Safe Speed Forums

http://www.youtube.com/watch?v=XRx5sBCE3HI

I don't think your link says it all, there's some more on here

http://www.youtube.com/watch?v=70-G0eBx ... ed&search=

Not so much of a giggle perhaps.

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COAST Not just somewhere to keep a beach.

A young loner on a crusade to champion the cause of the innocent, the helpless, the powerless, in a world of criminals who operate above the law.

The former video shows how vacuous the latter is, Handy.

That last one is from our own Road Traffic Authority here in Victoria. Please note just how the one that hits at 5kph actually has to lift off the brake so it can hit the truck.

Of course other options, such as trying to avoid the truck, are not explored.

The entire campaign over here is based on this sort of "science". Unfortunately, this sort of advertising and the use of a 3kph tolerance on speeding and covert cameras everywhere (no law requiring it to be visible over here), has resulted in traffic speeds being dramatically reduced. In an 80kph zone you can expect to travel at between 65 and 70kph, even in light traffic in the middle of the day.

Has our road toll decreased - NO, because they are simply dumbing down drivers with the "slow down and you are a safe driver" BULLSHIT!

Driving in Melbourne at any time of the day in simply a pain in the proverbial because it takes so damn long to get anywhere!

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Ross

Yes I'm a hoon, but only on the track!!!!

we could go on all night comparing pro-speeding and anti-speeding videos. Oh the joy of youtube.

If you think the video is vacuous, do the maths ... if 2 cars start braking at EXACTLY the same point and one is going at 70mph and the other is going at 100mph, how fast will the faster vehicle be travelling when the slower one reaches a complete standstill?

Do the maths then post your answers.

Don't try to bring in obfuscating issues (the usual Safespeed response) that the faster driver is paying more attention or the slower driver is looking at the speedo ... I'm looking for a MATHEMATICAL PROOF.

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COAST Not just somewhere to keep a beach.

A young loner on a crusade to champion the cause of the innocent, the helpless, the powerless, in a world of criminals who operate above the law.

Note the quote underneath the video

"Something seems odd about this"

Couldn't agree more, how can this statement make sense

"You knock off half your speed in the last 5mtrs of braking but the 65kmh car hits at 32kmh but the 60kmh car hits at only 5kmh ?????"

I am only a simple man, but that does not in any way stack up to me ????

Does that make any sense to our more brainy people??

EFC

As I'm sure you know, I'm not saying the maths in the video are wrong. I'm saying they're irrelevant.

The calculation made in the comedy video is mathematically correct too. The faster driver really would cause less damge in that exact scenario, for the sole reason that the scenario has been cherry-picked to fit the assumptions, and does not represent the real world.

In the real world all drivers should keep to a speed where they can stop if needed. That speed won't always be 60kph or any other given number.

However, if an even faster driver had concentrated on what he was doing, observed what was going on around him, anticipated the likely hazard, and given himself enough space and time to deal with with any hazard before it became it problem, he wouldn't have hit the truck at all.

To parody the Prof Ian Johnson - a second at the beginning can mean an eternity at the end. So spending a second staring at your speedo is much more dangerous than driving 5kph quicker. Here's the same scenario but with the slower driver starting to check his speedo at the same time the hazard appeared. At 60kph, he would have travelled 16 metres in the second that check took. Now Prof Johnson says that you wipe off half your speed in the last 5 metres of braking, which means that the intially faster driver would be going considerably slower at impact than the initially slower one. As Proj Johnson said, "at this end the difference is remarkable" because they'd have carried the intially slower driver off in a body bag while the initially faster, but more attentive driver, would likely have got a way with a minor injuries.

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Will

it means the 65km/h car would have stopped a further 5m down the road had it not hit the truck.

LISTEN TO THE VIDEO, the speed difference was 3MPH not 30mph, nobody will convince me of the difference in those impacts at only 3mph difference.

I didn't say the 70 vs 100 was a reference to the video, I stated it as a mathematical comparison.

Still waiting for someone to do the maths and post the answer.

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COAST Not just somewhere to keep a beach.

A young loner on a crusade to champion the cause of the innocent, the helpless, the powerless, in a world of criminals who operate above the law.

The maths is probably right, I don't know how to do the calculation offhand but it is consistent with what I've worked out before.

The real problem is the margins of error are ENORMOUS compared to the tiny input difference. If the lorry had pulled out 2 metres closer to the cars then they both would have been flattened. If it had pulled out 2 metres further downrange they both would have been OK. If one driver had started braking a tenth of a second earlier he would have stopped easily in time.

The solution to this apparent paradox is that in the real world of driving we allow generous safety margins and only very irresponsible drivers would be cutting it so fine. So a starting speed difference of +-5 mph or so still leaves you well within the safety margin. It has to, because other random factors have even more effect than the +-5 mph.

What maths? I smell a strawman.

perhaps you should find yourself a quiet road and try it out then. For starters they're allowing for a fixed amount of thinking time. That's 2m/s difference right there.

If You're looking for physics calculations, it won't help anyone, because road safety isn't a problem in physics.

Like all 'accidents' (i.e. not just on roads) the foundations are in human error and it's the human error aspects that we should be discussing.

If you want 'mathematical proof' that the physics is subservient to the psychology, look at the rarity of injury crashes - 200,000 annually amongst 32million drivers. An average of one injury crash caused per 160 driver years. What's happening for the rest of the time?

Or look at the scale of crash severity and frequency:

3 million damage onlys,
300,000 injuries
30,000 serious injuries
3,200 fatalities

You can't get that 'log' scale of crash severity out of a physics model (however much you torture it).

And of course, DfT just told us that only 5% of crashes involve a vehicle exceeding a speed limit while DfT also tells us that 60% of vehicles are speeding at saml=ple sites on most road types. How come speeding is so massively under-represented?

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

I've been through the maths.

The numbers can work, with the following assumption:
braking rate was 0.9G (very reasonable)
thus leaving a reaction time reaction time of 1.2 secs - resulting with both cars travelling 35.6m from hazard to impact.

It is VERY interesting to note that for the numbers used in this example, the reaction distance (~21m) is significantly greater than the deceleration distance (~15m), hence my comment on Youtube

Using the maths from this website

http://www.hintsandthings.co.uk/garage/stopmph.htm

The stopping distance is 12ft between the two cars speeds 65kph is 40mph and 60kmh is 37.5mph.

I fail to understand the staggering difference between 32kmh and 5kmh, as suggested I may just have to try it out myself

EFC

And of course as human beings we don't estimate braking distances at all. What we do estimate and get very good at estimating are braking times.

The time estimates are linear and entirely predictable unlike the 'sharp end' of the braking distance curves.

Have a look at http://www.safespeed.org.uk/braking.html

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

It does make sense when you consider the rather large reaction time applied. The faster driver will obviously cover more distance before he starts to brake (compared to the slower ), therefore he has less distance to actually scrub off his speed (assuming both will travel equal distance before impact), hence he ends up impacting disproportionately faster.

I've kept the spreadsheet if anyone wants to examine it.

I’ve got it. Assuming no impact, a car will come to rest in a distance of 5 meters from the point of where it was doing 32.5kph (32.5kph being half the speed of 65kph) if decelerating at a rate of 0.831G.

Revisiting my earlier maths, a braking rate of 0.831G would necessitate a reaction time of 1.275 seconds for all the numbers to stack (total distance = 38.17m).

No I’ve not spent all night doing this…