Safe Speed Forums

I asked the question in all seriousness - no wind-up intended.
I think you're not quite right about the speed of bikes though, I can get up to 30 on my plain, ordinary bike on the level, albeit for short bursts, and I have been overtaken in my car (undertaken, rather) on a steep downhill by a racing bike - whilst doing 40!

I understand that bikes killing pedestrians are not such rare events in countries with high bike usage - and pedestrians are more likely to walk in front of bikes as well, because you can't hear them coming.

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

I can do 30 on a bike over a short distance without a great deal of difficulty, and I'm not really one of life's great cyclists. It takes a damn sight longer to stop on a bike too (well on my cheapo machine anyway).

Put a GPS on your bike and see if you can average 30MPH over any significant distance. If you can start racing!! My mountain bike spins out around 30 (ie I can not pedal quick enough to make it go quicker). My road bike spins out quite a bit higher (i guess at around 36). The only place I can spin it out for any distance is down some of the biggest hills on the downs.

I do not doubt you can do 30+ down a hill or even on the flat for a few minutes but its so hard that I think you can safely ignore it as a risk to pedestrians.

Another thing about a bike is it is unlikely to accelerate our now mythical kid as effectively as a car or truck. From my years of riding through central London (i used to commute from Barnet to West Hampstead and later Canary Wharf on my mountain bike) I think bikes are at much more risk from everybody else. I have hit pedestrians how walked out and what happens is they spin off and you bounce down the road.

The point is that mass has little bearing on a vehicle vs child or vehicle vs adult accident. The acceleration of the victim and the speed their head hits the ground is what kills.

Given the only way to save the mythical but suicidal kid is to not hit him at 30 it seems speed limits past schools are ridiculous. What is the point of a 30 MPH speed limit when pretty much every kid will be killed at 30? A much better idea would be no parking within 300M of a school and a 20 MPH speed limit applied with a rod of iron. Schools are places where there ought to be both speed cameras and CCTV. No parking means no 4x4s for kids to pop out from behind and 20MPH gives the kids a reasonable chance of survival. I do not subscribe to the schools are only dangerous between 8-9am and 3-4pm weekdays. There are a lot of out of hours events and I do not believe a motorist can know for certain there is nothing going on at any given time.

Did you know the height your head is on a push bike is high enough that if you come off and head butt the ground you will die? Given this is only a few inches higher than where your head is when walking you see you do not need much additional energy to be dead.

If I understand correctly, what Paul was saying was that, if you take the KE of all the vehicles driving around in any period (say a day), the KE given up in crashes will be miniscule, as the overwhelming majority of vehicles (we hope) do not crash. Most of the KE is given up voluntarily by braking. What the relevance of this is I'm not quite sure, perhaps an indirect way of making the point that most accidents occur due to errors other than just speed?

Clearly in the car vs pedestrian situation, KE dissipation is not the issue, as the car gives up very little KE. But if you consider a head on car vs car collision, almost all the KE will be dissipated deforming the cars and their occupants. Don't know what the moral of all this is. Just don't crash in the first place I suppose!

Noob,

The 'moral' as far as I'm concerned is that we must look at the parts of the system that already work well, and build on those if we want long term sustainable improvements.

Clearly KE given up in crashes fails to meet the pattern, and causes a serious risk that we might missallocate resources to the wrong safety factor.

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

This is only true if the vehicle is brought to a complete stop by the person it hits.

You just failed O level Physics.

So - lets see some calculations for the KE of the pedestrian after the impact, at different speeds.

That's pretty useless isn't it? Shouldn't we jump direct to pedestrian mortality at various speeds?



That sort of thing?

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

Another way to think about this is to consider the kinetic energy expended during the collision (this is not the same as the kinetic energy transferred to the pedestrian). The momentum of the system will always be preserved, but the net kinetic energy will be reduced, this difference is the energy dissipated (in a destructive form) during the collision; in this case, the energy which deforms (crushes) the pedestrian. A quick Excel spreadsheet shows that vehicle weight has no significant weighting in the equation.

Yet another way to think of 'mass x speed' is with the mass of the pedestrian The kinetic energy transferred and the destructive energy given to the pedestrian will be in proportion to pedestrian weight (but the extra layer of fat may absorb some of this energy).
So should all pedestrians be forced to lose weight (to increase their chance of survival) just in case they get hit by a car?

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