This is a very helpful analysis. I was independently undertaking a similar analysis, and it's nice to have this for comparison. I hadn't thought to exclude pedestrians, pedecyclists, and other non-occupants, nor of excluding single-vehicle crashes.

I think a some important pieces are missing from this analysis, as follows.

1) The final number, 548, is the number of miles that I must drive to accrue one micromort for all passenger vehicle occupants. But I am more interested in how many miles I have to drive to accrue one micromort for myself. The average (mean) car has 1.5 occupants. Assume that the average (mean) crash of interest involves two cars, and deaths are distributed roughly equally between each person involved in a crash, total of 3 people on average. So you would need to drive or ride 3 times as many miles as claimed by Josh's analysis to accrue one micromort for yourself. A micromort would accrue to any other passengers in your vehicle as well.

(If excluding all accidents with one car, then the assumption of the average accident including only 2 cars is actually off, as some accidents will include more than 2 cars.)

https://css.umich.edu/factsheets/personal-transportation-factsheet (source for average passengers per vehicle)

https://www-fars.nhtsa.dot.gov/Main/index.aspx -- starting point for further honing this factor

2) If a passenger car gets in a crash with a larger vehicle such as a semi truck or a bus, likely the occupant of the passenger car will be injured much worse than the occupants of the larger vehicle. (I didn't look this up but it seems reasonable.) This would bias the results in the opposite direction from excluding motorcycles and pedestrians. More generally, we should be aware of cherry-picking which risk factors we remove from the analysis.

3) Deaths per 100M motor vehicle miles driven stayed approximately constant in a range of 1.0-1.2 per 100 million miles from 2009-2019 but went up dramatically in 2020 to 1.37 deaths per 100M miles (73 miles per micromort) and has stayed at a higher level or even increased in 2021 based on preliminary data.

https://www.usnews.com/news/health-news/articles/2021-06-04/traffic-deaths-increased-in-2020-despite-fewer-people-on-roads-during-pandemic
https://www.nhtsa.gov/press-releases/usdot-releases-new-data-showing-road-fatalities-spiked-first-half-2021

4) Taking out 50% of deaths due to alcohol impairment, so as to account only for the other driver being drunk, seems fine. But I'm less confident that the average lesswrong reader does not drive distracted or drowsy. This seems like an area where we should be careful about being too confident due to self enhancement bias. On the other hand, there are other ways to drive more safely than the average driver. One of the most important of these is maintaining a safe following distance.

5) Depending on the context, we might be more interested in the micromorts per mile of interstate highway type driving in particular. This risk is about half the mortality risk per mile as compared with all driving.
http://www.bast.de/EN/Publications/Media/Unfallkarten-international-englisch.pdf?__blob=publicationFile (assuming "motorways" has a similar meaning to interstate-type highways)

This figure is relevant, for instance, in answering a question such as "What is my risk of highway death from taking a 1000-mile road trip, as compared with my risk of death from covid over the course of the same vacation?"

Josh's analysis mutiplies the initial figure of 91 miles per micromort by a factor of roughly 6.

Jost includes factors of approximately
A) 1.51 for excluding motorcycles, bikes, and pedestrians
B) 1.78 for excluding single-car crashes
C) 1.86 for seatbelt wearing
D) 1.24 for drunk, distracted, and drowsy driving. This is dominated by drunk driving

I would reduce factor A, as I think it's balanced by cases where a passenger car hits a truck. More research on this could be helpful. For now, I'd say to get rid of half of it, make it 1.25
I would get rid of factor B, but replace it with a discretionary safe driving factor (see below)
I would keep factor C
I would keep most of factor D, maybe reduce it to 1.2.

Add (or rather multiply) in the following factors

E) Factor of 3 to account for only micromorts accruing to one traveler, not to co-passengers or occupants of other vehicles. This is based on an average of 3 people per crash, a figure that would benefit from further research.

F) Factor to account for being a safer than average driver (beyond the effect of wearing a seatbelt and not driving drunk). I think that it might be reasonable for especially safe drivers to use a factor of 2 or 3 here (which would be comparable to eliminating single-car crashes plus more). A factor of 10 seems like it would be too much. For myself as driver, I think I would be conservative and keep it at 1, absent further analysis. I am safer than average in some of my driving practices but less safe than average in other practices and skills.

One way to get a start at estimating this factor might be to look at your auto insurance rate and compare it to the average rate for comparable coverage of a comparable vehicle in your state. Insurance companies are in the business of rating risk after all.

G) Factor of 2 if considering only interstate highway driving (or other divided limited-access highways).

I would start with 73 miles per micromort, using the rate from 2020. Overall this gives me

73*1.25*1.86*1.2*3 = 611

611 miles per micromort for each vehicle occupant, or 1222 miles per micromort considering only interstate highway driving, to be adjusted further by a factor of up to 3 depending on one's beliefs about one's relative safety as a driver.