speed doesn’t cause fatalities, or even cause collisions, it increases severity in the case of a collision. collisions cause (pedestrian) fatalities, and distracted driving is the leading cause of collisions.

Well no; speed does increase severity in the case of a collision as you say, but it also directly impacts your stopping distance, even at the same level of alertness/reaction time/etc. If a kid runs out in front of your car, you are both more likely to hit them at 30 mph than 15 mph (due to not being able to stop in time) and more likely to kill them having hit them.

you see, that's exactly the kind of marginal thinking that gets us into these kinds of shut-in situations for children in the name of "safety". that exact scenario, where only the speed difference rather than a myriad of other factors, is material to life and death, is a tiny, and probably an undifferentiable, portion of collisions. putting forth such imagined scenarios as if they present significant risk is poor rationale.

support traffic calming measures (like narrowing car lanes, adding streetside trees, converting parking to bike lanes, etc.), not for specious reasons like this, but because they reduce distracted driving, and thereby reduce collisions and injury/death. it's at best misdirection to talk about reducing speed, and at worst, leads to poor policy that not only doesn't address the problem (reducing injury/death) but creates unintended consequences (like traffic and more distracted driving).

And focusing just on distracted driving similarly leads to campaigns that are little more than "you, driver, pay attention" PSAs while ignoring the structural reasons that drivers incorrectly feel safe enough to do so.

Of course they both contribute to the problem. So why not both? Traffic calming is the answer, either way.

no, that sounds like a reasonable compromise, but it's just appeasing poor thinking, which is exactly how we get so many bad policies and regulations. addressing distracted driving with traffic calming is a reasonable solution for a correctly identified problem. psa's for distracted driving is a bad solution for the right problem. traffic calming for speed is a bad solution for the wrong problem. it's how we get stupid speed bumps and unproductive stop signs as an energy and time tax on everyone rather than targeting the reckless directly, which is the (other) right problem to address. while excess speed is indicative (but not conclusive) of reckless driving, a focus on reducing speed is a safety theater red herring. just like "think of the children" and ceqa (environmental) challenges to housing, it's used as a brainless cudgel to get a pet outcome approved, not to improve societies.