What happens if everyone goes when the light changes to green?

If everyone accelerated at the same rate when a stop light turns green, what would the result be? I know it won’t happen in the real world but in fantasy land, it seems that traffic jams would be a lot less frequent.

It’d speed up traffic, but I suspect most jams are caused by things like accidents and gridlock.

Good ol CGP Grey made a video about this:

CGP Grey - The Simple Solution to Traffic (YouTube)

A huge rise in the rate of traffic accidents. Cars that are stationary in a queue can be bumper-to-bumper, but if they are moving they need space between them — the space required rises in proportion to the square of their speed. Hence when a car in the queue moves off, the car behind it needs to wait until an appropriate gap has opened up before it moves off, and as both cars pick up speed it needs to allow that gap to open up — i.e. it needs to accelerate more slowly than the car ahead of it. And so on, for every car in the queue.

If everybody in the queue started at the same moment and accelerated at the same rate, there would be an immediate and rapidly rising risk of collisions, as nobody would be driving at a safe distance. And, other undesirable consequences aside, lots more collisions would of course mean lots more obstruction of traffic.

It would be like a train starting up with the cars appearing to be physically coupled. But if one car doesn’t keep up with the one behind it you get an 80s comedy car chase pile up.

Ever see the start of a Formula 1 race?

But, in our world, if people did this, what would happen if the light turned green, and someone ran the light on the cross-street? Not only would the first car get into an accident (which might have happened anyway), but all the cars all the way back would smash into the car ahead of them…

Have you been to Vietnam? Because that’s exactly how it works there. Everybody goes. Nobody stops, they just don’t hit each other and all go around. Like Ho Chi Minh City has very, very few traffic lights, for a sprawling capital city because, why bother?

Most amazing, you can cross any very busy road anywhere, as long as you know how. The trick is, move at a slow and steady pace, do not run, race or hurry. The racing vehicles need to predict where you’re going to be. And I kid you not, they all just go around you. Confidence is everything. Lose your nerve and race across and you’ll surely get hit.

Very unnerving at first, but you get used to it. Then again, at rush hours, in jams, motorcyclists, of which there are a zillion, just take over the sidewalks. They don’t go any slower or cede to pedestrians. Better to sit on tiny chairs and have coffee till it passes.

When you first arrive it seems so chaotic, wicked fast and dangerous in the extreme. But after a couple of months you just get used to it. Don’t bat an eye, just step off the curb into the mayhem! It’ll be fine, no worries!

More disturbing yet, I saw a video of how automated cars would move about, how it would improve flow, blah, blah. Ends up it’s exactly the same structure. Everybody goes, everybody goes round everybody else.

It very well could happen in the real world if every car at the intersection was self driving and their AIs were networked

It is the same in Indonesia in the big cities. Just walk with confidence. Totally scary until you get used to it, then only mostly scary.

A friend hired a motorbike in Vietnam. He took a short test drive and was reprimanded for looking back over his shoulder. The hire place guy told him to always look forward, the people behind will sort themselves out.


IMO - that’s gonna be one of the main inner-city advantages of FSD-cars: they throughput of intersections and traffic-lights will be much higher


Vietnam has 3.4x the number of traffic-related fatalities per motor vehicle than the USA, and even the USA has 3-5x the number of fatalities of more civilized countries like Norway, Switzerland, Japan, the UK, and Germany. Not good.

Those simulations ignore pedestrians and cyclists and focus only on motor vehicle traffic. Roads aren’t just for cars.

This was my thought with self-driving cars. They would simply time things to avoid each other without stopping, intersections would be like those stunt car shows or a Mounties musical ride, where crossing streams would time things right so each car threaded with perfect timing between two cars crossing in front of them. They would even warn each other “watch out for the red Corvette! That’s a human driving!” They would all manage to avoid the human who simply drove like there was nobody else on the road, ignoring traffic rules, and the other cars get out of his way… until he collides head-on with the only other human driver on the road.

I think the point is that with FSD cars, following distances can be shorter because a following FSD car can respond to the leading car’s decel and hit the brakes (to avoid rear-ending the leading car) in a few milliseconds instead of the half-second that’s typical for a human driver. To the extent that the car in front is obstructing the view of an upcoming obstacle that needs to be steered around, interconnections can communicate the need for evasive steering from the lead car to any following car with an obstructed view.

That was my first thought, too. But train cars don’t veer off along the way or do much starting or stopping. It’s more like automatic parcel handling or highly automated baggage sorting but, instead of belts and diverters, with self propelled vehicles.

And, to get technical (this is this SDMB), even train cars don’t all start up simultaneously. There’s slack in the couplings that needs to be taken up, carefully, before the full train is moving and can accelerate up to speed.

It’s good enough for the hypothetical which fails as soon as a car goes the wrong speed or doesn’t follow directly behind the one in front of it.

In a small train like a commuter train the slack is taken up so fast it may not be noticeable at all. But yes, it has to be mentioned.

The hypothetical roadway traffic in OP’s ‘fantasy land,’ taken to its logical end, would have vast sensor suites coordinated by, if not centralized cloud control, then local comms between nearby vehicles.

It’s coming:

I wonder if this will happen before or after we all have flying cars.

Basically what militaries do. When they say “march” everyone starts walking at the same time. If they didn’t their formations would be stretched out and vulnerable and sloooow. Not good.

But, of course, I cannot assume the person in front of me in traffic will be paying attention and move when the light turns green (I’m sure we have all been behind that person who is distracted and doesn’t go when they should…and to be honest we’ve probably all been that person a few times). I have to wait for them.