Reduced Speed Limits

pwa
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Re: Reduced Speed Limits

Postby pwa » 1 Sep 2020, 11:27am

mjr wrote:
Pete Owens wrote:No need for speedometers for speed limits to apply. The fact that speed limits do not apply to cycles is just an anomaly - basically due to the fact that it probably didn't occur yo the legislators that we were capable of exceeding them.

No, it's because the limits were introduced in the Locomotive Acts primarily to reduce damaged roads from fast cornering by heavy vehicles and neither bikes nor horses are that heavy. The purpose has changed but that underlying framework hasn't as much.

However, where 20mph limits are applied we should abide by them even if we are physically capable of going faster. We really shouldn't be blasting through urban areas as fast as we can pedal and pose every bit as much danger as a car driven at the same speed.

No, less mass means less danger but it's still a good idea to interpret 20mph limit as a general caution. I rarely get above second gear in the town centre.

I think we can all agree that it is possible for a cyclist to ride too fast for the situation and to put others in danger by doing so. My elderly mother, who uses a "walker" to keep her steady as she goes to the shops, has had youngsters on bikes weave around her at speed in a pedestrianised area, and it would only take a minor miscalculation to bring her down and put her in hospital. My elderly father died after a fall and the complications that followed. So it very much does matter that cyclists keep their speed "appropriate" around pedestrians. I know both of you generally think the same.

Pete Owens
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Re: Reduced Speed Limits

Postby Pete Owens » 1 Sep 2020, 9:32pm

Cyril Haearn wrote:
Pete Owens wrote:..
We really shouldn't be blasting through urban areas as fast as we can pedal and pose every bit as much danger as a car driven at the same speed.

Much less danger, cycle + rider weigh much less than a motor vehicle


While that is true for less massive objects it isn't the case once you reach a similar mass as the thing you are hitting - the amount of kinetic energy transferred cannot exceed that required to accelerate the hit object to the speed of the object colliding with it. When the mass of the two is equal a perfectly elastic collision will transfer all the kinetic energy (see Newtons Cradle https://en.wikipedia.org/wiki/Newton%27s_cradle) When they are unequal it is the mass of the lighter object that matters.

What is critical is the speed of the collision (KE goes up with the square of speed) So in the case of being hit by a vehicle it makes no difference whether that is a bicycle, a car, a truck, an ocean liner or a planet - it is the speed of the collision that matters not the mass.

However that just covers the basic physics of the impact energy. The other important factor when it comes to being hit is the energy absorption. The front of cars are blunt forgiving shapes designed to bend on impact to soften the blow, whereas the front of bicycles are consist of sharp solid metal objects. Getting hit by a cyclist is going to hurt more than getting hit by a car at the same speed.

To understand this intuitively imagine you are trapped on the second floor of a burning building. Your only means of escape are jumping out of a window. You have a choice - under one is a parked car, under the other is a locked bike. Either way you are going to hit the vehicle at the same speed. Which do you choose?

drossall
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Re: Reduced Speed Limits

Postby drossall » 1 Sep 2020, 10:55pm

That's helpful on energy transfer, although of course, in your analogy, the car and the bike both have the extremely massive earth behind them. In a real collision, I think I'd choose the bike because (a) there's a fair chance of actually colliding with the rider, who is a lot softer than either car or bike, (b) the rider may well swerve, resulting in a glancing blow, which is possible but less likely with cars because they are so much wider, and (c) cars tend to hit you, accelerate you to their speed, and then run you over, whereas bike and rider tend to be lying in the road by that point.

But the summary, on which I think we're all agreed, is that cyclists still have a responsibility to others to ride with reasonable caution.

thirdcrank
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Re: Reduced Speed Limits

Postby thirdcrank » 2 Sep 2020, 7:37am

One important argument in favour of reduced speeds in residential areas is the problems some vulnerable pedestrians have in estimating the speed of approaching traffic. This tends to keep them off streets - or in the case of children means their parents/ carers keep them off the streets. I'm only surmising, but I suspect that the unconscious assumption that a cyclist will not be moving faster than motor traffic will add an extra layer to the technical problem of estimating speed.

Put another way, at 30+ traffic speeds, the speed of cyclists is irrelevant to things like children playing out. Not so on residential streets.

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mjr
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Re: Reduced Speed Limits

Postby mjr » 2 Sep 2020, 8:33am

Pete Owens wrote:When the mass of the two is equal a perfectly elastic collision will transfer all the kinetic energy

A road collision is never perfectly elastic. The size of the impact area also matters.

[...] the front of bicycles are consist of sharp solid metal objects.

The leading edge of mine is a soft rubber tyre! Next are a mudguard edge effectively shielded by the tyre and handlebars wrapped in soft cork-like tape. Where's the sharp metal? Are you riding on bare rims? :lol:
MJR, mostly pedalling 3-speed roadsters. KL+West Norfolk BUG incl social easy rides http://www.klwnbug.co.uk
All the above is CC-By-SA and no other implied copyright license to Cycle magazine.

drossall
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Re: Reduced Speed Limits

Postby drossall » 2 Sep 2020, 8:57am

mjr wrote:A road collision is never perfectly elastic. The size of the impact area also matters.

Pete Owens was demonstrating the difference between physics and engineering. In physics, collisions are perfectly elastic, beams have infinite length and zero width, and so on. In engineering, they start to have real dimensions and properties.

However, as Pete's analysis showed, the physics is really fundamental in understanding what's going on. Then you need to apply the engineering to specific situations. I still say that Pete's post was helpful.