Safe Speed Forums

Here is something to consider. The killing potential of a vehicle is related to it's mass x speed (momentum or killing units). Lets say for argument that a car weights 1 tonne and a lorry 20 tonne at 50mph the car has 50 killing units and the lorry 1000 killing units. it is therefore very hard for the two to coexist safely on the same road. To achieve parity the car would have to be allowed to travel very fast indeed, or the lorry would have to be limited to about 30mph. But when was the last time you even heard the this vital factor being discussed in the debate on speeding?

Regarding higher speed motorway smashes you might have a point but it's not as straight forward as that. What about motorbikes? By the same token - can they be allowed to do 200mph?

However, given that the mass of a person is almost insignificant compared to that of a vehicle, the vehicle will still be doing roughly the same speed after the collision. I reckon a pedestrian wouldn't be able to tell the difference between being hit by a car or a train at equal speeds!

_________________
Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

KE is almost irrelevant to the road safety argument because the proportion of vehicle KE expended in crashes is almost immesurably small.

We have to ask: "What parts of the system disappate all that KE safely?"

It's mainly appropriate driving.

And: "How do we improve the parts of the system that are ALREADY protecting us from all that KE?"

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

The "killing potential" or put correctly the kinetic energy of a moving vehicle is given by the formula .5 x mass x velocity squared.

Therfore a vehicle going twice as fast as another will have an energy of 4 times the slower one.

Put another way assuming a car weighs 1.5 tons and an artic 40 tons then at 70mph the car will have the same energy as the truck travelling at 13.5mph.

Or, perhaps more usefully, at 40 tonne truck at 40 mph has the same KE as a 1.5 tonne car at 206.6mph

Or a 40 tonne truck at 56mph has the same KE as a 1.5 tonne car at 289.2mph

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Or a 200kg bike at 792mph..........

_________________
Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

So if this rule is generally applied massXvelocity(speed)=killing potential then a 25g bullet at 700mph is only 0.0175 on this rating, or a small container ship (100,000 tonnes) at 0.5mph is a rating of 50,000. I know which I'D rather be hit by!!
Points similar to this - have been aired in the past, normally about vehicle to vehicle impacts, and vehicle stationary object, mass & impact speed.

There are too many variables, that as yet are largely untested. Velocity, Mass, Shape, Size & Density of impact area etc. What is impacted, where, weight etc. One person could be killed outright by a cyclist, and another may walk away from being hit by the number 9 bus. Assigning a 'number' to safety or danger levels is akin to 70=safe, 71=unsafe.

Sorry if I sound as if I'm having a pop at you, I'm not!! Just my opinion.

The stiffness of the impact point also has a big influence, which is why crumple zones are beneficial.

Paul, why do you say ?

Are you referring to the fact that many crashes occur after the vehicle(s) have been braking hard and so lost much of their KE?

Obviously driver behaviour is key in preventing / minimising the impact of a crash in the first place, but surely the KE expended in a crash is one of the main factors determining its severity?

No. In a collision with a pedestrian the car will lose practically no speed as a result of the collision, and will still be possessed of almost all the kinetic energy it had before the collision.
The only kinetic energy dissipated by the actual collision is the proportion it takes to accelerate the pedestrian up to whatever speed, which for a 75kg ped vs a 1.5 ton car is around 5%. And, unless the vehicle has an unyielding flat front, some of that 5% will deform the car's bodywork, and only a portion of the pedestrian's weight will be accelerated - because the ped will likely be somersaulted and so will not reach anywhere near the speed which the car was doing.

Putting it anouther way - two scenarios: 1) A car hits a stationary pedestrian at 20mph, and b) a runner sprints into the front of a stationary car at 20mph. Which scenario does the most damage? Answer: They'll both do the same amount of damage, even though the moving car has very much more kinetic energy than the sprinter.

_________________
Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

Much more. Imagine how much motor vehicle KE in a week must be given up in a planned way (routine slowing for example) compared with that given up in crashes.

And then imagine how much it would be if all drivers shut their eyes for 30 seconds.

We're constantly avoiding crashing...

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Pete317 is correct to say that there is no difference (in energy terms) between a pedestrian running into a car and a car running into a pedestrian. This is due to "The Law of Conservation of Energy". Simply put, the energy of the pedestrian and car combined will be the same after the collision as before, this is how the hanging balls on a Newtons Cradle work.

The point about a crashing car is that the energy available to cause damage is proportional to the square of its speed so that at double the speed you have the potential to cause 4 times the damage.

All is not lost however as all vehicles have a system to transfer kinetic energy into heat, otherwise known as brakes. They also have a device to allow the vehicle to avoid an obstacle (commonly called a steering wheel) and lastly they have a driver who is, hopefully, ensuring that the vehicle is travelling in a manner suitable for the road / conditions. The last point however assumes that the driver is concentrating on observing the road conditions rather than watching out for cameras.

ssilvie defined his "killing units" as momentum, not KE. They are different things. KE though is probably a more important measure regarding the damage dome in a collission.

I do not understand this at all. Maybe I am missing something?

Anyway here is some nice physics for you.....

Think about a motorbike, a car and a truck all hitting exactly the same child at 30MPH. Say the kid weighs 25Kg, the Bike 275Kg (inc rider), the car 1500Kg and the truck 45000Kg. 30Mph is roughly 15M/s so I am going to use 15M/s as the impact speed. For the pedants 15M/s is 33.555MPH, if anybody wants to argue that the diference between 30 and 33.555MPH are going to make much diference then feel free.

Using conservation of momentum we know the total momentum before and after will be the same. The vehicle will be braking but this is not relevant as we are only interested in the absolute instant in which the impact occurs. It does not actually matter if the vehicle is accelerating, braking or remaining at a constant speed.

The bike slows by 1.25M/s or roughly 2.5MPH. Not enough to make the blindest diference. The Kid is still dead.

The car slows by 0.25M/s or 0.5MPH. Not enough to make the blindest diference. The Kid is still dead.

The truck slows by 0.01M/s or 0.02MPH. Not enough to make the blindest diference. The Kid is still dead.

Can we see a pattern in the above? The only way for the Kid to not be dead is to not be hit at 30MPH

Say we assume the childs brain will be accelerated to 30MPH in the distance the head swings and any shock absorbant material in the vehicle. Say 30cm to go from 0-15M/s, thats about 375 M/s or around 40g. Thats going to smart.

For the sake of completeness, how about including a bicycle travelling at 30mph

_________________
Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

You get a bicycle to do 30 MPH for 30 miles and then I will do the maths for you!

Why does it have to be 30 miles?

Because 30MPH is very quick on a bicycle. And to be honest I doubt anybody has been killed by the impact of a bicycle. Oh ok then here you go.....

Bike and rider weigh in at 75Kg, kid weighs 25Kg bike is doing 15M/s. Result is the bike doing 11.5M/s or roughly 23MPH. Kid might survive but I would not bet on it.

Sorry, what I meant was why does it have to be for 30 miles (not 30mph)?

No its m yfault, I was being obtuse. What I meant was there is virtually no way a bike will hit someone at 30MPH. You can probably just do 30 down a steep hill if the gearing on your bike is quite high. So bikes at 30MPH are rare and thus bikes hitting pedestrians at 30 is very rare.

55mph for me (highly geared mouhtain bike with carefully calibrated speedo on a steep incline - and I've got surprisingly strong legs) and that was in...... [edit: probably didn't want to say that]

_________________
Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign