Add cameras, but even more so add average-speed camera. They suck but are absurdly effective. They record your plate from two places and if you got there too fast it means you went over 50 and are fined.
I don't understand the reasons, but the use of average-speed cameras, and in fact, the entire practice of enforcing speed limits based on measuring time between two marked points on a road, is not just not used in California, but is outright illegal.
This is particularly depressing, because they work amazingly well, and seem far fairer than an arbitrary (or perhaps worse, discriminatory) choice, made by a handful of police, of targets to give de facto large penalties, after extra fees and insurance are considered, for a practice that everyone is doing.
With average speed cameras, you can exceed the speed limit, even significantly, for short moments when it's important to do so (eg, to avoid an accident, to pass someone safely, etc), because average, not instantaneous, speed is what matters. You can know with near certainty that you'll get a (comparatively minor, in many cases) penalty if you speed for significant periods of time. But traffic will also be going the speed limit, not 20mph faster as in California, because everyone knows this, and no one feels they won't get caught if they're just careful / friends with the police / etc.
Driving in average-speed-camera-heavy parts of Europe is quite pleasant by comparison to the madness in California.
