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Comment by WalterBright | Hacker News Reader

I knew about the brake wave back in 1980, as I could see it on I405 on my way home from work. Due to a curve in the highway, I could see about 3 miles ahead. The wave heading towards me was pretty obvious.

I found I could "break" the wave by letting a gap grow in front of me that would absorb the wave, and I wouldn't have to brake to a stop (and so my clutch would last longer).

It never occurred to me to write a paper about it :-)

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>letting a gap grow in front of me

How do you prevent someone from just filling the gap? If it's large enough to absorb the wave, someone will fill it just out of spite.

I sometimes do what the gp said - let a gap grow in front of me if I'm in stop-and-drive traffic and get annoyed when someone selfishly rushes to fill the gap in front of me only to have to break 60 seconds later.

On further reflection, that's not necessarily selfishness, more that they don't understand what I am trying to do, but there's no way to communicate complex information car to car on a highway.

I started a job in Fresno in 2013, driving up from LA on Sunday and back down on Friday.

Bluetooth had started to become standard in all cars, and as I was working on a train project, the idea occurred that if we could link all the freeway cars in series, and treat it as one long train, then I would essentially just have to steer, and we all would avoid brake-waves.

One car only needs to be able to talk to the one car in front of and the one car behind itself.

That's what I thought too until I started practicing this. Probably 75% of the time no one comes into the lane. It takes some practice to find the correct gap size.

Besides saving your clutch, you can do this type of brake-early maneuver at stoplights. Decelerating early and coasting at a lower speed means you don't have to stop and can increase your gas mileage significantly.

I do it for stop signs, too, so I only have to stop once. Especially if it is uphill.

Sometimes they would. But most of the time, they didn't.

A good example of why allowing for stress tolerance can make a system more efficient in the long run.

"by letting a gap grow in front of me that would absorb the wave" would cause the freeway to carry fewer cars if everyone does it. My oldest one used to argue with me on this point when I asked him to keep a bigger gap from the car in the front for safety reasons.

Not if they all go faster. The human induced delay is the problem. Plus it causes accidents, which really slows down the system.

Ever since driving an electric car, I have come to believe that a gas-throttle car’s laggy response during stop and go traffic may be responsible for the elasticity of the delay propagation. From zero, a piston pumper goes through a complicated series of windups which seems like days compared to an electric motor.

Throttles have only become drive by wire over the last 15 years, prior to that it was cable actuated and was instantaneous. Like a lawnmower. One could argue carburetor cars were even (a fraction of a second) more responsive.

Also if you WOT a modern car you get the instant response without the ecu calculating pedal angle, comparing to accelerator history, humidity factors etc etc but kiss fuel efficiency bye bye. There are mods and tunes that reprogram the fuel map to cut out this processing time

I also drive a big turbo family sedan and am used to saying 2 Mississippi before the engine pulls at full boar

Though drive-by-wire throttles absolutely do modify and potentially hold back your throttle input for reasons of both economy, comfort and enforced driveline sympathy, there is also an inherent response difference between petrol and electric engines and also slop in the driveline system. That difference varies on the engine and transmission type.

Unfortunately I cannot find an easy good reference on what exactly that difference is quantifiably. Also many of the Electric cars right now are higher end even sporty vehicles, and more likely to have a throttle mapping that is more direct.

But it could arguably increase the throughput (easier to on and off).

Which is better: more vehicles on the freeway at lower average speeds - or fewer, faster ones?

According to analysis from engineers, or Esther from a guy I used to work with who studied break waves for the state transportation board, less cars with larger gaps is better. That's why you see semaphores on highway entering ramps sometimes, ie 101 around SV or certain European big city inner highways.

This is a problem that can probably be solved someday with self driving cars collaborating to make highways huge coordinated conveyor belts.

Actually, this is a problem that is already solved by smart motorways in the UK (Europe too maybe?). They have a variable speed limit that dynamically changes according to traffic that stop brake waves happening.

Do you not have them in the US?

Not only do we not have them, but I don't think I've ever read a freeway speed limit sign at all.

> I don't think I've ever read a freeway speed limit sign at all

Lol, why adopt the UK's variable speed limit when we have already de-facto reinvented the Autobahn....

If my experience with smart motorways is anything to go by they seem to increase brake waves because there are often times that speeds get reduced significantly for arbitrary reasons that not everyone chooses to follow. You then get larger closing speeds which causes people to slam on the brakes.

Additionally the removal of hard shoulders is just dangerous.

Seattle adjusts the speed limit on one of their highways down to 45 or 35 during rush hour which increases the throughput as the cars can be closer together. Not sure if that has been picked up by other cities.

You should read about the "Fundamental diagram of traffic flow" and it's corresponding diagram. It's very well explained and shows that throughput can increase when having fewer cars on the road

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