Before turning is the law. e.g. Maryland Code "(d) Where signals to be given.- When required, a signal of intention to turn right or left shall be given continuously during at least the last 100 feet traveled by the vehicle before turning; except that a bicyclist may interrupt the turning signal to maintain control of the bicycle."

There are actually many states where using the turn signals to change lanes isn't even legally required though, so things are more mixed when it comes to changing lanes vs. making turns.

> There are actually many states where using the turn signals to change lanes isn't even legally required though, so things are more mixed when it comes to changing lanes vs. making turns.

Arizona is one state where it is required judging by a warning I got from a friendly highway patrol officer who chased me ~5 miles while I was having fun on the old ninja during morning rush hour.

Yes of course, but it is a _signal_, not a request. You should _not_ put your signals if there is no room, just to "ask" other drivers to clear the other lane.

Suppose you're in the center lane during some stop-and-go traffic. You want to take the next exit. Do you really just drive along in quiet desperation, hoping for a gap?

I—and everyone I know—would leave their blinker as they creep forward, "requesting" that someone leave a gap big enough to take the exit.

Of course, the vast majority of drivers would do so (including me). That would just be incredibly inefficient to do otherwise. Nevertheless, in my home town, it is against the law (Ontario btw, so I confess I cannot speak for the US).

Same thing goes for the left turn on yellow light. If there is a gapless traffic coming from the other direction, you will probably just creep in the intersection and turn at the end of the yellow, when the other cars have stopped. Nevertheless, by doing so, you are doing two illegal things: 1) going further than the stop line and 2) passing on a yellow (or even a red) when you could have stopped (easily, you were already stopped). Is it a bad thing that people (again, including me) do this? Probably not, doing otherwise would be a huge loss of time for everyone and it does not really increase the accident rate. Yet, it is against the law.

Here's the text of Ontario's Highway Traffic Act: https://www.ontario.ca/laws/statute/90h08#BK232

“The driver or operator of a vehicle upon a highway, before turning to the left or right at any intersection or into a private road or driveway or from one lane for traffic to another lane for traffic or to leave the roadway, shall first see that the movement can be made in safety, and, if the operation of any other vehicle may be affected by the movement, SHALL GIVE A SIGNAL PLAINLY VISIBLE TO THE DRIVER OR OPERATOR OF THE OTHER VEHICLE OF THE INTENTION TO MAKE THE MOVEMENT."

I guess one reading of this suggests that you must first check if it would be safe to make a maneuver, and if so, then and only then, must you use a turn signal. I think you could also read this as requiring safety checks and indicating (in either order) before turning, but I'm not a lawyer. I can't imagine that people are actually prosecuted for signaling out-of-order though.

The driving handbook, however, tells you to check your mirrors, then "Signal that you want to move left or right." (here: https://www.ontario.ca/document/official-mto-drivers-handboo...) which seems to suggest some level of intention-indicating is okay.

>If there is a gapless traffic coming from the other direction, you will probably just creep in the intersection and turn at the end of the yellow, when the other cars have stopped.

While this is illegal in some states, it is not in others. Furthermore, the general rule on reds (again, this might vary by state) is not that you should not be in the intersection, but that you cannot enter the intersection. If you are in the intersection, when it turns red, you clear the intersection.

Isn't that exactly the discussion?

It is against the highway code to do that, so can't be programmed in, as this would be ultimate premeditation of breaking the law.

On the other hand, in the real world, this is how people sometimes use turn signals, so autonomous cars are at a practical disadvantage and can't merge when a human driver could.

For large sections of many highways at rush hour, there is essentially never a space to comfortably merge into. You have to essentially make spaces to merge into whether because someone sees you signal and lets you in or someone isn't really paying attention and has let a gap open up.

Really? My understanding was that the signal is a request to the vehicles behind you in that lane to yield and create space to let you in. That's how I always see it working here in Vancouver and it's a good system. People are very good about creating space in front of them when someone needs to be let in.

Edit: This depends a lot on context. The relative speeds of the two lanes, whether there's a merge, and so forth. If there already had to be enough space to safely change lanes without any other cars slowing down or reacting at all, then turn signals would become unnecessary. Often there's an opening, but to really make it safe the car behind you in your target lane does need to yield to make it safe, and the turn signal is a request that they do so (or at least not speed up).

It is a signal that I am going to get over into that lane one way or the other. You can choose to keep me from doing so in which case I'll get in front of one of the following vehicles and merge there. I don't know where you drive but that is absolutely normal and expected behavior where I do.

It seems like just one of the challenges for autonomous operations. Driving that is pretty much necessary in some locations (and to do otherwise would cause other drivers to route around you in unsafe ways) would often be seen as hopelessly aggressive in others.