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Comment by mc32 | Hacker News Reader

There are three trends: bigger mass, larger sizes and taller vehicles.

Which one of the above is the most consequential?

I'm guessing height. at 2500 or 5000 lbs, neither is going to be pleasant when you bang in to it. size maybe there is more surface area to get hit by, but that's unlikely to be significant.

mass does influence stopping distance though, so that could be, but at the same time we now have large disc brakes with anti-skid.

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Also, shape. Go look at the front end of a car like a ford F150. The hood is flat and sits 4 feet high. Now go look at a more compact car like a prius. The hood is low, and is angled like a wedge smooth into the windshield. When a prius hits you, its liable that you will just roll up and off of it and not get much of the force from the impact as a result. When an F150 hits you with its 4 foot tall wall of a grill straight in your chest, I don't think you will be rolling up and over. Maybe your head would just pop off from the kinetic force, sparing you from a slower death?

I once managed to barely jump and slide over the hood of a car that came up behind me and turned - at considerable speed - right into the pedestrian crossing I had been traversing on a green light. The driver looked right through me, didn’t even realize it until my - corner of the eye induced - leap. I would be dead for 15 years now if that would have been a SUV. Lots of people really don’t seem to have a good grasp of the sheer physics involved while driving these tanks especially.

> driving these tanks

They are not tanks. They are suburban armoured personnel carriers.

Not even. You see them in cities themselves. Like why? Finding large parking lots is hard and it’s by definition urban and you can’t possibly have mud or ditches to traverse in your 4x4!

Even worse is that they're becoming increasingly common in European cities that really don't have much space.

Pedestrian visibility is also compromised. My SO is not a tall person. We've tested - she's quite invisible standing in front of these trucks and SUVs, because their front hood is so high and square. A small child would be impossible to see even some meters away. Worse if the driver of the vehicle is not tall.

> A small child would be impossible to see even some meters away.

A US local News did a segment on that recently, they lined up sitting children in front of a truck, I think only the 7th or 8th child in the row was visible and even then barely. You basically can’t see anything shorter than the hood for 10 feet in front of you, and that’s assuming you look at the road from over the driving wheel not between the dash and wheel arch.

Edit: turns out “recently” is optimistic (or more likely this is a recurring segment), found this from 3 years back: https://youtu.be/NDH3FDfVQl0

Tho it’s not the segment I was thinking of.

This also creates a huge problem for visibility around parked cars. Most US cities I've visited allow parking right up to pedestrian crosswalks on corners (yes, I know this is stupid and dangerous, but the street parking lobby is very strong here). When someone parks a massive truck or a huge SUV on the street, pedestrian, cyclists, and even cars trying to cross the street are stuck gambling on a blind spot.

This shouldn't even be a problem; these massive vehicles literally don't fit in street parking spaces, where they're designated with paint or verge on a bike lane.

I once had a Toyota land cruiser start climbing onto the hood of my Hyundai accent because the driver couldn’t see me over the hood of his engine as he driving up out of a parking garage.

At some point, there needs to be a separate driving test for these gigantic vehicles. A guy on my block is always parking halfway on the curb and has hit more than a few cars when parking. It really takes a different set of skills than driving a regular sedan or maybe even a crossover SUV. Never seen him haul or tow anything, either.

Making driving tests more strict and mandating a refresher every few years would probably go a long ways towards creating more walkable and bikeable cities. Some folks would choose to drive unlicensed, of course, but it'd be silly to let that imperfection destroy the good.

I've hauled all sorts of trailers over the years and driven F750s. They always belonged to friends or work though. When I finally bought my own little 2 horse a ways back, I was just kind of floored that I didn't have to prove to anybody that I knew what the hell I was doing with it. Not long after, I was driving through the mountains in the southwest and realized that I was surrounded by retired people who didn't have to prove they could safely drive their giant RVs.

Don't get me started on cheap CDL training programs.

Driver's testing in the US is a joke.

As a driver of an oversized vehicle I am always paranoid about driving in the city and almost never do it in my truck. If you walk in front of my truck I literally cannot see you. The tires are almost 4 ft tall and it is easily possible to simple drive over smaller cars. I keep this diesel monstrosity where it belongs, out in nature off roading and buying groceries in my local suburb.

> buying groceries in my local suburb

Ah yes, the natural environment of the 4ft tall tired diesel monstrosity truck.

I understand that you want this vehicle for offroading. But in my opinion it should be treated like a dirt bike or a 4 wheeler: you should have to tow it to the trails. If you live in the wilderness, feel free to drive it right onto dirt roads and trails. But it just shouldn't be allowed at all in a suburban or urban environment. A truck like that cannot possibly fit in the parking space at the grocery store. Not being able to see over the hood is a danger to anyone walking or biking in your suburb, and probably contributes to the fact that nobody walks or bikes in your suburb.

I was tongue in cheek. It is not even very useful for off roading except out in very open areas, which it does do okay at, it has a great suspension setup. Too wide to fit on most of the mountain trails here in Colorado, though. And it does fit in a parking spot, if you are very careful, but I generally just park away from other vehicles so no one has to deal with it. I drive smaller vehicles in congested areas. And FWIW a ton of people cycle in my area and I am almost always one of the few people giving cyclists appropriate space out here.

I do at least ponder the moral implications of it. It was fun for a while, I will probably sell it soon, it is rather impractical :)

As a former Coloradan myself, up your off roading game and get yourself a mountain bike! Especially if you live on the Front Range, there are a ton of trails much closer to home. You might even be able to bike to them!

Yeah, we do all our "real" Colorado activities in our 4Runner and or with our off road camper. My son loves mountain biking when we go camping. I just don't enjoy mountain biking, but I do live in Golden where you have some great trails minutes away. In all seriousness I actually just use the truck to tow trailers and move cars and other things around, and occasionally take the camper to the more open areas :)

A complaint I have and I've heard from others is the airbags in the A pillars are a menace to pedestrians because they block the drivers view.

Yes, cyclists too. When approaching an intersection which is not at right angles, it can be especially dangerous for cyclists when the speed of the car and cyclist is just right for the cyclist to be obscured for a long period of time. [1]

I also noticed when driving a certain Kei car in Japan with no a-pillar airbags and a so-called “split” or “double” a-pillar with a window between 2 skinny pillars, that the visibility is so much better. It feels great when you realize you don’t need to move your head around at all to see past them, the parallax affect of having 2 eyes makes them basically disappear. [2]

[1]https://usa.streetsblog.org/2016/12/02/study-diagonal-inters...

[2]https://en.m.wikipedia.org/wiki/Daihatsu_Tanto#/media/File%3...

Tom Scott has a good video for anyone looking for a visual of this phenomenon:

https://www.youtube.com/watch?v=SYeeTvitvFU

He shows off a rural (and thankfully now fixed) crossroads in the UK with seemingly great visibility and an unusually high rate of cyclist fatalities - the roads are at such an angle that any reckless driving speed can align with a reasonable cycling speed such that the driver's door pillar can block visibility of a cyclist with right of way right up to the point of impact - and it's compounded by the fact that the car is necessarily approaching from behind the cyclist, so a passing cyclist will be unaware of the danger possibly until they get hit.

Of course if drivers in the area actually followed the stop signs no one would get hit - but if you watch the video in a five minute period genuinely only about half of the cars passing through actually come to a complete stop at the stop sign. You need more than signage and quietly hoping that people to do the right thing.

I've had an entire Mazda 2 hidden from view in a roundabout, by the A-pillar and associated airbag of a VE commodore. Pedestrians and bicycles required much more effort to see in that car than any other I've ever driven.

A good rule of thumb is, if you are in dense complex traffic, to move your head around a bit when looking out and in the mirrors. Better coverage of dead angle in side mirrors, adding a bit more 3D into perspective.

Its sort of defensive driving - expect troubles, idiotic clueless drivers, people having heart attack or just turning head to yelling kid on backseat / nice lady walking nearby. Can't save it all but definitely helps.

Driving in country where people, when averaged, drive like crap is a good learning experience (albeit stressful). For me, whenever I enter France from Switzerland (on average 1x per week), its night and day and training (with cursing) resumes.

The density or complexity of traffic really has nothing to do with whether a car has been designed in such a way that it significantly impedes vision where it is most critical. In this case, a quiet roundabout with one other car almost directly in front.

Some passenger cars also have a nifty feature where if an impact is detected, the hood on the side of the windshield is instantly propped up by some springs to provide a cushioning effect to whoever ends up on the hood.

I've seen it action when a guy who I was riding motorcycles with got hit on the side by a car (his fault, low speed, he was fine). My car also has the feature.

Obviously impossible if the hood's too high like with an SUV or pickup.

The 2012 Volvo V40 (now discontinued) did one-up that by including an airbag under the hood which covers the A-pillars when detecting pedestrian impact.

The Tundra now has a front camera so you can see what’s in front of you.

I've had an idea for a long time that A pillars should have screens that show through them (in addition to reverse cameras above).

I drove a Prius only once, for around ten minutes. I used to dislike the Prius, but following that short experience I now hate it from the deepest of my heart.

The Prius is an abomination, because it is too low to be comfortable and because of its wrong, arcuate shape. The natural and right shape for a car includes only vertical walls and horizontal surfaces, apart from the windscreen which is allowed to be slanted, but not too much.

"Just roll up and off of it" makes it sound like a pleasurable experience. The majority of serious injuries happen not upon collision with the vehicle, but when hitting the ground afterwards. This applies for any vehicle shape, but in different ways.

Yeah, but landing on the round hood of a VW Polo vs being hit by the equivalent of a rolling brick wall makes a difference.

If you look at the formula for acceleration (a = ∆v/∆t) and acceleration force (F = a⋅m) the only two things we can truly influence here are speed (slower collisions are less dangerous) and time (collisions where the force transfer takes longer produce less force).

The latter is what makes the difference between getting hit with an cushion and an equally heavy rock. The same difference exists if you run against a brick wall and against an angled grassy hill (or: a bendy car hood). Not only will the elastic hood (and the roll) absorb part of the collision energy, the angle of the hood stretches the acceleration over longer time and reduces the force transfered on your body.

So if you get the choice as a cyclist, the choice is pretty clear.

Mass also affects force transferred, during contact.

Newton's 3rd law, implies the more mass you have the less recoil you feel.

For example hitting a massive box will hurt a lot more than a light box, because light box will get some acceleration of its own vs massive box.

Though at the mass ratios of car vs pedestrian/cyclists, the difference on the car side makes no significant effect. It doesn‘t really matter if the ratio is 1:10 in favor of the car or 1:20. The acceleration that the car will feel is negligible either way.

I neglected mass here, because I don't think it really makes much of a difference in the deadliness of the collision (car is always much heavier than cyclist).

One thing it would definitely affect tho is the time within which the car could brake, provided they react in time.

Been hit a couple of times, I’d be dead if I went under instead of rolling over the cars

> The majority of serious injuries happen not upon collision with the vehicle, but when hitting the ground afterwards.

That claim right there is begging for a source. It's hard to imagine landing from a person's height is worse than hitting a brick wall moving 40 km/h, unless that brick wall also threw you more than 6 meters up into the air, which point at which you'd be moving faster than 40km/h when you hit the ground.

A few months ago, I saw a pedestrian get hit by an SUV going ~40mph (64km/h). He flew far enough that when he came to a stop he was a good ways down the road. If you made it through the sudden acceleration to 40mph, I could also see "the road braking you from 40mph to zero" doing some damage.

As another example, here's a locally-high-profile crash [0] (the driver, a city councilwoman and the daughter of the county commissioner, left the scene and didn't report the crash for hours). Red lights and sightlines aside, even though not much momentum is imparted to the cyclist, the crash was enough to flip him over and knock him into the air, which changes all of the calculus for landing damage from what normally happens when you fall off a bicycle. If you land straight down on your head, it might be worse for you than getting hit in the torso at a higher speed.

[0] Content warning: this is a video of someone getting hit by a car https://youtu.be/X7V_H9bADDM?t=54

I got into a head on collision with an SUV that came off two wheels while rounding a curve and crossed the yellow divider right into my little car. In that case it was definitely the height that did us in.

Glad to hear that you lived to tell the tale.

Sounds like a horrendous collision.

My car crumpled perfectly and my only injuries were scrapes from the airbag. The other driver broke a few ribs.

I will never forget the split second I recognized there was a huge mass of metal coming straight at me at a high speed and I resigned to the fact that it would probably mean life altering injuries. Good thing I wore my seat belt!

Note that minivans have been around longer than SUVs, but do not appear to be as disproportionate in fatalities. I suspect part of this is that they are typically lower to the ground and, more importantly, tend to only weigh about 500 lbs more than the average car at ~4500 lbs. Contrast this with full size SUVs, which are often taller and weigh on average closer to ~5900 lbs.

The Chevrolet suburban from 1947 was clearly a large SUV. (the suburban was introduced in 1935, but you could argue those models were station wagons, by 1947 they were clearly SUVs). The VW microbus - was from 1950.

The minivan didn't become popular until 1980s, while SUVs remained the domain of niches like families too large for a minivan, or those who regularly drove off road for another decade or two. However the SUV is clearly the older of the two.

E=mV^2/2

60 km/h car will kill you either way, 1 km/h car won't kill you either way, but somewhere in between there's a velocity range where a SUV will kill you and a compact car won't - simply because of the mass difference. I'd argue it's the velocity range in which a lot of collisions happen (especially in city traffic).

This is the wrong way to think of it. At some point the mass is irrelevant. If an inelastic lead object the size of a car hits you, the energy transfers is the same as if a lead 747 hits you at the same speed.

Both objects have an overwhelming amount of energy and momentum compared to a nearly stationary 100 kg person. They'll transfer enough energy to bring the person up to their speed, likely mangling them in the process.

Trauma depends on how quickly this energy is transferred to the individual.

More to your point, both a car and a truck have more than enough energy to not be meaningfully slowed down by a pedestrian. The difference is strike location, contact area, and crumple zones to slow the transfer.

Its the momentum that kills you p=mV, not the kinetic energy. So I would guess that mass ia much more important.

A typical locomotive at 1 mph has more momentum than a Ford F-150 at 45 mph, but the latter is far more likely to kill you in collision so it doesn’t look like it is momentum that kills.

Why would that be? Isn't it the energy spent tearing your body apart that is relevant?

You are both wrong, It's the acceleration that kills you.

Doubling the mass of the car assuming everything else (crumple zones etc.) stays the same - doubles the acceleration experienced by the pedestrian.

    X_p0 = X of the pedestrian before collision
    X_c0 = X of the car before collision
    X_p1 = X of the pedestrian after the collision
    X_c1 = X of the car after the collision
    m_ = mass
    v_ = velocity
    a_ = acceleration
    t = time

    //conservation of momentum:
    m_p * v_p0 + m_c * v_c0 = m_p * v_p1 + m_c * v_c1
    m_p * (v_p0 - v_p1) = m_c * (v_c1 - v_c0)
    v_p1 - v_p0 = m_c * (v_c0 - v_c1) / m_p

    // a = delta v / t
    a = (v_p1 - v_p0)/t = m_c * (v_c0 - v_c1) / (m_p * t)

Acceleration, which is caused by a transfer of energy?

It is not just the mass, the design is critical as well. Check pedestrian impact tests, on regular cars pedestrians can roll over while with tall cars or SUVs they are simply splashed against it.

I think that band where the light car is not lethal is a narrow bad. I could be wrong but I have a feeling it's sitting up high in an SUV/truck that is causing some of this.

Don't forget that crashes aren't just an oblivious driver smashing into a pedestrian at full speed. Many, perhaps a majority involve some degree of braking.

I'll always choose height for my car because I want comfort.

My top priority when driving is safety for everybody, as in doing all my best to prevent accidents.

My second priority is personal comfort, which helps in guaranteeing the first priority to be respected.

Higher weight also requires thicker pillars so that the car doesn't smush you when it rolls over leading to worse visibility

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