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Custom24
Joined: 15 Feb 2008
Posts: 62
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 Posted: Tue Jul 22, 2008 2:56 pm Post subject: Wet stopping distance |
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You're on a DC or M'Way at 70 and it's dry, so you are leaving HC guided 96 metres to the vehicle in front, possibly judging by the marker posts down the central reservation or in the verge, which are 100 metres apart.
Then it starts to rain, mildly moist at first, then eventually quite wet.
HC says stopping distances should be at least doubled in the wet. My question is what do you guys do in practice? I find it quite difficult to leave any more than about 130 metres to the Veh in Front, either because I feel pressurised by the guy behind or vehicles from adjoining lanes start to take the space. I've tried leaving 200 metres and I find it very difficult - is it just me? |
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7db
Joined: 06 Apr 2006
Posts: 1691
Location: London
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| Posted: Tue Jul 22, 2008 3:18 pm Post subject: |
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I usually look at 2s in the dry, 4 in the wet, 8 in snow/other.
On a motorway, it's thinking distances that are the bother - as both things need to brake. Elsewhere it's stopping distances as the hazard may well be already stationary.
I have to confess I've never had much time for the HC stopping / thinking distances and prefer my judgement based on experience. I test braking distances when I get a chance on an empty road - it's well worth it. |
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Horse
Joined: 05 Feb 2007
Posts: 495
Location: Darkest Berkshoire
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| Posted: Tue Jul 22, 2008 3:29 pm Post subject: Re: Wet stopping distance |
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| Custom24 wrote: | | because I feel pressurised by the guy behind |
One of the times when an longer buffer zone ahead of you might come in useful.
| Custom24 wrote: | | or vehicles from adjoining lanes start to take the space. |
So you arrive at your destination 200 meters 'later', 60mph = 88 feet per sec, so about 7 seconds.
Old mnemonic for the system was:
Can
My
Safety
Be
Given
Away?
IIRC
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crr003
Joined: 11 Sep 2005
Posts: 1140
Location: Wirral
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| Posted: Tue Jul 22, 2008 5:09 pm Post subject: Re: Wet stopping distance |
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| Custom24 wrote: | You're on a DC or M'Way at 70 and it's dry, so you are leaving HC guided 96 metres to the vehicle in front, possibly judging by the marker posts down the central reservation or in the verge, which are 100 metres apart.
Then it starts to rain, mildly moist at first, then eventually quite wet.
HC says stopping distances should be at least doubled in the wet. |
Well, can I refer you to the IAM driving manual - "How to be a better driver" page 53.
"Braking distances increase significantly in the wet, especially at speed. In testing a Z-rated high-performance tyre at 70mph took five metres longer to stop in 1mm of standing water than in the dry."
So where are you guys getting this twice the distance rule? (And don't say HC).
Or have the IAM made a small typo? |
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Custom24
Joined: 15 Feb 2008
Posts: 62
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| Posted: Tue Jul 22, 2008 5:25 pm Post subject: Re: Wet stopping distance |
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| Horse wrote: | | Custom24 wrote: | | because I feel pressurised by the guy behind |
One of the times when an longer buffer zone ahead of you might come in useful.
| Custom24 wrote: | | or vehicles from adjoining lanes start to take the space. |
So you arrive at your destination 200 meters 'later', 60mph = 88 feet per sec, so about 7 seconds.
Old mnemonic for the system was:
Can
My
Safety
Be
Given
Away?
IIRC |
Yes I do increase the distance if there is someone up my tailpipe or even not as far back as I would like (ie their own stopping distance) but thinking about it, that would require 200 metres for the wet, plus a further increase because the guy behind is too close. Oftentimes, I will find a way to let them past.
CMSBGA - someone did tell me this recently. Presumably it was something like Check Mirrors, Speed, Brakes, Gears, Acceleration? What happened to Position? |
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jcochrane
Joined: 25 May 2008
Posts: 66
Location: Croydon
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| Posted: Tue Jul 22, 2008 5:31 pm Post subject: Re: Wet stopping distance |
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| Horse wrote: |
CMSBGA - someone did tell me this recently. Presumably it was something like Check Mirrors, Speed, Brakes, Gears, Acceleration? What happened to Position? |
Think you'll find the C stands for Course. |
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Custom24
Joined: 15 Feb 2008
Posts: 62
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| Posted: Tue Jul 22, 2008 5:31 pm Post subject: Re: Wet stopping distance |
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| crr003 wrote: |
Well, can I refer you to the IAM driving manual - "How to be a better driver" page 53.
"Braking distances increase significantly in the wet, especially at speed. In testing a Z-rated high-performance tyre at 70mph took five metres longer to stop in 1mm of standing water than in the dry."
So where are you guys getting this twice the distance rule? (And don't say HC).
Or have the IAM made a small typo? |
I don't have the latest IAM manual, but what you quote here does not give enough information to work out the ratio, only the difference. In other words, we don't know what the dry or wet distance was, only that the wet was five metres longer than the dry.
But presumably you are suggesting that the HC twice the stopping distance is excessive according to the latest IAM guide? |
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Custom24
Joined: 15 Feb 2008
Posts: 62
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| Posted: Tue Jul 22, 2008 5:37 pm Post subject: |
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| 7db wrote: | I usually look at 2s in the dry, 4 in the wet, 8 in snow/other.
On a motorway, it's thinking distances that are the bother - as both things need to brake. Elsewhere it's stopping distances as the hazard may well be already stationary.
I have to confess I've never had much time for the HC stopping / thinking distances and prefer my judgement based on experience. I test braking distances when I get a chance on an empty road - it's well worth it. |
I also use 2s rule as well as marker posts. What I don't like are the "Keep apart 2 chevrons" they don't seem to allow you the HC stopping distance even for the dry, so I think they confuse people.
I'm not sure you can assume that a hazard on a m'way will not be stationary when you react to it, but I take your point.
I have tested braking distances before, but I've not had the nerve to try it from 70mph, because I am worried about someone who is not yet visible in my mirror coming at my with a relative speed of 70mph+. I probably should try it on a track, if I ever get the chance. |
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crr003
Joined: 11 Sep 2005
Posts: 1140
Location: Wirral
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| Posted: Tue Jul 22, 2008 5:38 pm Post subject: Re: Wet stopping distance |
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| Custom24 wrote: | I don't have the latest IAM manual, but what you quote here does not give enough information to work out the ratio, only the difference. In other words, we don't know what the dry or wet distance was, only that the wet was five metres longer than the dry.
But presumably you are suggesting that the HC twice the stopping distance is excessive according to the latest IAM guide? |
Well, you're right, but if HC dry is 96M then HC wet is about 200M. IAM is saying (unless I'm missing something) that the difference is only 5M???? |
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ScoobyChris
Joined: 01 May 2006
Posts: 1033
Location: Andover, Hants
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| Posted: Tue Jul 22, 2008 6:54 pm Post subject: Re: Wet stopping distance |
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| crr003 wrote: | | But presumably you are suggesting that the HC twice the stopping distance is excessive according to the latest IAM guide? |
Well, you're right, but if HC dry is 96M then HC wet is about 200M. IAM is saying (unless I'm missing something) that the difference is only 5M????[/quote]
I'm pretty sure that must be a typo, although I suppose it could be feasible as a Z-rated tyre will likely be very good at shifting water. On a related note, I was quite surprised how badly (read conservatively!) I estimated stopping distances when trying to brake to a cone at the end of a runway.
Chris |
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ROG
Joined: 08 Oct 2006
Posts: 955
Location: LEICESTER
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| Posted: Tue Jul 22, 2008 6:54 pm Post subject: |
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Hang on a mo - ain't we forgot something here 
If we are taking about MOVING hazards in the wet then they will also have a greater stopping distance.
A car following a car at a 2 second gap in the dry - just be able to stop in time - distance covered by both cars whilst stopping, example 200 metres
A car following a car at a 2 second gap in the wet - just be able to stop in time - distance covered by both cars whilst stopping, example 400 metres
The doubling of stopping distance in the wet applies to stopping for static or near static hazards - or those coming from the opposite direction
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crr003
Joined: 11 Sep 2005
Posts: 1140
Location: Wirral
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| Posted: Tue Jul 22, 2008 7:09 pm Post subject: |
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| Custom24 wrote: | | What I don't like are the "Keep apart 2 chevrons" they don't seem to allow you the HC stopping distance even for the dry, so I think they confuse people. |
They're not absolute stopping distance - they're thinking distance.
The chevrons are 40M apart - nearly twice the 70mph HC thinking distance.
And thinking distance isn't wetness dependant, so the chevron distance is quite conservative? |
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StressedDave
Joined: 24 Sep 2005
Posts: 532
Location: Cambridgeshire
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| Posted: Tue Jul 22, 2008 7:15 pm Post subject: Re: Wet stopping distance |
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| crr003 wrote: | Well, can I refer you to the IAM driving manual - "How to be a better driver" page 53.
"Braking distances increase significantly in the wet, especially at speed. In testing a Z-rated high-performance tyre at 70mph took five metres longer to stop in 1mm of standing water than in the dry."
So where are you guys getting this twice the distance rule? (And don't say HC).
Or have the IAM made a small typo? |
I'd say a little bit of a large one... Wet stopping distances are a bit of a nightmare area. Doubling the stopping distance would be about right for a locked wheel, although in practice it's easy to achieve the same rates of deceleration as the highway code stopping distances in the dry. A doubling of stopping distance actually equates to a rate of deceleration about 71% of dry values, which doesn't actually sound like much.
One of the exercises I do with clients at Millbrook involves their ABS testing area which is a mixture of polished concrete and grippy tarmac permanently wetted. From 30 mph you can get a shorter stopping distance (about 1 car length) by doing something that is absolutely counterintuitive - letting go of the steering wheel. |
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jont
Joined: 07 Jul 2006
Posts: 750
Location: Bristol
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| Posted: Tue Jul 22, 2008 7:35 pm Post subject: Re: Wet stopping distance |
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| StressedDave wrote: | | One of the exercises I do with clients at Millbrook involves their ABS testing area which is a mixture of polished concrete and grippy tarmac permanently wetted. From 30 mph you can get a shorter stopping distance (about 1 car length) by doing something that is absolutely counterintuitive - letting go of the steering wheel. |
I'm assuming thats to do with the wheels trying to lock/unlock at slightly different rates playing havoc with the ABS systems? So you end up with the steering wheel juddering which if you fight and try and hold straight ends up using braking grip for a wasted amount of steering?
How do clients without ABS get on with that surface? |
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7db
Joined: 06 Apr 2006
Posts: 1691
Location: London
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| Posted: Tue Jul 22, 2008 9:40 pm Post subject: |
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| Custom24 wrote: | | I have tested braking distances before, but I've not had the nerve to try it from 70mph |
Do try it somewhere safe. If you drive at 70, you should feel what it feels like to stop from 70.
I've stopped from 120, but it wasn't nice as I locked up on the way down. The amazing thing is how long it takes you to get down to 60. |
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