Speeding in a thirty limit

[ger147]

As far as braking distances, as opposed to overall stopping distances, are concerned, they are calculated on a skid to stop basis. With a locked wheel the type of brakes are a complete irrelevance, the road surface and the tyres will have a much bigger effect.

So these calculations must pre-date ABS which is standard on the majority of modern road cars which no longer lock wheels and skid?

If not it just shows how irrelevant they actually are as ABS ensures drivers don't skid any more.

 

[Deleted member 15344]

So these calculations must pre-date ABS which is standard on the majority of modern road cars which no longer lock wheels and skid?

If not it just shows how irrelevant they actually are as ABS ensures drivers don't skid any more.

You can still skid with ABS

 

[ger147]

http://www.sdt.com.au/safedrive-directory-STOPPINGDISTANCE.htm

Quite a spread even amongst ordinary cars...

 

[Deleted member 15344]

http://www.sdt.com.au/safedrive-directory-STOPPINGDISTANCE.htm

Quite a spread even amongst ordinary cars...

Not many in that list are " ordinary"

And even then the difference between most is 2 metres bar one which is 4 metres

 

[ger147]

A BMW 330D is 7 metres less than a Suzuki Alto, neither of which are particularly unusual or uncommon cars on our roads.

 

[MadAdey]

People are now getting into the pointless argument. The facts on the braking distances are just an average as you can't take into account a Fiesta with brand new brakes against a Range Rover with knackered old brakes. I do agree that different cars and people will have an effect on these stopping facts.

When I had an Impreza I was never worried about hitting something in front of me as my car stopped on a sixpence. I was more worried that the car behind would not stop before it went into the back of me.

I will put a question to you...... would you prefer to be hit at 20MPH or 30MPH

 

[ger147]

People are now getting into the pointless argument. The facts on the braking distances are just an average as you can't take into account a Fiesta with brand new brakes against a Range Rover with knackered old brakes. I do agree that different cars and people will have an effect on these stopping facts.

When I had an Impreza I was never worried about hitting something in front of me as my car stopped on a sixpence. I was more worried that the car behind would not stop before it went into the back of me.

I will put a question to you...... would you prefer to be hit at 20MPH or 30MPH

So they are not facts, just approximations which was my point. And any real world test invariably shows that, on average, real cars in the real world need less distance to stop than the officially published approximations.

 

[Blue in Munich]

http://www.rospa.com/about/currentcampaigns/publichealth/info/rs4-casestudy-20-mph-zones.pdf


http://www.roadsafetygb.org.uk/misc...in Residential Areas by Shops and Schools.pdf

Point 8 in a long list at the bottom of the second link is that reduced speed reduces pollution. Which is directly contradicted by this article;

http://www.theguardian.com/environment/2010/apr/19/ask-leo-20mph-speed-limits-pollution

So it seems the winner is whoever pointed out earlier in the thread that whatever your point of view, there is always an expert and a statistician prepared to back it up.

 

[MadAdey]

So they are not facts, just approximations which was my point. And any real world test invariably shows that, on average, real cars in the real world need less distance to stop than the officially published approximations.

But a car going 20 still need less stopping time than if it was doing 30.

 

[Foxholer]

http://www.rospa.com/about/currentcampaigns/publichealth/info/rs4-casestudy-20-mph-zones.pdf


http://www.roadsafetygb.org.uk/misc...in Residential Areas by Shops and Schools.pdf

Neither of which support Hobbit's numbers!

Here's the (or at least a) Factsheet relating to the Royal Society for Prevention of Accidents summary that you quoted. It's the link I added to my challenge of Hobbit's numbers.

http://www.rospa.com/roadsafety/advice/highway/info/20-mph-zone-factsheet.pdf

Dorling's paper is actually a proposal/making a case rather than the analysis that the RSfPoA doc is. He also quotes Ashton and Mackay study (of 1979) which was the old (ancient actually) doc I referred to earlier. Those 35 year old figures are now unreliable imo.

 

[ger147]

But a car going 20 still need less stopping time than if it was doing 30.

Of course, but that same car at 30 may be perfectly capable of stopping in a shorter distance than another car doing 20.

 

[Blue in Munich]

So these calculations must pre-date ABS which is standard on the majority of modern road cars which no longer lock wheels and skid?

If not it just shows how irrelevant they actually are as ABS ensures drivers don't skid any more.

If I remember correctly from the point of view of the calculation of the braking distances, a wheel on the point of locking as per one with ABS and a locked wheel are so close as to be treated the same way. However the vehicle with ABS will take longer to stop than one without, as the ABS releases pressure on the brakes in order to keep the wheel revolving then reapplies the pressure, whereas full pressure is maintained in the case of the locked wheel.

ABS is not a braking accessory, it's a steering accessory as you can't steer with a locked wheel but you can with a revolving one.

http://www.autoguide.com/auto-news/...houldnt-upgrade-to-carbon-ceramic-brakes.html

Read the paragraph just above the bold heading "Why is Carbon so squishy" if you don't believe me on the tyres and road grip factor.

 

[Ethan]

In the old days when cars were limited to 4mph and you needed a bloke with a flag walking in front, I guess there were fewer fatalities. Perhaps we should return to those days. Can't be too careful.

 

[Deleted member 15344]

Of course, but that same car at 30 may be perfectly capable of stopping in a shorter distance than another car doing 20.

But you can't dictate speed limits based on the supercars that can stop quicker

You can't say - well you have a Ferrari with ceramic breaks so you can go 30 in a 20 because you can stop quicker -

 

[Blue in Munich]

http://www.sdt.com.au/safedrive-directory-STOPPINGDISTANCE.htm

Quite a spread even amongst ordinary cars...

A BMW 330D is 7 metres less than a Suzuki Alto, neither of which are particularly unusual or uncommon cars on our roads.

And when you read the caveat in the paragraph above the table you quote, it concedes that there are so many variables in the test conditions as to make the results worthless;

[FONT=Arial, Helvetica, sans-serif]The table below lists the BRAKING DISTANCE of various cars from 100 km/h. These cars were tested at different locations on different days.

[/FONT]
[FONT=Arial, Helvetica, sans-serif]Be careful comparing results as test results can vary depending on many factors including the road surface, how the speed was measured (as various cars have differing speedometer accuracies), the tyre pressures, fuel load and whether the car had only the driver or had additional passengers.[/FONT]
[FONT=Arial, Helvetica, sans-serif]
[/FONT]

 

[Deleted member 15344]

So do people want to be able to drive quicker ?

Do they want residential areas back from 20mph to 30mph because some figures are about a couple % out

Do people want to drive quicker through residential areas ?

 

[ger147]

But you can't dictate speed limits based on the supercars that can stop quicker

You can't say - well you have a Ferrari with ceramic breaks so you can go 30 in a 20 because you can stop quicker -

I am not suggesting that you can or should do such a thing. I simply don't accept that you can state as a fact that the overall stopping distance for a car from Speed A is X as there are FAR too many variables involved.

 

[ger147]

And when you read the caveat in the paragraph above the table you quote, it concedes that there are so many variables in the test conditions as to make the results worthless;

The table below lists the BRAKING DISTANCE of various cars from 100 km/h. These cars were tested at different locations on different days.


Be careful comparing results as test results can vary depending on many factors including the road surface, how the speed was measured (as various cars have differing speedometer accuracies), the tyre pressures, fuel load and whether the car had only the driver or had additional passengers.

That is my point - there are many variables in the real world which is why I simply will not accept that there is 1 figure for a stopping distance for a car doing speed A. Quite simply, it depends...

 

[Hobbit]

Neither of which support Hobbit's numbers!

Here's the (or at least a) Factsheet relating to the Royal Society for Prevention of Accidents summary that you quoted. It's the link I added to my challenge of Hobbit's numbers.

http://www.rospa.com/roadsafety/advice/highway/info/20-mph-zone-factsheet.pdf

Dorling's paper is actually a proposal/making a case rather than the analysis that the RSfPoA doc is. He also quotes Ashton and Mackay study (of 1979) which was the old (ancient actually) doc I referred to earlier. Those 35 year old figures are now unreliable imo.

Foxy, they're not my figures, they're the SDT figures. If you're that hung up on them Google them. One thing this exercise has shown is that there are a number of sources that quote different figures for survival rates but the figures that are diffinitive is the significant reduction in fatalities where 30's have been changed to 20's. BTW Foxy, they're your figures.

 

[Captainron]

I have a mate who drives like he is ferrying miss daisy about on the motorway. Never gets above 60 but going through villages he maintains the same speed and breaks the limits by 20mph or so. Drives me bloody insane.

I would love all cars to be limited by a satellite system that would adjust the cars limit accordingly but that's not going to happen is it.