So, it’s more like you observe somewhere in the range of 5-10 cars for the duration of the commute depending on how densely you’re surrounded on average? That’s really interesting, and if you see 1 mistake per commute, then the average driver is actually quite a bit worse than the gp assumed!
The unit I'd use is car-minutes. In a 30-minute commute you might observe 200 car-minutes (this is a total shot in the dark to be clear) closely enough to notice a mistake if it happened. So if you notice 1 mistake per commute on average, and you yourself make a mistake every 200 minutes of driving, you could estimate that you're an average driver.
