Speeding in a thirty limit

[ger147]

Er. No! There's a formula. The fastest a moving car can reduce velocity (actually Kinetic Energy) to zero (minimum distance) is a function of the Friction between the Road and Tyres, the (square of the) Velocity and the Mass of the vehicle (that's the laws of physics that BiM is referring to). More efficient brakes/braking will mean any extra distance is minimised. Having a lower initial velocity is the best way to significantly reduce stopping distance.

So all cars have the same mass? Thought not.

And all brakes being properly maintained is an assumption which is not the case in the real world. Physics will not prevent an Ariel Atom with perfect brakes and brand new tyres stopping in a shorter distance from 30 mph than a 15 year old Range Rover with a leaky slave cylinder and tyres which have seen better days from 20 mph.

 

[Deleted member 15344]

Read the Factsheet that I linked to and you will see how

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

It also provides the answers as to why you were wrong! I welcome any correction when I am wrong and am prepared to admit it when I am. The same doesn't seem to apply to you, though you are not unique!

And the chart on Page 1 provides the evidence CheltenhamHacker seems to be looking for - though, imo, it's pretty self evident in this case.

Here is a little story for you

About 8 years ago I was walking to my mates through a residential area which was still a 30 -

A little girl about 100 yards in front of me ran out into the road chasing her dog - she was 7

She got hit by a car doing 30mph - the driver reacted and skidded but still hit her , we all tried our best at the scene but it was clear she had a head injury as well as legs being broken

Unfortunately the little girl passed away - it was my mates little girl.

I had to go to the coroners as witness and listened to all the reports

The driver wasn't breaking the speed limit but they worked out that he hit the car at 24 mph

And that she died of a brain trauma because the force of the collision through her onto the floor where she hit her head.

The police gave a report that the driver reacted normally around a second and conditions were good on the road

They also did a study that if the driver was doing 20mph there was a 80% chance that she would have survived because the collision speed would have been well under 10mph - maybe not even hitting the poor girl

I sat there and watched the driver face who was in bits - he was doing the speed limit but still killed someone

They did a study of that road and within 3 years they introduced a 20mph as well as speed bumps on corners and a chicane.

Thankfully since then no one has been killed - a person has been hit by a car but the car was doing the correct speed limit and he survived with a badly bruise leg and broken ribs

I have no doubt if the limit was 20 mph my mates little girl would be getting ready to do GSCE's right now

So I don't care about any reports you want to post to try and prove a point to ensure you make sure everyone is wrong I sat there and listened to a coroner and other state that a little girl would be alive now if the speed limit was 20mph

So if reducing the speed limit in urban and residential areas saves just one child's life then it has my full backing because I have seen the devastation caused by someone getting knocked over

Now you can abrasive tell me I'm wrong - but that will just show your character or you can act a bit better and say "sorry I see your point but I will have to disagree with some points" you might get a better response

Enjoy your day

 

[Smiffy]

The limits are lowered so that if someone gets hit at 20mph they are more likely to survive than being hit at 30mph

Incorrect.
The limit has been lowered to 20 as it has been proved that four scousers can be trained to remove a set of alloy wheels at this speed.
At 30 they struggled, and only managed one.

 

[Deleted member 15344]

Incorrect.
The limit has been lowered to 20 as it has been proved that four scousers can be trained to remove a set of alloy wheels at this speed.
At 30 they struggled, and only managed one.

Cheeky bugger

 

[PhilTheFragger]

I don't think that anyone would disagree that a collision at 20 mph will have a better outcome than one at 30 mph

But I'd like to turn it around and look at it from another angle and forgive me if I appear to be playing devils advocate.

A collision is caused when 2 objects traveling at variable speeds and in different directions ( forget rear Enders in this instance) meet at a specific point in time.

Now altering the variable speeds will effect the position of the vehicle at this key point in time.
Decreasing speed will put the vehicle further back on the timeline and the collision may be avoided

However the collision may also be avoided if the vehicle is traveling faster as the vehicle would be beyond the collision point on the same timeline.
Additionally a slower vehicle is in the danger ( residential) area for longer than a faster vehicle , therefore there is more time for a collision to occur.

Now I agree that this thought process is a bit left field and I'm not putting it forward for serious discussion, I'm just making a point

Unfortunately bad stuff happens, we have some of the safest roads anywhere and you cannot ever eliminate all risk
I can see the rationale for a 20 limit outside schools etc but think that 30 is ok for residential areas.

I'd also like to see the limit increased on motorways to 80

 

[Imurg]

I'd also like to see the limit increased on motorways to 80

Some problems with that..
A lot of the time 80 is relatively safe but..
With the limit at 70, if you stick to it, you rarely get out of the inside lane as everyone else is doing 80
With the limit at 80, the majority will do 85+.
As everyone's going the same direction, there shouldn't really be any incidents at all, barring blow-outs/breakdowns - but there are!
At higher speeds there is less time to react to incidents so it follows that there is a likelihood for more.
You also have the problem of HGV's. Although limited by law to 60, many, especially on inclines, can't do that speed. Many drivers approach HGV's and leave their overtaking way too late, either having to pull out sharply or brake heavily.
Add another 10mph to approach speeds and it's easy to imagine the outcome.

 

[Foxholer]

I don't think that anyone would disagree that a collision at 20 mph will have a better outcome than one at 30 mph

But I'd like to turn it around and look at it from another angle and forgive me if I appear to be playing devils advocate.

A collision is caused when 2 objects traveling at variable speeds and in different directions ( forget rear Enders in this instance) meet at a specific point in time.

Now altering the variable speeds will effect the position of the vehicle at this key point in time.
Decreasing speed will put the vehicle further back on the timeline and the collision may be avoided

However the collision may also be avoided if the vehicle is traveling faster as the vehicle would be beyond the collision point on the same timeline.
Additionally a slower vehicle is in the danger ( residential) area for longer than a faster vehicle , therefore there is more time for a collision to occur.

Now I agree that this thought process is a bit left field and I'm not putting it forward for serious discussion, I'm just making a point

Unfortunately bad stuff happens, we have some of the safest roads anywhere and you cannot ever eliminate all risk
I can see the rationale for a 20 limit outside schools etc but think that 30 is ok for residential areas.

I'd also like to see the limit increased on motorways to 80

The above has been considered in at least 1 study I have seen - I could dig it up, but suggest you do that yourself.

From memory, it concluded that while there can be occasions where additional speed might avoid a collision, these are rare and almost never in the situations where a 20mph area would be considered. They are, by their nature, always in circumstances where one of the parties has sufficient warning to take evasive action. That's almost invariably not the case in the candidate zones - around Schools etc.

And while the likelihood of an event may be reduced linearly, the effect of a collision is increased exponentially (to the square of the increased speed). And that's without considering the point at which survival of an 80lb human is unlikely when hit by a 3500lb metallic object!

 

[PhilTheFragger]

Yeah I know, that's why I said it wasn't a serious suggestion, just looking at it from another angle

 

[Deleted member 15344]

I don't think that anyone would disagree that a collision at 20 mph will have a better outcome than one at 30 mph

But I'd like to turn it around and look at it from another angle and forgive me if I appear to be playing devils advocate.

A collision is caused when 2 objects traveling at variable speeds and in different directions ( forget rear Enders in this instance) meet at a specific point in time.

Now altering the variable speeds will effect the position of the vehicle at this key point in time.
Decreasing speed will put the vehicle further back on the timeline and the collision may be avoided

However the collision may also be avoided if the vehicle is traveling faster as the vehicle would be beyond the collision point on the same timeline.
Additionally a slower vehicle is in the danger ( residential) area for longer than a faster vehicle , therefore there is more time for a collision to occur.

Now I agree that this thought process is a bit left field and I'm not putting it forward for serious discussion, I'm just making a point

Unfortunately bad stuff happens, we have some of the safest roads anywhere and you cannot ever eliminate all risk
I can see the rationale for a 20 limit outside schools etc but think that 30 is ok for residential areas.

I'd also like to see the limit increased on motorways to 80

Well I guess it depends on how much faster you want people to go - yes if he was doing 50 he would have been well past - 40 and the poor girl prob would have walk straight in front as opposed to giving him a second to break


So you can see the limit outside schools for kids - but aren't residential areas full of kids ?

Does going 10 mph slower really effect people's journeys ? Arriving 30 secs to 1 min later that much of a deal

Is it that harmful to people to just go a little slower through housing estates just in case that little boy or girl runs into the road

Is arriving home that minute quicker worth it ?

And increasing the speed limit on motorways ? Won't that just snowball speeds ? Won't we then see people flying along in the 90's and 100's

Why the rush ? Why the need to go quicker in a car when the difference is minimal

 

[Blue in Munich]

Er. No! There's a formula. The fastest a moving car can reduce velocity (actually Kinetic Energy) to zero (minimum distance) is a function of the Friction between the Road and Tyres, the (square of the) Velocity and the Mass of the vehicle (that's the laws of physics that BiM is referring to). More efficient brakes/braking will mean any extra distance is minimised. Having a lower initial velocity is the best way to significantly reduce stopping distance.

Foxholer, I believe you are mistaken on mass being involved in the equation, unless I'm forgetting something; I'd confirm it but the necessary notes are buried in the loft somewhere.

So all cars have the same mass? Thought not.

And all brakes being properly maintained is an assumption which is not the case in the real world. Physics will not prevent an Ariel Atom with perfect brakes and brand new tyres stopping in a shorter distance from 30 mph than a 15 year old Range Rover with a leaky slave cylinder and tyres which have seen better days from 20 mph.

Physics will not prevent anything, but the laws of physics will dictate how quickly either vehicle stops. Yes in your highly skewed example the Ariel Atom may stop quicker than the Range Rover, but if you have to skew the facts that far you haven't really got a cohesive argument have you? A vehicle with brakes will stop quicker from any speed than a vehicle without brakes…..

 

[Foxholer]

.....
Does going 10 mph slower really effect people's journeys ? Arriving 30 secs to 1 min later that much of a deal

Is it that harmful to people to just go a little slower through housing estates just in case that little boy or girl runs into the road

Is arriving home that minute quicker worth it ?

And increasing the speed limit on motorways ? Won't that just snowball speeds ? Won't we then see people flying along in the 90's and 100's

Why the rush ? Why the need to go quicker in a car when the difference is minimal

You really can't see past the extremely blunt instrument that is 'limit' can you! If you as concerned about it as the vast number of (repetitive) posts indicate, I suggest you do some reading of some of the studies rather than simply regurgitating the same argument all the time. Did you check that link I provided earlier?

Indeed, going faster does not affect travel time unduly in residential areas - and reduced speed can actually improve travel times in some circumstances! But increased speed on long distances does make a significant difference - up to almost 20% I believe. Apply that to your 4 hour round of Golf and it becomes not much over 3 hours!

 

[guest100718]

Is it ok to speed on the motorway if I need to get to the pub that has just opened in the services?

 

[Deleted member 15344]

You really can't see past the extremely blunt instrument that is 'limit' can you! If you as concerned about it as the vast number of (repetitive) posts indicate, I suggest you do some reading of some of the studies rather than simply regurgitating the same argument all the time. Did you check that link I provided earlier?

Indeed, going faster does not affect travel time unduly in residential areas - and reduced speed can actually improve travel times in some circumstances! But increased speed on long distances does make a significant difference - up to almost 20% I believe. Apply that to your 4 hour round of Golf and it becomes not much over 3 hours!

Read back on my earlier post where I explained exactly what I think of studies and prefer to listen to expert witnesses in a coroners
case - I have no interest in your studies

And yes in circumstances a journey can be reduced if you go quicker. I'm going to guess those circumstances will never really exist in our roads

Here is another story for you from a copper

A policeman had a call about an accident and they needed two cars in attendance - one was a high powered car the other was a land rover restricted to 70mph - the journey was about 9 miles maybe a bit more

High powered car when flying off at over 80 mph but due to our roads was obviously having to slow down all the time because of other users - he arrived at the scene and 10 secs behind him arrived the Land Rover he did a steady just under 70 all the way

A sat nav calculates a journey using national speed limits - ignoring traffic incidents how times has anyone ever really got to a destination more than 3-5 mins quicker than sat nav ? I haven't

If people really have the need to go quicker then have track days , go live in Germany

I would actually be happier if they reduced the limit to 60-65mph - along the same lines as continental Europe ( bar Autobahn )

There will very rarely is ever be an occasion where you "have" to break the speed limit - it's all done by choice

 

[stevie_r]

Indeed, going faster does not affect travel time unduly in residential areas - and reduced speed can actually improve travel times in some circumstances! But increased speed on long distances does make a significant difference - up to almost 20% I believe. Apply that to your 4 hour round of Golf and it becomes not much over 3 hours!

Care to explain how travelling slower can improve your travel time?

Increased speed on longer distances makes a significant difference- and in other news, Sky grey grass green.
Your 20% figure is plucked from the air and fairly meaningless, yes it's accurate if you average 72 instead of 60 or 60 instead of 50; it isn't if you average 70 instead of 50.

 

[guest100718]

what are you all arguing about?

 

[Pin-seeker]

what are you all arguing about?

Time travel I think
Just don't question it!

 

[stevie_r]

I would actually be happier if they reduced the limit to 60-65mph - along the same lines as continental Europe ( bar Autobahn )

Why would you be happier? Our motorway speed limit is years old and based on the standard of car at the time. The motorway limit could actually be raised to about 85 without any appreciably greater risk.

 

[Foxholer]

Foxholer, I believe you are mistaken on mass being involved in the equation, unless I'm forgetting something; I'd confirm it but the necessary notes are buried in the loft somewhere.

Kinetic energy. E= 1/2MV**2. M is Mass. So Mass is definitely 'involved'. As the Mass of the vehicle also contributes to the Friction, I agree that it's not a 'simple' linear link though (I don't believe I actually stated that it was, if I did, I).

Should be pretty intuitive too - Think FT=MV (or its integration across the stopping period). Increased Mass requiring more effort to stop - so larger brakes for more leverage. Imagine a Fiesta with Range Rover brakes! Or, more frighteningly, the other way around!

Don't you just love the way Physics (and Maths) can describe 'everything'!

 

[Blue in Munich]

Here is another story for you from a copper

A policeman had a call about an accident and they needed two cars in attendance - one was a high powered car the other was a land rover restricted to 70mph - the journey was about 9 miles maybe a bit more

High powered car when flying off at over 80 mph but due to our roads was obviously having to slow down all the time because of other users - he arrived at the scene and 10 secs behind him arrived the Land Rover he did a steady just under 70 all the way

Or the driver of the Land Rover was a more competent driver than the driver of the car.

As to the 20mph limit, I have no issue with it if it is used as it was intended to have been used; in areas where there is a particular risk to vulnerable road users. Unfortunately my experience is that it is being used as a blunt instrument in what I would consider to be an unnecessary manner, and in certain instances a manner which actually leads to the speed limit being disrespected. The guidance that I am familiar with is this;

http://assets.dft.gov.uk/publications/circular-1-06/circular-1-2006.pdf

Before anyone tells me it has been updated, I am aware, but this is the version that was in place when a number of the ones I have issue with were put in, hence reference to it.

If this was adhered to fine, but there are too many examples of abuse of it or inappropriate use for my mind to be supportive of blanket 20 mph limits. n the right place and engineered in the right manner yes, but there should be nothing wrong with 30 mph as a limit in most residential areas if the drivers adhere to it as a limit AND drive appropriately for the prevailing conditions, and this document on speed limits echoes that.

 

[Robobum]

Its hard not to let your speed drift up a bit, especially when trying to read this thread whilst driving