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Comment by sytelus | Hacker News Reader

This is great video to study on many levels beyond self-driving car. Some observations:

* The car behind didn't show turn signal so probably driver wasn't paying attention.

* A small car escaping with not too much damage while SUV rolls over many times

* From physics point of view, the car behind produced almost perfect torque that started horizontal spin on SUV

* Collisions with barrier at high speed can produce lots of rolls. Many time we see cars upside down in accident and wonder how did that happened. This is how.

* The middle barrier did its job wonderfully even against massive inertia of SUV. Cars on other side remained unscathed!

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The biggest thing I noticed was just how easily SUV's flip over. This has been a problem forever I'm just surprised that regulators don't take more notice of it. Rollovers are far more likely to result in injury or death.

Most cars just don't flip, all the traction their tires have is simply not enough to do it on flat ground. Yeah anything could flip over once you go off the road but the SUV in this looks like a rolling death machine.

Because people think these SUVs are "safer" due to their size. And then it becomes an arms race; you see the highway full of large SUVs, and you feel vulnerable in a tiny compact car, because you think they will crush you in a collision, so others start buying SUVs to "protect" themselves. These land-beasts might be comfortable but they are unsafe, take an excess and unfair amount of space on the road, consume too much fuel, and trigger arms race. They need to be regulated to death for most users.

Yes. IMHO insurance costs should have to figure in the externalities to society of their increased risk to other vehicles on the road, even when the SUV driver isn't at fault. Crashes will happen, but even when the root cause is another vehicle, an SUV in the mix increases the likelihood of a crash due to worse braking distance and increases the severity of the damage to other vehicles due to its mass. SUVs make the road measurably worse for all other users of the road due to their negative externalities.

This is one of the reasons I ride bicycles and motorcycles. I take on more personal risk, but the overall effect is a reduction in accident, injury, and mortality rates for road use.

Anecdotally, my former gf managed to flip my Taurus by striking a parked car on the road at 15mph. Police verified that she wasn't moving faster because she practically flipped it in place, rather than ending up a significant distance away. Just a case of hitting at just the right angle and riding it up, I guess.

I was wondering how did SUV rolled because for that to happen you need force getting applied on car that is on Y axis (the +X axis being the forward direction) which will ultimately produce the torque causing the roll. After looking at video at low speed, here's how we got that force vector:

1. Hatchback hits on tangent producing torque and spinning SUV by 90-degree.

2. SUV collides with barrier which produces force vector that is normal to collision surfaces and slightly off forward direction.

3. The driver now seems to have pressed on breaks really hard which produces another force vector. But this vector is now quite off original forward direction because car had been rotating.

So the moral of the story is that if SUV is rotating and you press on breaks really hard, you will get roll. This is purely because SUV has higher clearance and so even small force can produce enough torque that will cause roll. It looks to me if driver hadn't pressed on breaks really hard, the rolls could possibly been avoided or reduced.

I've seen a crash that was very similar. A tiny car hit the rear bumper of a SUV, on the right half, not really even a hard hit. The SUV got turned to the left and hit the slightly raised center of the road (maybe 10 cm high?) and spun and slid on its roof. Everybody was okay in the end.

I was in a very bad accident when a GMC pickup t-boned my 98 Subaru Impreza 2.5RS at ~55mph. My car crumpled and was totaled but took the hit and went for a 4 wheel slide across the road. A nice balanced car takes hits really well. And I'm very glad I wore my seatbelt.

That also depends on the momentum (weight ratio, impact speed angle, etc.) The lesser the weight, the worst off during impact - we've seen that too often in NHTSA crash tests.

Flipping depends entirely on geometry. SUVs have high suspension, have a lot of weight at their top, and have an overall higher height to width ratio. Also, their aerodynamics do not push as much down as on lower cars.

I'm feeling rather dumb, but can't really intuitively understand the physics here. I see a small car hitting a pretty huge one and yet spins the huge one several times. I always thought those bigger cars are safer / stronger / more resistant to hits?

Is it purely because it hit it at the corner, so all the force went into the "spinning axis"? (did I say I feel dumb?)

> I always thought those bigger cars are safer / stronger / more resistant to hits?

Physics would have you start with a spherical, uniformly dense car, but cars aren't that. Crumple zones engineered for passenger safety requirement mean that intuition falls short unless you really dig into it and add stuff like center of gravity to the model.

Also think about why you believe that - looks can be deceiving, and marketing, especially for rare expensive purchases like cars, very much plays on emotional appeal rather than actual science. ("Ultimate driving machine"? Pft, really?). The original SUVs were designed off truck platforms and did quite poorly in safety tests, and their higher center of gravity doesn't help at all. (Though it looks like a minivan in the video.) Just like two objects falling will fall at different speed in the real world due to air resistance, bigger != better with cars. You can't see crumple zones just by looking at how pretty the car is; vehicle safety is one area where government regulation has made cars safer by requiring all new cars sold in the US to meet a very high safety bar, and that's improved over the years, evident in this video of an intentional crash between a 1962 Cadillac vs a 2002 model. https://youtu.be/O-WYKYrq5FI

SUV center of mass is probably higher and easier to tilt.

Both cars braking, center of mass shifting toward their front and lowering. Meaning the small car momentum was peak at the collision point, while the SUV rear end was already lifted thus lighter.

Direction of the collision helped transfer the energy into torque. The barrier provided the final help to start the car tumbles.

The SUV had forward momentum, yet it was suddenly facing sideways due to the side-back impact from the smaller vehicle. That means the tires were skidding and "gripping" the car at the base rather than "rolling", while the whole body wanted to go forward. Hence, it toppled and kept rolling because there was nothing to stop it.

The way i see it the small car just got the SUV to ram the middle barrier. The spinning momentum resulted from that crash and facing normal to the dominant moving direction.

Take two cereal boxes. Put one flat and the other on its end. Smack the flat one into the tall one. Which one begins to roll? This is severely exaggerated but you get the idea of what's happening between the car and SUV.

Look on two things:

A) main that SUV happened (just before starting sommersaults) to position itself orthogonal to its previous way of moving. In this position all wheels of SUV created massive 'resistance' to continue move forward

B) it happened partialy because of the middle barrier. WOuld it be missing, SUV would probably move (on the arc) without sommersaults.

Change of orientation was caused by small car, but look that up to moment A) SUV more less was moving fine.

Torque is

r X F

meaning that small force applied over a long length can produce huge torque! You can move a huge rock if you use a long enough metal stick for leverage. This is why Archimedes said,

Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.

Large angular acceleration due to large torque is then directly result of Euler's equations.

The SUV would have spun if it didn't hit the crash barrier.

The SUV was pushed into the crash barrier and small car wasn't, so it's a highly unfair comparison.

We did a lot of safety research before buying our current round of cars, and you need to not generalize about cars based on classifications like "SUV". Look at actual data from actual cars on actual roads, e.g.

http://www.iihs.org/iihs/topics/driver-death-rates

Note that you can find vehicles like the Honda Pilot that are enormously popular and have in effect ZERO fatalities. What's horribly unsafe to everyone is passenger cars built on truck frames.

> What's horribly unsafe to everyone is passenger cars built on truck frames.

Have any examples? The vast majority of new SUVs (which the one in this incident appears to be) are unibody rather than body-on-frame ("truck frame" style).

> Have any examples? The vast majority of new SUVs [...] are unibody rather than body-on-frame

Apparently there are 18 models of Body-On-Frame SUVs that are being sold in the US in 2016:

  Cadillac Escalade
  Cadillac Escalade ESV
  Chevrolet Suburban
  Chevrolet Tahoe
  Ford Expedition
  GMC Yukon
  GMC Yukon XL
  Infiniti QX80
  Jeep Wrangler
  Land Rover LR4 *
  Lexus GX460
  Lexus LX570
  Lincoln Navigator
  Mercedes-Benz G-Class
  Nissan Armada
  Toyota 4Runner
  Toyota Land Cruiser
  Toyota Sequoia

Source: http://www.goodcarbadcar.net/2016/02/body-on-frame-suv-produ...

Any idea which of those are "passenger cars built on truck frames?" I am not sure if OP is talking about all body-on-frame SUVs or just a subset.

Those are mostly the biggest ones. Maybe 25% of market at best?

Other than the Jeep Wrangler, few car-size vehicles are still built on truck frames. About 10% of SUVs still are, mostly the larger ones.

I'm not sure what you mean by "the car behind didn't show turn signal".

If you're referring to the pink hatchback that ultimately rear-ended the SUV - the pink hatchback seems to be making a lane change to the right after passing. The driver seems to have signaled and looked to the right to clear his/her blindspot at the same moment the SUV ahead slams on the brakes. It really seems like bad luck, that traffic would suddenly stop at the exact moment the driver of the pink hatchback looks to the right to change lanes.

> It really seems like bad luck, that traffic would suddenly stop at the exact moment the driver of the pink hatchback looks to the right to change lanes.

-Pet peeve of mine: "Bad luck" is incredibly rare in traffic. This accident occurred because the hatchback was too close to the car in front of it; not because of bad luck.

At 0:00 the pink hatchback is well behind the car it's trying to pass. At 0:02 the brake lights of the SUV illuminate; the pink hatchback is still not quite caught up to the car it's trying to pass. Instead of braking, the hatchback keeps accelerating trying to beat the closing gap. No signaling at any time. Looks much more like bad judgment than bad luck.

No signaling at any time.

You're just missing the signal. Look at the upper-right of the hatchback at 4 seconds.

Right you are; I was looking in the wrong part of the car.

Well no -- that wasn't bad luck at all. He simply was way too close for the speed.

Back in the day when I got my driver's license we were told to have 3 seconds of distance. One second to react one second to brake and one second for safety, if I remember correctly.

Around here they recommend 2s. As part of a science fair project, I measured the following distance of cars on a nearby highway. It's been about 15 years but IIRC, the mode was ~0.5s.

> The car behind didn't show turn signal

Watch the video again; the little red car turns on their blinker and does a very slow shift to the right before clipping that stopped SUV. You're right that he wasn't paying attention, but his reaction time is incredibly slow - it looks like that car had just enough time to slam on their brakes or swerve to the right, yet they did neither.

I disagree that the pink car driver was not paying attention - he or she was likely turning to look over their right shoulder before making the lane change. The drift to the right isn't an avoidance maneuver executed lazily, but just a normal lane change. Bad luck that the SUV ahead slammed on the brakes at the exact moment the pink car driver checked his or her blind spot.

not sure what the speeds these cars were traveling at but assuming this is regular highway speed is slow reaction time really a good excuse?

aren't you supposed to have a wide enough space between you and the car in front of you? to compensate for any slowness in reaction.

If it's any indication, you can read 113 km/h on the dashcam overlay. Besides, the Opel Corsa, at least for the few seconds before the crash, seems to roughly maintain the same distance from the Tesla, so it should be travelling at a very similar speed...

It's odd that the turn signal is only located at the top of the car. I guess the mid height one was broken ?

I think the situation is not entirely unclear, it seemed that there was a problem ahead of the SUV, that made a bunch of cars slow down altogether. I still don't understand the smaller red car driver decisions.. maybe he didn't know what to do and hesitated and tried to swerve as a last resort. Or maybe he was just not watching. The SUV break lights were on quite a long time..

It looks like a 2001 Vauxhall Corsa, that is just where the indicators are.

Shit that's bad, it's really not very large and "originally" placed.

> Many time we see cars upside down in accident and wonder how did that happened. This is how

Living in India, I have seen a fair share of accidents where the car is upside down. I always used to wonder how that happens, now I know.

It's also "a thing" in New Orleans. Check this out: https://www.reddit.com/r/NewOrleans/search?q=flip&sort=new&r...

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