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Comment by kosmet | Hacker News Reader

I'd be impressed if I see a self-driving car making a left turn in a crowded junction without a specific left-turn signal at LA. The only way to turn left in such roads is to make an illegal turn after the signal just becomes red. I wonder how the self-driving car would do in such a situation.

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In California (and most places I've lived), this is actually legal. To complete the turn, you first enter the intersection while the light is green, then wait for traffic to clear. If it only clears after the light turns red, you're still in the right - because only when you fully entered the intersection (on the green light) is considered. See here: https://patch.com/california/sanbruno/ask-a-cop-should-i-pul...

Of course, this only allows one car (or maybe two, if the intersection is particularly large) per light cycle, which isn't much. People who enter the intersection on the yellow or red light (tailing the person who was in the intersection waiting to turn left) are turning illegally.

In my country there is very specific provision along the lines "it is illegal to enter intersection if an obstacle would force a stop in intersection and block traffic" which means that in a "turn left or go straight" lane you enter intersection and cannot turn left due to traffic and block traffic for cars going straight making that technically illegal. Human drivers, of course, do this all the time.

Are you sure that moving, oncoming traffic is seen as an obstacle under that definition?

The real trick against gridlock is to equate the switch to green not as "go", but as "go, once the previous wave has cleared the intersection". The "don't enter before there is room for your car on the far side" is only an optimization on top of that.

I am not a lawyer so I may not be 100% correct. As far as I remember my driving classes and other resources (cannot quickly find exact provision), anything that is not part of road infrastructure (there are intentional obstacles to guide traffic) and causes vehicle to change speed/direction is an obstacle, so moving traffic kind of is. I understand that this particular provision is specifically there to prevent gridlocks and is never enforced in this particular situation.

The way traffic works is somewhat dependent on region. I can go to another city an hour away and already feel a bit alien traffic wise there. This is relevant in discussion on autonomous vehicles: in any foreseeable future we need them to coexist with human drivers and abide by unwritten "everyone drives like that here" rules.

If you come to LA, you'll see there's no way to avoid it. The majority of lights are unprotected left turn lanes. The cops here even break the laws and no one judges. Sure, everyone could make 3 right turns instead, but I'd imagine if everyone did that the damage from wasted fuel, smog, and extra traffic would outweigh the few accidents that happen due to cheating the left turns.

In situations in which my country has that same provision, it does not treat moving traffic that crosses your path as an "obstacle", so if you're turning across traffic, you're allowed to enter the intersection and wait there. It might be worth double-checking whether your country has a similar subtlety.

You are allowed to enter the intersection on yellow in California.

21452. (a) A driver facing a steady circular yellow or yellow arrow signal is, by that signal, warned that the related green movement is ending or that a red indication will be shown immediately thereafter.

But CA law also says you can't enter an intersection without having clearance to vacate it. In other words, if the cross street itself is backed up such that its traffic backs up or into that intersection and prevents you clearing the intersection as you complete the turn, you violate the "anti gridlock" law.

That's not relevant to this situation. Waiting for oncoming to traffic to clear, then turning while the light is red is legal, and orthogonal to whether "there is sufficient space on the other side of the intersection". If there isn't sufficient space to turn, whether the light is green, red, or yellow, then you can't turn and are violating the anti gridlock law. If you entered the intersection before it turn red, and there is space to turn, you can turn on green, yellow, or after it turns red.

Most jurisdictions do not consider it blocking the box if you enter the intersection to turn and it turns red after. They consider it blocking the box if you enter with the intention of going straight while it is not clear to do so.

This is the case for NY state, see VTL 1175: http://www.safeny.ny.gov/rowa-vt.htm

CA Vehicle Code 22526(a): "Notwithstanding any official traffic control signal indication to proceed, a driver of a vehicle shall not enter an intersection or marked crosswalk unless there is sufficient space on the other side of the intersection or marked crosswalk to accommodate the vehicle driven without obstructing the through passage of vehicles from either side."

https://leginfo.legislature.ca.gov/faces/codes_displaySectio...

This does not seem to be relevant to the situation here (waiting for the oncoming traffic to clear): usually, there's plenty of space to turn into, but one has to yield to oncoming traffic before occupying that space.

Turning after the light turns red isn’t illegal, it is what you are supposed to do to clear the intersection. Otherwise you’d be sitting there blocking traffic.

Ideally, isn't the point to have only autonomous vehicles and these situations become obsolete?

Maybe after all cars become autonomous. Earlier there will be a hybrid period where both manned and unmanned cars drive together I think.

Even all cars become autonomous, the problem will still be hard I believe. Hard coded rules may solve the problem but relying on communication and cooperation will not be easy.

And computer-driven pedestrians, too - the car had to stop multiple times for turning left/right across a pedestrian crossing which was green at the time. (If you have watched the video, you have seen that most potentially dangerous situations did not originate from cars.)

In vancouver the general rule in dense traffic seems to be that two cars can left-turn through a stale orange light, right as the light is changing and before the opposing traffic has a chance to start rolling into the intersection from their newly green light.

Sometimes only one car, if the timing works out that way. Three is almost certainly too many. The ability of a car to judge the light timing requires near 180 degree vision and head pivoting on the part of a driver.

Equally impressive would be four self driving cars turning left (in right-hand traffic) at equal priority intersection. At least in my country the rules are muddy in this case and no car has explicit priority. The most general "priority in right" loops around and cars should be stuck there indefinitely :)

I like to imagine to avoid such situations there would some kind of control on the dash that allows a stalemate to be overridden (not the steering wheel :P). It would be great if it were a "give way" button rather than a "I'm going now" button.

Doesn't matter how the controls look - you can't have the cake of Level 5 autonomy and eat it too by having a human giving control inputs.

(GP: That's actually easy: random exponential binary backoff. It works for other domains quite well.)

In this particular instance yes, that is easy. The core of the problem is that hard rules force traffic into this deadlock. To avoid this deadlock autonomous vehicles must be taught to sometimes bend the rules. Which brings its own can of worms - how do you teach a robot under what specific circumstances rules can be bent.

Not a good example - "Yield right _in the order of arrival_" is still a hard rule, but practically deadlock-free.

The rule in California is “first to arrive”, with “yield right” as fallback for simultaneous arrival. Deadlock is unlikely (and a tertiary fallback by compass direction would solve it completely.)

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