The scenario illustrated in the GP happens because the the rightmost lane is subject to merging traffic from onramps and offramps, and also tends to be the lane where trucks and vehicles obeying the speed limit (or both) congregate.

Meanwhile, the leftmost lane attracts people wanting to drive faster than traffic, the people who just want to get one car ahead, as well as the people who just want to drive without having to let cars in all the time.

If there's only two lanes -- like on most rural interstates -- and sufficient volume exists to prevent the left lane from actually clearing out cars, this creates a tragedy-of-commons situation in the left lane where there's more empty space in the right lane than in the left. A subset of people will then use the empty slots in the right lane to try to get ahead, making the situation even riskier.

But this poster's right; the root cause is not enough lanes to ensure a high level-of-service.

Totally agree, but the US is huge!

It would be much more informational to take into account things like average number of cars traveling between various locations, number of lanes between said locations, residential density...

Another issue is mobility in America. It's not uncommon for people to move across the country for work. So cities like Portland or Austin don't expect the sudden boom in population and increased traffic that entails.

> Most people in most places will obey the left passing rule only so long as there are enough lanes that the traffic fits in lanes - 1 so one can always be open.

By that logic, that would mean at 11pm at night on US-101 between SF and San Jose, I shouldn't need to pass anyone on the right, but I guarantee you there's always drivers lingering in the left-most lane.

If the road is usually congested, frequent traversers of that road will just get in the habit of staying left until they are near their destination. I do it on PA-22 all the time (albeit not in a condition where people need to pass me on the right), even when the traffic is light, because its what people expect behavior wise on that road.

highway 5 between sf and la is notorious for this. it's a 2-lane highway and the right lane is filled with trucks, so all cars go in the left lane. passing is some crazy game where you try to get around slow cars in the left lane while trying not to get stuck behind a truck. it's just awful. rush hour makes it even worse.