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From: Clive George on 17 May 2010 15:04
On 17/05/2010 19:50, Phil Bradby wrote:

> If they do work and they do stop such a heavy vehicle moving so quickly
> in such a short space, how could the driver possibly survive the impact?
> It must be like hitting a brick wall to stop dead in a few feet in those
> conditions.

How long do you think they are?

Which one are you thinking of - maybe google maps or streetview has some
pics.
From: Mortimer on 17 May 2010 15:09
"Clive George" <clive(a)xxxx-x.fsnet.co.uk> wrote in message
news:POmdnXKMm_QaDmzWnZ2dnUVZ8qGdnZ2d(a)brightview.co.uk...
> On 17/05/2010 19:50, Phil Bradby wrote:
>
>> If they do work and they do stop such a heavy vehicle moving so quickly
>> in such a short space, how could the driver possibly survive the impact?
>> It must be like hitting a brick wall to stop dead in a few feet in those
>> conditions.
>
> How long do you think they are?
>
> Which one are you thinking of - maybe google maps or streetview has some
> pics.

Yes the ones I described above are probably a couple of hundred yards long.
Of course, your brakes might fail just as you reach the bottom end of the
escape lane...

From: Halmyre on 17 May 2010 15:29
In article <hsrue0$g4m$1(a)speranza.aioe.org>, nospam(a)nospam.com says...
> ...the ones on steep downhill runs.
>
> Just curious - seems a bit unlikely to me that a few feet of sand could
> stop a massive lorry hurtling down a 1 in 4. How often do they get used
> in anger?
>

I don't think they're used in anger. Terror, maybe...

--
Halmyre

This is the most powerful sigfile in the world and will probably blow your head clean
off.
From: Adrian on 18 May 2010 14:03
Phil Bradby <nospam(a)nospam.com> gurgled happily, sounding much like they
were saying:

> About 100ft by my ruler. The ordinary stopping distance for a car at
> 56mph is well over 300ft

Bollocks it does.

The HC gives the total _stopping_ distance from 60mph as 73m/240ft or
only 53m/175ft from 50mph.

http://www.direct.gov.uk/dr_consum_dg/groups/dg_digitalassets/@dg/@en/
documents/digitalasset/dg_070645.pdf

That includes the thinking distance - irrelevant in this instance - of 15m
from 50 or 18m from 60, giving a braking distance of 38m or 55m. Now
remember that those braking distances are reputed to be based on a '60s
Ford Anglia with unassisted drums the size of shirt buttons and crossply
tyres. Any modern car will stop in considerably less distance.

> - presumably a lot more for a heavy lorry.

The wagon itself - or the load? It's all a factor that the driver of a
wagon who actually needs the trap will take into account. B'sides, I've
never yet seen a sandtrap escape road on an m'way - which is the only
place 56mph is legal in a wagon.
From: Mortimer on 18 May 2010 14:23
"Clive George" <clive(a)xxxx-x.fsnet.co.uk> wrote in message
news:m9WdnXgwGMjbSW_WnZ2dnUVZ7sWdnZ2d(a)brightview.co.uk...
>>> There's one.. Not that I'd know either way about the physics of it all
>>> :)
>>
>> About 100ft by my ruler. The ordinary stopping distance for a car at
>> 56mph is well over 300ft - presumably a lot more for a heavy lorry.
>
> And the ordinary stopping distance is actually a fairly gentle
> deceleration on the scale of things.
>
> 56mph, 300ft = about .35g
> 56mph, 100ft = about 1G, which is about the maximum braking a car can do.
> That doesn't begin to hurt the driver.
>
> Double, triple that - still not going to be a problem. Think about the
> films you see of car crash tests - 30mph to zero in about a metre, which
> is 8G+. That'll give bruising, but it's survivable.
>
> Lorry vs car makes no difference - it's the deceleration which matters.
>
> If it was going to be like driving into a brick wall, it would need to be
> zero length. 100ft is nothing like that.

OK, so stopping in 100 ft would produce a survivable deceleration. But would
100 ft of sand drag be enough to produce sufficient deceleration force to
slow a 40-tonne HGV to zero so it didn't run off the end of the escape lane
or hit an earth bank or whatever? Would the retardation force be as great as
if the lorry had been able to slow using its brakes?

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