Here's some recent data, via CityLab, suggesting that Americans are walking less and driving less, but killing more people when they do drive around. (The report is based on data from 2019 to 2022.)
My first reaction to these high-level findings is that they seem to make sense. This time period was the pandemic. And people were locked away at home (though I used to take some seriously long walks around downtown during this dark time).
So I don't know, I'm not sure we can conclude that walking less is truly a structural phenomenon. Similarly, I'm not sure that we can immediately conclude that cars are becoming increasingly more dangerous.
According to Wikipedia, deaths per capita, deaths per billion vehicle miles traveled, and total deaths, have all been generally declining in the US since the 1960s.
However, I do wonder if there's some sort of correlation between people walking less and car-related fatalities. The most dangerous streets, in my mind, are often the ones that don't have a lot of pedestrians.
That's why, broadly speaking, it feels safer walking around Manhattan than it does Los Angeles. So maybe less people walking is enough to trigger an increase in pedestrian fatalities.