santa Monica mountains a lot. I see rattlesnakes on the trail all the time. They are almost always very considerate and let you know that they are there with a quick shake of the tail long before you are close enough to do it harm.
From my experience, rattlesnakes are slow. The only time they are fast is within their striking range. Which I would guess is somewhere around 3/4 of their body length. When I encounter a rattlesnake, I simply give it a wide berth when I walk by and put my hiking pole between it and me. If there isn't a whole lot of room, I've used my hiking pole to encourage them to move on.
I worry a lot more about bees than I do snakes.
I'm going to have to disagree with you here. I see rattlers in the local mountains at least once every other week during the summer, if not once a week and they are dumb as hell.
I have never, ever had one rattle ahead of time. The sequence usually goes like this:
1) I see a rattler laying on the trail and can't get by.
2) I kick dirt at it and nothing happens. I kick more dirt and nothing happens.
3) I throw little pebbles at it and nothing happens.
4) I find a long branch and poke it from far, far away or a rock the size of a dollar coin and throw it at the body. I'm not trying to kill the thing, that would be easy.
5) HISSSSSS, rattle, rattle, rattle.
6) I stand there exasperated for about 30 seconds as it lets me know it's pissed off.
7) It now starts to slink off into the bushes or I repeat step #4.
I've had one guy slink off without ever rattling. I've also had one guy lunge at me multiple times. It just did not want to move off the trail. I have never had one warn me ahead of time and I've had more snake encounters than I have fingers and toes.
I worry about ticks and lions.