Rear-end collision best practices

May 16, 2005 5:57 AM

If the car you're in is about to be hit from behind, what is the best thing to do to avoid injury? Assuming your car is stopped (i.e. at a stoplight or something) and you don't have time to do any significant acceleration, should you, for instance, apply or release the breaks? Should you brace against the steering wheel?

More generally, what are the factors that affect the dynamics of this collision? For instance, how do crumple zones influence things? How much force can brakes/tire friction counter? I'm also curious about how initial motion in the front vehicle changes things...it introduces the possibility of serious loss of control. I guess this is a bit open-ended, but I'm curious about this from both a practical view as well as a sort of engineering-problem view.

posted by abingham to travel & transportation (21 answers total)

2 things would worry me: getting whiplash, and being shunted into oncoming traffic. If I knew I was about to be shunted from behind, I would:

1. Leave the car in gear, but the handbrake on, clutch down. When I get hit, my car will stall the engine and stop the car from moving, and the brakes will also help stop me from being pushed into oncoming traffic.

2. Lean back against the headrest, and put my body flat against the seat, but mainly make sure my head is on the headrest. When the car hits, my head won't be jerked back by the impact, risking neck damage.

My understanding of crumple zones is that they lessen the shock of the impact by allowing the car behind to slow down over a slightly longer distance, so it will lessen the effect of the crash. Imagine driving into a solid concrete wall at 20mph, and the shock of the impact. Compare that to driving into a wall of mattresses. The speed is the same but you've slowed down over a longer time, therefore lessening the shock of the impact. Even the extra foot or so of distance that the crumple zone gives can make a huge difference to the survival rate of the passengers.

I should point out I'm not a doctor, engineer or car crash expert.
posted by gaby at 6:11 AM on May 16, 2005

This is hard to do, but the best thing to do in any accident is to relax all of your muscles and just go with it. Tensing up your muscles can cause more serious injury, especially if you lock your elbows and press on the steering wheel or grab onto something to "brace for impact".

Over 10 years of getting thrown onto Judo mats has taught me to be able to instinctively relax before my body impacts something hard so that I don't get hurt. I would imagine its going to be very hard for others to relax in the face of impact though.
posted by jduckles at 6:24 AM on May 16, 2005

When I was a kid, my mother's boyfriend told me a story of how he was sitting at a red light, the first car on the line, and two girls were crossing in front of him. When he checked the rearview he saw an 18 wheel truck that wasn't going to stop. He leaned on his horn -- the girls looked up, scooted out of the way -- and my mom's bf was rammed passed them and into oncoming traffic where he was hit on both sides. The truck driver was asleep. Mom's bf was hurt, but nothing permanent.

The story scared the shit of me so I'm gonna tell you what you probably don't want to hear:

I've taken motorcycle and car driving lessons and both teach you to leave a cushion of a car length or two in front of you and only slowly let that go when you're confident the cars behind you are stopping.

In other words, if you're paying attention you should never be in a situation where you "don't have time to do any significant acceleration". Know your exits before you stop and if at all possible, never stop (coast that space till the light turns green) unless you have to.
posted by dobbs at 6:28 AM on May 16, 2005

Never, never brace your arms. I was hit head on once, and because I braced my arms, they both snapped like twigs.

Two casts, one surgery, intensive therapy and six years later I still haven't regained full use of my left hand from nerve damage (I never will).

There's a reason why drunks tend to walk away from accidents and the people they hit get hurt. It's because a drunk will go limp, and the first instinct of any sober person is to tense up and brace for impact. That's what causes a lot of the injuries.

Also, IIRC, the problem with whiplash isn't the forward motion of the neck so much as the backwards after. If your headrest is adjusted properly that'll reduce the injury a lot, too.
posted by Kellydamnit at 6:33 AM on May 16, 2005

While not an entirely technical perspective, I once read an article discussing how it's best (if time permits, which it probably doesn't under normal circumstances) to consider what's directly in front of you vs. your attachment to your vehicle.

e.g., you're driving a POS and intent on mitigating damage to other cars and/or there's a kindergarten class crossing the street in front of you, brake and brace yourself against the steering wheel and left foot - absorb the impact.

If you just pulled out of the Mercedes-Benz parking lot with still-wet ink on your loan paperwork (or whatever reason), however, and there's no cross traffic, let off the brake and still brace against the steering wheel - absorb less of the impact.

Using the steering wheel as a brace absorbs a lot of the initial forward-bounding that will happen to your body when you get rear-ended, because the steering column on almost all cars is collapsible, and will rebound into the dashboard upon impact assuming you're pushing hard enough on it.

In any collision hard enough to cause your car to move from a stationary position (ie: skidding forward) and in which you're braking reasonably hard (as you likely would if you knew you were about to be hit), brake traction is going to trump tire traction, as you're going to skid x distance based on how hard you're hit. Also, ABS would in theory reduce the actual distance, as it would let your wheels actually turn rather than skid (rolling tires stop faster than skidding ones) although I don't imagine it'd be by much, as you're starting from 0mph w/ brakes applied fully, and ABS on most vehicles operates only above 5-10mph.

In most circumstances, though, given safety belts, airbags, and crumple zones in modern vehicles, I'd say brake and brace for the impact - in an serious accident, there's no time to weigh options like these.
posted by porntips guzzardo at 6:53 AM on May 16, 2005

I should clarify that by "bracing for impact", I don't mean locking up your elbows; a firm grasp on the steering wheel with bent arms is what I was going for.

As any weightlifter will tell you, fully locked joints are no good. Shattered limbs are bad news.
posted by porntips guzzardo at 7:00 AM on May 16, 2005

Heh.

This happened to me a few months ago. Traffic on the freeway had come to an unexpected halt. Because I hadn't expected it, and because I had come dangerously close to rear-ending the car in front of me, I checked the rear-view. Sure enough, there was another vehicle cruising up behind me.

I angled the steering wheel to the right (I was in the right-most lane of the freeway) and then, as somebody else suggested, just relaxed. I didn't brake. I didn't brace myself.

The impact was not as severe as it might have been because the other driver realized her mistake (though to late to completely miss the collision). Still, my van was pushed onto the shoulder (and would have hit the car in front of me if I hadn't turned the wheel).

The other driver was mortified — she was over fifty and this was her first accident — but I tried to assure her I was fine, that I'd seen her coming, and that I had nearly done the same thing myself. It was just one of those unexpected freeway things...
posted by jdroth at 7:06 AM on May 16, 2005

While bracing your arms against the steering wheel is definately a bad idea, if you have the time and enough wits about you, you could try crossing your arms over your body (left hand holds right shoulder, right hand holds left shoulder). At the very least, it won't make things any worse.
posted by Civil_Disobedient at 7:13 AM on May 16, 2005

Do _not_ brace against the steering wheel/dashboard with your arms, or the pedals/floor with your legs.

For one thing, that's like trying to jump off the elevator floor just before it hits the bottom of the shaft--as Kellydamnit's story makes clear, you're _completely_ overestimating the strength of your muscles and bones versus some enormous physical forces.

Even worse, any rigid connection between your body and the car's frame is going to transmit forces to your body. Even in a moderate impact that doesn't total the car, whiplash, strains, and all those things are going to be much worse. You want as little rigid connection between your body and and the car frame as possible.

Those things are all true whether you totally lock out your joints or not. Any rigid part of your body that receives the force of impact is going to preserve and transmit it until some other part of the body like your neck joints simply cannot bear it, and gives. A limp arm or leg basically acts like a sac of fluid--relaxed muscles can actually absorb and dissipate an enormous amount of force, compared to a clenched one, which will just pass the force along.

You can _try_ to do something like throw your arms over your face to protect it, but unless the impact is pretty moderate, the odds of actually keeping them there are pretty slim. You're much more likely to be surprised at how easily the impact pulls them right off your face, but at least maybe they'll still be between your face and anything else if that becomes an issue.

Any newer car has several systems in place to match physics with physics, like airbags and especially seatbelts. Your best bet is to let go of the steering wheel, get your feet up off the floor, and let them do their job.
posted by LairBob at 7:18 AM on May 16, 2005

Why all this bracing against the steering wheel anyhow? If you are stationary and something rear-ends you, you (the person) will move backwards in relation to your vehicle. The only thing to do is relax back into your seat with your head against the headrest.

It's the same reason I always choose a rear-facing seat when on a train - if it crashes into something, people will move forwards in relation to the train, and sitting with your back against the seat facing backwards reduces the chance of being flung across the carriage.
posted by benzo8 at 8:23 AM on May 16, 2005

(I have this exact question, nearly verbatim, saved in a text file in my "To AskMeFi" directory. Kudos for hacking into my system and reading my mind).
posted by Hankins at 8:31 AM on May 16, 2005

If you had a completely open space in front of you, benzo8--like you're the first car in line at a sparse intersection--then it's as simple as you've described. But the real-life situation is very likely to be more complex. What if there's more cars already stopped in _front_ of you, or pulling across the intersection? In any of those cases, you're going to get shoved back into your seat from the rear impact, but then you're also very likely to get whipped forward on a second impact.

The basic point, though, is that it doesn't matter exactly what might happen--what direction in what order. Not only can you not predict where the forces are coming from, but even if you could, you just don't want to adopt a position that's going to transmit those forces to your body.
posted by LairBob at 8:37 AM on May 16, 2005

That's true, LairBob, but in the situation the questioner describes (and in fact, in any rear-ender situation I can imagine), you're going to get thrown backwards first, and it's the rapid snapping backwards on an unsupported neck that most likely to cause permanent damage in such a situation, so I stand by my answer - back against the seat, head against the headrest and relax...
posted by benzo8 at 8:45 AM on May 16, 2005

Okay, I am and engineer and have been involved with some crash testing. And I have been rear-ended in a very small car by a much larger one. As most people here said: Do not brace yourself rigidly against the rest of the car (wheel, dashboard, etc.) Let the seat do that. If you see someone in your rearview mirror about to rear-end you, put your head back to the headrest and relax. If your hands are on the top part of the wheel, move them to the sides or bottom so that the air bag does not push them into your face. I put my manual shift car in neutral and applied light brake force (just enough to prevent the car from rolling on its own). The result was that I was moved forward about a half-car length, and two or three of the clips that held the rear bumper fascia broke. The car suffered no other damage and I'm still driving it 8 years later.

To summarize:
Relax; don't brace.
Keep the air bag in mind.
To the greatest extent possible, let your car absorb the impact, either through crumple zones or getting pushed.
posted by Doohickie at 10:05 AM on May 16, 2005

I'm not 100% sure if it is the snap back that is the most likely to cause injury... After all, there is a safety device back there, but there isn't one in front (unless you have an airbag).

Even if there are no cars in front of you you are subject to the secondary impact because your breaks are locked. Whatever G-load your tire grip is able to produce is going to happen to you in the throw-you-forward direction immediately after you suffer the G-load from the impact.

A couple of people in my immediate family have been rear ended, both described the sense of being thrown forward, not back at all...
posted by Chuckles at 10:09 AM on May 16, 2005

IANA engineer but I used to read their Ax reports every other week. Doohickie is of course right about the best course of action.
But even then, although the initial head movement is backwards (because the rest of your body is being shunted forwarded and even the strongest neck would have virtually no chance of keeping the heavy head aligned) you still find people will end up with whiplash of possibly equal severity to a fowards crash.
As the vehicle you are in stops the after an impact from behind, your head will still jerk forward potentially causing injury. Your car weighs most and has wheels and road/friction and thus ceases forward movement first. If your body is harnessed by a seatbelt, it will be secured, whereas your head will keep moving forward as there's nothing to stop its momentum, save for the neck attachment.

But in real life, most people find it really difficult to stop themselves from bracing strongly against the steering wheel. It's kind of human nature to brace yourself before any forseen trauma except as noted above, when you're drunk or out of it.

If the road is clear ahead and you apply the brakes before impact, the damage to your vehicle and yourself is likely to be greater. The force from the car behind is absorbed by the crumple zone at the front of their car and the rear of your own car AND by the forward movement of your vehicle. If you stand on the brake heavily, you prevent some forward movement of your vehicle upon impact so the energy is absorbed and not dissipated.

All of this requires of course that you have plenty of time to judge the speed and distance of the approaching vehicle from your rear mirror, taking account of the area ahead of your car and remembering to relax back against your headrest. In reality - very very rarely do people get the opportunity or if they do, they are too panicked to do anything effective.
posted by peacay at 10:42 AM on May 16, 2005

When stopped for a left turn, always keep your wheels straight until the moment you start turning. If you are ever hit with your wheels angled left, you will be pushed into a head on collision.
posted by weapons-grade pandemonium at 12:28 PM on May 16, 2005

Your head jerking forwards is far less worrying than your head jerking backwards. It's nothing to do with safety equipment (ie: the headrest) and everything to do with range of motion - your spine is perfectly designed to allow your chin to rest on your chest, but don't try doing it the other way!
posted by benzo8 at 12:31 PM on May 16, 2005

Bones aren't the only thing to protect. An awful lot of tissues, including organs, muscles, nerves, tendons, blood vessels, etc. can get badly damaged even when nothing's broken. Even a very low speed collision transmits enormous forces to the vehicle. If the vehicle doesn't absorb that force (e.g. crumple zones), it WILL be passed on to the passengers in the form of a fast hard yank (rapid acceleration/deceleration) that far exceeds our bodies' design specs.

Your head jerking forwards is far less worrying than your head jerking backwards.

Either way, a sudden forceful jerk is bad news. Whiplash, for instance, can happen whether the head is yanked forward, back, or both. Brain injury can happen even when there's no direct hit to the head (again, because when the container doesn't dissipate the jolt, contents get slammed around; in this case, the brain against the skull itself.)
posted by nakedcodemonkey at 9:56 PM on May 16, 2005

If its highway, hit the gas, or you'll have your back end going through your body. Not long ago an entire family was killed here in seattle because they were in a compact and stopped on the highway...the other driver was flying in a SUV and ...wait for it...talking on the cell phone. There was nothing left...so to the engineer who was hit but is still driving his car...you couldn't have been but lightly touched in that "accident", good for you, but don't mislead people please, your happy result is not always true.
posted by uni verse at 10:50 PM on May 16, 2005

hit the gas

If you have enough time to hit the gas and actually accelerate, you have enough time to avoid the accident all together. If you see an accident coming, you generally have just a moment to take whatever action you're going to take and your car can't possibly accelerate quickly enough to minimize an impact.

Yes, my accident was a "light touch" comparatively speaking, but like I said, it still moved over a ton of metal forward about 10 feet in small fraction of a second.

Oh, and headrest/forward whiplash thing: In rear impact, you are stationary and the seat will move forward into you. It is far better to have your head already resting on the headrest than to have it even an inch away; the relative speed of the headreast to your head is what does the damage; if that relative speed is zero (i.e., they are touching), the damage will be less. The swing forward afterwards looks nasty on film, but that's because it takes so long (in the timescales involved). The longer you can spread out absorption of an impact, the less damage it does (as a general rule. Of course, your mileage may vary. Not valid in Hawaii or Puerto Rico.)
posted by Doohickie at 4:48 AM on May 17, 2005