[k1t]

Sometimes liberties are taken here, such as California originally separating carpool lanes with yellow lines (when they should have been white).

Presumably they felt that even though yellow was incorrect, it sent a clearer "do not cross" message to drivers.

They were later ordered to change them to white.

[toast0]

Were there mandatory prescriptive federal guidelines on HOV lane line colors when California introduced carpool lanes, or did they only come after California had been using yellow lines?

Personally, I do find yellow lines are a stronger border, but then I grew up in a culture of yellow carpool lines. I feel like white lines are almost always ok to drive over, and yellow lines usually aren't (although yellow dotted lines are a sometimes drive over (if safe, to pass on a two lane undivided highway), and yellow solid + dotted is also ok to drive over (if safe, to get into a shared median lane for an unprotected left turn)

[ubercore]

I think it's because yellow is supposed to indicate traffic moving in the opposite direction. That's why it's a stronger signal (in theory), but not "correct" for an HOV lane moving in the same direction.

[jrockway]

California should have closed the lane once a year for one minute to run a single vehicle down the carpool lane in the wrong direction. Then they could justify that the lane marking was the right color (since in theory, the lane can be reverse-flow at any time, and hey sometimes they even do it).

[nybble41]

> I feel like white lines are almost always ok to drive over…

Single and double solid lines parallel to the flow of traffic, of any color, almost always mean "do not cross".

A single dashed line, or a double line where one or both halves is dashed, can be crossed under at least some circumstances. The rules for crossing these vary depending on context, e.g. one-sided passing and center turn lanes use similar markings but with the solid and dashed lines flipped. (You pass from the dashed side but enter the turn lane from the solid side.)

White lines separate traffic going the same direction. Yellow lines separate traffic in opposite directions. This is independent of whether the lines can be crossed.

[lsaferite]

My funny anecdote around that: The road in front of my house has a dual center yellow line, but the reflective additive they used in the paint shows up white. At night all of the lines are white. VERY confusing.

[ichydkrsrnae]

That sounds like something you should report to an authority. It could even be a whistleblower issue if the contractor shorted the city with substandard glowy stuff and it's causing serious safety issues.

[jimmygrapes]

I've driven through places where the lanes change directions or get closed based on traffic flow, most often near sports/entertainment arenas, and they tend to have one or two lanes with dashed white lines which are always open and the rest of the 3+ lanes are dashed yellow. I've always wondered if this is an exception to a rule, or if there even is a definite DOT rule (or if localities can override DOT regulations, such as in the case of rainbow crosswalks). I do recall in my driver's education classes 20+ years ago that a dashed yellow is "allowed to enter and exit but not for travel" but again that may have been a local rule.

[matt-attack]

I noticed that change recently and assumed it was motivated by Tesla. There are certain things we all take for granted when driving. A huge one that we probably don’t give much conscious thought to us this:

How do we know if the lane to the left is ok to go in, or if it actually is for carrying oncoming traffic?

You’d like to think that one way though not the only way, is to look for a double yellow line. As in, double yellows always separate opposing traffic so never cross them.

In CA though, double yellow was also chosen to separate the carpool lane as OP mentioned. So there was actually a really mundane case where driving to the left of a double yellow was actually fine. Seems very confusing to an autonomous car that’s operating at the mental-level of an 18mo old.

[tobyjsullivan]

You are giving way too much credit to autonomous vehicles in this theory. Line colours are standard throughout all of North America (that I’ve seen so far). Yellow means what it means. This carpool line experiment would have been confusing to any driver seeing it for the first time.

Imagine a tourist on that highway for the first time, at 3am, when there’s no traffic. Can they use that lane? Can they pull a sneaky u-turn and start driving the opposite direction?

What if I’m driving down the highway, effectively on mental autopilot, and suddenly become aware that I’m on the “wrong” side of the yellow line? How would I be likely to react?

The opportunities for misunderstanding by normal humans seem plenty.

[mikestew]

I noticed that change recently and assumed it was motivated by Tesla.

My '11 Nissan Leaf couldn't even have LED taillights because the Feds said "no". Tesla isn't going to march into some DOT office and demand that the color of the lines be changed to accommodate them (I mean, they could, but the laughter would drown out the conversation).

CA changed it because not only was yellow against federal guidelines, but also because I personally believe it was a stupid choice to begin with (for reasons others have already listed).

[throwawayboise]

Yellow lines separating oncoming traffic is not universal though? I believe in the UK that the center lines are white?

[tannhaeuser]

Living in Germany and having only driven in Mexico (City) as far as the Americas go, I admit to not understand what yellow lines signify to US drivers, like at all ;) At my place, they're used for temporarily "overriding" regular white lines eg at construction sites, and only for that purpose.