The numbers seem a bit alarmist on the fatality front, seems like it would make more sense to account for fatalities as a proportion of accidents overall. In absolute numbers, we're talking tens of deaths and thousands of accidents.

As a visitor (periodically throughout the whole timespan) it's seemed to me like there's massive growth in population in the metro area and more densification inside the Seattle downtown area. Tough to tell what geography this exactly captures. Assuming the numbers are valid, I do wonder if there's a significant demographic or exurb shift, where older drivers became a higher proportion of all drivers where they already lived, and a bunch of others either stopped entirely or moved outside the city boundaries.

If memory serves, I feel like there's also a tendency to accidentally end up committed to a toll bridge crossing by getting stuck on an exit/on ramp off one of the highways, which might make people panic and bail at the last second erratically, but that idea seems a bit farfetched