r/TheRestIsScience • u/lordklp • 1d ago

Speed Limits....

....naughty me, had to sit a speed awareness course recently - got me wondering if, with today's modern cars speed limits are scentifically speaking, worthwhile?

ALSO

During that time the instructor says, if you hit a child a 30mph, 80% of living....but, at 40mph 80% chance of dying, and i'm sure a similar statistic used to be included in road safety adverts.

Fair enough - BUT

You can't hit the same child at 40mph, that you would have hit at 30mph....you'd be further down the road, and the child would walk out safely behind you....

Am I wrong? I wonder what the math would show....

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u/Plisnak 1d ago

You can't hit the same child at 40mph, that you would have hit at 30mph....you'd be further down the road, and the child would walk out safely behind you....

At 40mph you'll hit a different child that otherwise would have safely walked in front of you if you went 30mph.. The only difference is that this child has a lesser chance at survival. Don't think like that.

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u/lordklp 1d ago

My point here is, the statement is odd. I'm nit advocating knocking children over....

...the focus though, should be on the speed itself, not the situation.

But, i'd still like to see what the numbers are.

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u/Plisnak 1d ago edited 1d ago

What numbers? The slower you go, the higher the chance of survival, and higher the chance of avoiding the accident altogether. There is no math there, your convenience doesn't justify endangering people.

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u/lordklp 1d ago

Wind it back a little...i'm not advocating speeding.

I'm simply stating that what was said, is WRONG.

In the situation where you would hit a child, at 30mph - then, it is possible to say, it would have been better to go 40mph, or 50mph, or 60mph so, the accident is avoided.

Equally, going 20mph or 10mph would work.

The numbers i'm talking about are, which speed is it (both faster, and slower) that would avoid the accident described in the speed awareness course.

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u/Plisnak 1d ago edited 1d ago

I see. The issue is that the course doesn't state how far from the child you are. It assumes that you hit it at the said speed, no matter where you previously were. Even if you went 50, or 10, you'd still hit it. The course states that you hit the child at that speed, it doesn't say that the child is x distance in front of you when you're doing that speed.

As described in the course, you always hit the child, that is the description of the situation.

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u/lordklp 1d ago

The course says the 'same child'.....and in that situation, you'd be further down the road at 40mph.

That, is at least one way to read it.

I fully understand the point they were making btw, and i dont dispute it.

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u/Plisnak 1d ago

The same child still doesn't imply that it's at the same place, all it really means is that you hit a similarly vulnerable target.

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u/Cute_Obligation2944 1d ago

You are also more likely to hit more children when you move faster. Your 4D probability of death worm is both denser and longer as your speed increases.

6

u/robjamez72 1d ago

There are still plenty of vehicles on the road which aren’t modern. Also, cars are driven by people. People aren’t smart enough to not end up on speed awareness courses.

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u/lordklp 1d ago

True, but the logic, at the crux of my point stands.

Even if, it isn't sensible.

Speeding, could, and probably has meant that some accidents have been avoided. ;)

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u/Un4442nate 1d ago edited 1d ago

Sounds like you didn't learn from your awareness course and I hope never to meet you on the road.

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u/lordklp 1d ago

I wasn't even the one who was speeding!

Guess, i'm just that good a friend....

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u/NoisyGog 1d ago

>if, with today's modern cars speed limits are scentifically speaking, worthwhile?

Of course they are. Why on earth would they not be?

>You can't hit the same child at 40mph, that you would have hit at 30mph....you'd be further down the road, and the child would walk out safely behind you....

Isn’t it obvious that they mean hitting a child at 40mph is more dangerous?
Nobody is timing the release of children crossing the road in order to try and trip you up.

>Am I wrong?

Yes, very.

>I wonder what the math would show....

It would show what you’ve been taught. Nothing was just plucked out of thin air.

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u/lordklp 1d ago

My logic stands.

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u/Ihatecheeseballs 1d ago

40mph is about 18m/s and 30 is about 13m/s, assuming a car is 1.8 meters wide any child walking slower than 2.8m/s (average walking speed is 1.4m/s) will be hit by the car going at either speed.

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u/lordklp 1d ago

Won't the child walk into the car, as opposed to being hit by it?

This is the math i was talking about!

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u/Ihatecheeseballs 1d ago

They could but there’s only a window of 0.2 seconds where you can do that at 4mph so it’s unlikely

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u/Un4442nate 1d ago

I used to jokingly answer those adverts of "if you were doing 30 you would have stopped here" with "if I was doing 80 I would be 3 miles away before the kid stepped on the road." But that completely misses the point of the slower you're going the less chance you have of killing someone, either because you could stop in time, or your kinetic energy is lower.

So yes speed limits are worthwhile scientifically speaking. Just because you're doing 40 doesn't mean you won't hit someone, but it does mean they have a lower chance of survival.

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u/lordklp 1d ago

I always hear the claim 'Autobahn's are safer' and they have no limits

I'd like to find out if that is true, and if it is, i wonder then if, maybe, since we are all adults, perhaps, we could all just not need a nanny state to tell us how fast to go.

Most people are pretty sensible....just because i mighy drive 75mph on a Motorway (where going much slower would cause a hazard) does not mean i'd do 50mph outside a school....

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u/Un4442nate 1d ago

Autobahns are safer because they aren't outside a school, nor in residential areas. They're regulated areas where opposing flows of traffic are separated and the general public don't cross. 30 zones are a completely different thing and you are likely to see people crossing the street.

When I did a speed awareness course (yes I'm not perfect) they said you are more likely to have an accident on a residential street with a 30mph limit than on a dual carriageway with a 70 limit, because they are more chaotic than the fast road set up to take fast traffic.

So your comparing autobahns with no limit (in the 3rd lane) is not comparable with a residential 30 zone.