You’re right that streets should be designed such that low speeds feel inevitable and not something you have to think about, and that they should serve one purpose and not two (no stroads). And highways should completely bypass cities, because the idea that they should cut through them is just absurd.

Where are the cameras catching tailgaters, people who don’t signal, people cutting others off, people cruising in the left and not passing, people blatantly running stop signs, people texting or doing makeup?

The technology to do this is more challenging then detecting speeding. Red-light cameras are also very common, because they are relatively easy to implement. I believe there is some tech for texting while driving at least, but I’m not sure how automated it is.