"Hit me at 30..." Advert BS

[twister]

BOF said:

What I tell my IAM punters is...

At 30 in a 30 zone, if a kid steps out 75 feet ahead, you will maybe bruise the kid or scare the sh1t out of him...

At 40 in a 30 zone - same kid, same 75 feet, you are still travelling at 27 MPH when you kill the kid...

tongue-in-cheek mode enabled
Ooh, how dare you contradict the government message that hitting a kid whilst doing 30 is OK...
disengage tongue from cheek

Personally as a driver I wouldn't be even doing 30 down a street where there was a chance of someone suddenly running out in front of me, but then neither would I as a pedestrian be dumb enough to suddenly run out in front of someone - that was one of the invaluable "how to avoid killing yourself by taking responsibility for your own safety" lessons drummed into us in the early 80's at primary school over and over again, both through the official safety education visits (Green Cross Code, British Rail, Northumbria/BT Police etc...), and through informal reminders during lessons with certain more enlightened teachers who understood the need to reinforce the official messages.

I think the problem today is that we have a combination of drivers who don't understand the need to slow down except where they see a yellow box on a stick (possibly influenced by the dumbed-down safety messages they've been hit with over the last decade or so) AND a generation or two of pedestrians who seem to think road safety is purely the responsibility of the driver. It's no good just telling drivers to slow down, because stupid pedestrians will still manage to kill themselves if they get in the way of the wrong kind of vehicle (e.g. bus/HGV doing 5-10mph, with you sandwiched between it and some other solid object...)

[streaky]

FishFace said:

It's only confusing if you make it so. The advert shows a child running out into the road. It doesn't appear it's the driver's fault as the child has just suddenly run out in front of him / her. The advert then claims that the child is much more likely to die if hit at 30 MPH instead of 40 MPH.

Seems quite reasonable to me. Children aren't perfect and will sometimes run into the road. They are taught the green cross code in school (at least the ones I know\0.

Obviously it is to do with speed limits so it must be evil and wrong...

Who makes it confusing? - Streaky

Perhaps we could have a sticky where FishFace gets muddled up and practices hyperbole - S

Edited by streaky on Monday 24th September 07:00

[BOF]

safespeed said:

BOF said:

What I tell my IAM punters is...

Why not educate them in the much more useful and realistic 'safe braking zone' instead?

See http://www.safespeed.org.uk/braking.html

Wish I had the time - I have had some at 18 years of age and up to 74 so far...average around 6 drives...some have computers, some not...I do have some print outs from Safespeed which I use.

I use the tools that make some impact on the average punter under the conditions...one day the Government will make the IAM level the minimum legal requirement? We still refund the £75 fee to under 21s and £40 to under 26s out of Group funds.

Driver education is what is needed - what would be the cost of funding further education after, say, 12 months driving compared to the cost of A+E Hospitals and funerals?

BOF.

Edited by BOF on Saturday 22 September 13:12

[s2art]

BOF said:

What I tell my IAM punters is...

At 30 in a 30 zone, if a kid steps out 75 feet ahead, you will maybe bruise the kid or scare the sh1t out of him...

At 40 in a 30 zone - same kid, same 75 feet, you are still travelling at 27 MPH when you kill the kid...

We can debate thinking time, braking time, tyre condition, age of driver,state of car, weather, medication, or whether it is 26MPH not 27MPH...the above is the nearest I have found for general consumption.

BOF

PS - This is long, but relevant...
STOPPING DEAD – another myth exploded

Mike Collins a former RoADA Chief Driving Examiner assembled the following from experiments conducted by the Thames Valley Police.

At an airfield a vehicle was driven in a straight line and at a given point the driver applied the brakes to 95% lock (the most effective). The point P where the vehicle stopped was marked. This exercise was repeated through varying speeds, the driver applying the brakes at the same place each time, and the final speeds at which the vehicle was traveling when it reached the point P were measured and are given below.

Readers may be surprised at some of the results measured. For example, a vehicle which stopped at P when braked from 30 MPH was still traveling at 11 MPH when braked from 32 MPH.
INITIAL SPEED FINAL SPEED
ASSUMING VEHICLE STARTS BRAKING AT 30 MPH
30 MPH 0 MPH
32 MPH 11MPH
40 MPH 26MPH
50 MPH 40MPH
ASSUMING VEHICLE STARTS BRAKING AT 40 MPH
40 MPH 0MPH
50 MPH 30MPH
60 MPH 45 MPH
ASSUMING VEHICLE STARTS BRAKING AT 50 MPH
50 MPH 0 MPH
60 MPH 33 MPH
70 MPH 49 MPH
ASSUMING VEHICLE STARTS BRAKING AT 60 MPH
60 MPH 0MPH
70 MPH 36MPH
80 MPH 53MPH
ASSUMING VEHICLE STARTS BRAKING AT 70 MPH
70 MPH 0 MPH
80 MPH 38 MPH
90 MPH 57 MPH
100 MPH 71 MPH

Edited by BOF on Saturday 22 September 11:30

Actually, if these were real tests this does surprise me. They look like calculated results to me. In the real world, with real cars I would expect far more variation. Some doing a bit better (some a lot better), many doing worse than those figures. So I am skeptical that these numbers are from real tests on real cars.

[Adrianq]

Better still why dont they educate the kids not to play on the roads and cross it safely then hopefully none of them will get hit.

[Jasandjules]

s2art said:

Actually, if these were real tests this does surprise me. They look like calculated results to me. In the real world, with real cars I would expect far more variation.

Given the weight of the car, we could possibly calculate the distances required for each speed (the braking force would be linear as all the same at 95% or whatever, and we can reverse engineer that figure from the distance to stop from 30mph..)..

[andmole]

Jasandjules said:

s2art said:

Actually, if these were real tests this does surprise me. They look like calculated results to me. In the real world, with real cars I would expect far more variation.

Given the weight of the car, we could possibly calculate the distances required for each speed (the braking force would be linear as all the same at 95% or whatever, and we can reverse engineer that figure from the distance to stop from 30mph..)..

The weight of the car is largely irrelevant, (only in so much as it potentially has an influence on deceleration), all you need to know is the maximum braking deceleration, i.e. is the vehicle braking at 1g, 0.9g, 0.8g, etc. Then you can use the equations of motion to calculate the distances quoted, (those figures do look suspiciously calculated by the way).

[s2art]

Jasandjules said:

s2art said:

Actually, if these were real tests this does surprise me. They look like calculated results to me. In the real world, with real cars I would expect far more variation.

Given the weight of the car, we could possibly calculate the distances required for each speed (the braking force would be linear as all the same at 95% or whatever, and we can reverse engineer that figure from the distance to stop from 30mph..)..

95% of lock up is not a constant decelleration for all cars (it would even vary in a single car over a speed range), it depends on many factors, tyres, suspension, road surface etc. And thats not mentioning how difficult it is to achieve precisely 95% lock-up over the braking manouver.

[GreenV8S]

Jasandjules said:

Given the weight of the car, we could possibly calculate the distances required for each speed (the braking force would be linear as all the same at 95% or whatever, and we can reverse engineer that figure from the distance to stop from 30mph..)..

Easily done, and in fact when you look into the physics you'll find the answer is independent of the vehicle weight, it only depends on how grippy the tyres are. For road tyres in ideal conditions you can probably assume Mu (grip factor) of about 1, some tyres will exceed this slightly but only slightly.

[s2art]

GreenV8S said:

Jasandjules said:

Given the weight of the car, we could possibly calculate the distances required for each speed (the braking force would be linear as all the same at 95% or whatever, and we can reverse engineer that figure from the distance to stop from 30mph..)..

Easily done, and in fact when you look into the physics you'll find the answer is independent of the vehicle weight, it only depends on how grippy the tyres are. For road tyres in ideal conditions you can probably assume Mu (grip factor) of about 1, some tyres will exceed this slightly but only slightly.

Actually vehicle weight can be relevent. The idea of friction being independent of weight is over simplified. You have to take strength of materials into account. At max braking the tyre itself is being torn apart (leaving the black marks), weight (mass really) matters here.

[Jasandjules]

s2art said:

Actually vehicle weight can be relevent. The idea of friction being independent of weight is over simplified. You have to take strength of materials into account. At max braking the tyre itself is being torn apart (leaving the black marks), weight (mass really) matters here.

Indeed, we used wieght to determine the co-efficient of resistance at College.. Not sure I even have the formulae any more but I could find them online I am sure...

And yes, I agree, those figures look too linear to account for tyres warming up, changes in the grip of the surface, brakes warming etc.. all of which can have a reasonable impact on deceleration.

Of course, no matter what, ANY set of braking figures are only relevent to the conditions at the time, and the vehicle etc.. as different cars with less weight and better brakes will stop faster (in theory!) and large 4*4s with 2 tonnes to slow down will stop in a greater distance (unless they have super duper uprated brakes etc.)??!

[bryan35]

seen that add. All very sensible and on average I can see the point,

what it misses is the fact that if the girl keeps running out in front of cars, it's inevitable that she's going to get hit by one, and even at 30mph there's a 1 in 5 chance she'll get killed.


it also misses the point that how hard you hit someone is based on Mass x velocity squared, and that in fact even carrying passengers increases the kinetic energy. going to try to do some quick maths...............

(relative values just for comparison)
1000kg car with a 75kg passenger = 1075
velocity = 30mph
30 squared x 1075 = 967500

1000kg car with 75kg driver and 75kg passenger
mass is now 1150
divided into 967500 is 841 (ish)

square root of 841 is 29.

so adding a 75kg passenger is the the same (kinetic energy wise) as adding 1MPH to your speed.

so 4 passengers would be 4MPH. or if they are 14 stone lads and not just under 12 stone, 5MPH to the speed.

ish......... sort of. But you get the idea.

then add in how full the fuel tank is - another 55kg. - nearly 1 MPH.

interstingly, as the velocity is squared, the weight difference is more significant at lower speeds, and yet is ignored but the 20's plenty lot.

[GreenV8S]

s2art said:

Actually vehicle weight can be relevent. The idea of friction being independent of weight is over simplified. You have to take strength of materials into account. At max braking the tyre itself is being torn apart (leaving the black marks), weight (mass really) matters here.

I'd lump all that under 'grip' i.e. coefficient of grip. To a first approximation you can consider it a constant of about 1.0 for a typical car on road tyres in good conditions. If you want to get really accurate you can consider the effect of different tyre compounds and sizes, temperatures and pressures, weight distribution, brake balance, driver skill, variations in road surface conditions etc. But for the purposes of this discussion I don't think you need to go into that level of detail and it's reasonable to pretend that Mu is a constant of around 1.0.

[delboy735]

I'd like to meet the Police driver who was able to apply 95% braking to the car many times over for those results. He must have been some kind of Robot.

Besides, nowadays, surely 100% braking is better now that we have ABS. Indeed, most drivers have never even had the ABS kicking through the pedal at them anyway.

[s2art]

GreenV8S said:

s2art said:

Actually vehicle weight can be relevent. The idea of friction being independent of weight is over simplified. You have to take strength of materials into account. At max braking the tyre itself is being torn apart (leaving the black marks), weight (mass really) matters here.

I'd lump all that under 'grip' i.e. coefficient of grip. To a first approximation you can consider it a constant of about 1.0 for a typical car on road tyres in good conditions. If you want to get really accurate you can consider the effect of different tyre compounds and sizes, temperatures and pressures, weight distribution, brake balance, driver skill, variations in road surface conditions etc. But for the purposes of this discussion I don't think you need to go into that level of detail and it's reasonable to pretend that Mu is a constant of around 1.0.

Sure. My point was simply that Mu will vary with weight. (imagine adding half a ton of load in a pick-up, even if the vehicle is not brake limited it may well become tyre limited).
I would be very interested to now what deceleration 0.95 lock up actually equates to on several cars, and how it could be precisely maintained during the braking manouver. (rather than the calculated figures we have been presented with)

[s2art]

delboy735 said:

I'd like to meet the Police driver who was able to apply 95% braking to the car many times over for those results. He must have been some kind of Robot.

Besides, nowadays, surely 100% braking is better now that we have ABS. Indeed, most drivers have never even had the ABS kicking through the pedal at them anyway.

Probably most ABS systems are only capable of approx 90-95% of max braking. A really skillful driver will do better, but there are not many of them. IIRC the best ABS systems can get close to 100% braking but not many cars have that set-up.

[GreenV8S]

s2art said:

I would be very interested to now what deceleration 0.95 lock up actually equates to on several cars, and how it could be precisely maintained during the braking manouver. (rather than the calculated figures we have been presented with)

It's not something that I've studied in any depth, but measuring back the skid marks after an emergency stop in the TVR indicated that I'd averaged about 1.15 G stopping from around 80 mph. I don't know my initial speed exactly so there is some margin for error, but the estimate is based on typical indicated speeds in that corner with a speedo that seems about right compared to GPS. This is a non-ABS car of course. The tyres were initially warm rather than hot, of course they were quite hot afterwards!

I think that 0.95G is probably the upper limit that you could expect a typical driver who gets everything about right to achieve in ideal conditions.

[s2art]

GreenV8S said:

s2art said:

I would be very interested to now what deceleration 0.95 lock up actually equates to on several cars, and how it could be precisely maintained during the braking manouver. (rather than the calculated figures we have been presented with)

It's not something that I've studied in any depth, but measuring back the skid marks after an emergency stop in the TVR indicated that I'd averaged about 1.15 G stopping from around 80 mph. I don't know my initial speed exactly so there is some margin for error, but the estimate is based on typical indicated speeds in that corner with a speedo that seems about right compared to GPS. This is a non-ABS car of course. The tyres were initially warm rather than hot, of course they were quite hot afterwards!

I think that 0.95G is probably the upper limit that you could expect a typical driver who gets everything about right to achieve in ideal conditions.

The Tuscan S is the best braking car I have ever driven, and that includes several Porsche models. I have never measured it accurately but I guess it gets well over 1G.
Personally I doubt many tin boxes driven by average drivers would get near sustaining 0.95G for the whole braking manouver.
Remember some of the brake tests the guys at Top Gear did? There was a huge variation in stopping distance between different cars.

[BOF]

delboy735 said:

I'd like to meet the Police driver who was able to apply 95% braking to the car many times over for those results. He must have been some kind of Robot.

Besides, nowadays, surely 100% braking is better now that we have ABS. Indeed, most drivers have never even had the ABS kicking through the pedal at them anyway.

The MAIN, if not the ONLY, advantage of ABS is that you can STEER under braking...I have not seen conclusive data that says you stop quicker on dry roads...happy to be corrected...

BOF.

Edited by BOF on Saturday 22 September 15:51

[s2art]

BOF said:

delboy735 said:

I'd like to meet the Police driver who was able to apply 95% braking to the car many times over for those results. He must have been some kind of Robot.

Besides, nowadays, surely 100% braking is better now that we have ABS. Indeed, most drivers have never even had the ABS kicking through the pedal at them anyway.

The MAIN, if not the ONLY, advantage of ABS is that you can STEER under braking...I have not seen conclusive data that says you stop quicker...happy to be corrected...

BOF.

I think some of the very best ABS set-ups will beat most drivers. Remember they banned it from touring car racing (and F1). Of course not many road cars will have those systems.