The problem with "pass on the left" is that it requires to drivers to consciously treat the left lane as a passing-only lane. That doesn't happen in California. The left lane in California is the "let me zone out and ignore all these cars passing me" lane.

I disagree with strictfp that "staying to the right" creates unnecessary traffic dynamics. What happens in California is worse. Someone holds up the left lane, so all the faster cars will pile up on the right lane and try to sneak in front of the slow car, but inevitably they'll cut someone off and set off a chain reaction of braking for everyone that was patiently queued up in the left lane.

(Granted on two-lane I-5 there are some legitimate a reason for this: the right lane is used heavily by trucks, so the road wears down quicker and becomes rougher in the right lane than the left. Naturally people gravitate towards smoother asphalt. And if you just passed a truck a few minutes back and are gonna pass another one again, why not just stay in this lane?)

This only happens on poorly designed roads that cannot handle their traffic flow. 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.

Since the US infrastructure system is a colossal piece of shit, highways built 40 years ago still stand exactly as they were then today having to accommodate multiple times more traffic than they were ever meant to. This breaks down the road rules when its bumper to bumper and every lane is backed up.

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.

It depends on the state. In many states passing on the right is illegal and considered rude, but California is a notable exception. Also, while there is a law here in CA that slower traffic should stay out of the left lane, I think it is more of a recommendation -- I can't recall ever seeing it enforced. I think most drivers make some effort to follow it, but not to the extent that people do on the east coast.

In most states (all?) "passing on the right" does not mean passing someone on the right in a regular traffic lane.

It refers to passing people while in an exit-only lane, turn-only lane, shoulder, etc.

In Massachusetts, passing on the right on a divided highway is legal. I'd argue that in a lot of cases, it's the fault of drivers on the left not moving out of their lane. But you also have situations where, with lanes of steady traffic, the right lane just gets moving faster.

Now, yes, you get situations where people veer from lane to lane because the overall flow of traffic isn't fast enough for their tastes. But, especially on three lane each direction roads, you get cars and trucks that don't want to deal with merging traffic at exits just sitting in the middle.

I believe that there it's ruled so that you have to use the rightmost lane that's free - so if it's possible to pass someone on the right, then it means that they're not in a position to honk and flip off because they're driving wrong themselves.

> Isn't that a thing in the U.S. as well?

Yes, but not normally observed in more densely populated areas. In Los Angeles, for example, it's use any lane you can to pass. I've personally witnessed people, who were driving slower than the flow of traffic, move all the way to the left to "get out of the way" of other drivers. It's well-intentioned, though poor understanding.

Licensing needs to be more rigorous, but many cities rely very heavily on commuting without a proper public transportation infrastructure in place.

The left lane is also, almost universally, a "I expect to speed" lane. When I need to use the left lane to pass and I'm driving the speed limit, I'm almost always tailgated.

Well at least you know why they're tailgating you.