CityCyclingEdinburgh was launched on the 27th of October 2009 as "an experiment".
IT’S TRUE!
CCE is 15years old!
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For drag:
the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW)
(from the Wikipedia drag article)
By extrapolation (a dangerous thing!):
At 20mph -> 0.64 horsepower to overcome drag
At 30mph -> 2.16 horsepower to overcome drag
Nice! Now who's going to collect all this into one concise and coherent debunking EEN comment... is there any point?
I wonder if it's something for that cycling myths / fallacies website...
Definite scope creep if cycling fallacies started going into driving fallacies
It's more 'fallacies about things that affect cyclists' though. Not toooooo creep-y.
@edd1e_h not that it matters but (being bored at work) I get I get 0.5Kw for 20mph and 1.7Kw for 30mph.
The Wikipedia figures seem to be based on a CdA (drag coefficient * effective drag area) of about 0.5 which would would be a reasonably aerodynamic car, I'd say more representative figures would be 0.6Kw and 2.2Kw respectively.
Either way, the difference is pretty marginal given all the other losses in a car. It's only when you get up to higher speeds that the it starts to be noticeable.
Interestingly, from a quick search CdA for a cyclist seems to be in the range 0.2 - 0.7 depending on position. Higher than a lot of cars if you're sitting up on the bars.
@urchaidh
Nice.
What fluid density (rho) did the wikipedia article assume?
Answered my own question:
I guess it would be 1.2kgm3 for air @ ~20degC and 1 atm pressure
I make the CdA to be 4.67 based on:
v = 13.89ms-1 (50kph)
rho = 1.2 kgm3
power = 7500W
Did I go wrong somewhere?
OK, now I get:
v = 22.22ms-1 (80kph)
rho = 1.2 kgm3
power = 7500W
CdA = 1.14
Still not right...
Time to go home
You could submit it to the Cycling Fallacies website and let them decide if it's off topic or not...
Talking of off/on topic, I was at the 20's Plenty conference today. The chap from Transport Scotland was talking about the Scottish Government's encouragement of 20mph zones and how they've updated the guidance but it's up to individual councils to decide and some are making progress while some are very anti. When it came to questions from the floor, an official from Glasgow asked why not make 20mph the default speed limit for urban streets and then make councils decide which roads *shouldn't* be 20mph. That way the onus is on objectors to make the case for faster speeds - and the cost is much lower. An official from Moray then chipped in to say they'd love to go to 20mph but they can't justify it because it costs too much and there aren't enough casualties to make it a high priority. And there was a fair amount of nodding from the other officials in the room. You could say that it's abdicating responsibility on the LAs part to basically be asking central government to take the issue out of their hands - but it was an eye opener how much these things cost, if each council has to do its own 20mph policy individually, with separate consultations, pilot exercises, advertising campaigns etc.
Obviously the officials whose councils are willing to send them even to a free conference on 20s plenty (each LA got a free place to attend) will be the more enlightened ones, but even so it was interesting ...
"You could say that it's abdicating responsibility on the LAs part to basically be asking central government to take the issue out of their hands"
Or you could say it's the SG's job to take the lead especially now that there is evidence that LA's want it...
OK, final OT-ish post on power calcs:
I made a mistake converting from force to power (I multiplied again by speed in mph rather than m/s). So, I now get I get 0.25Kw for 20mph and 0.86Kw for 30mph. (CdA = 0.55)
@edd1e_h - you're calculation looks correct I think, the numbers on the WP page match with a CdA of 1.14. The source document referenced from the WP page [16] that I looked at seems to use a CdA of 0.55.
WP Reference
"
Diesel emissions dominate urban road pollution with about ten times the toxicity of petrol fumes. As 20mph limits reduce the most toxic diesel fumes, setting a wide 20mph limit is equivalent to removing nearly half of all petrol cars. This massive air quality and public health gain must be implemented urgently and nationally to reduce the 30,000 annual UK deaths from air pollution.
"
"
The agency said last year discussions had taken longer than expected because of “numerous questions” raised by local people. Councillors in Largs objected to the plans as “unreasonable”. 20’s Plenty said Transport Scotland was taking the wrong approach in Langholm. Director Rod King said: “If you make the main road 20mph, it makes sense to make all streets the same. There is a mismatch with keeping side roads 30mph.”
Neil Greig, of the IAM RoadSmart motoring group, said: “20mph limits were unlikely to have made any difference to safety or congestion in these towns.”
"
"Neil Greig, of the IAM RoadSmart motoring group, said: “20mph limits were unlikely to have made any difference to safety or congestion in these towns.”
Is this guy a full time clown or only when he speaks about driving.
"where the simple fix of a pothole etc, etc could prevent most Road Traffic Incidents"
I think that might be my favourite comment ever. I'm not sure I've ever witness bad or dangerous driving that could be blamed on a pothole.
Neil Greig, of the IAM RoadSmart motoring group, said: “20mph limits were unlikely to have made any difference to ... congestion in these towns.”
Glad he can see that there is no effect on congestion and therefore no reason to object...
"I think that might be my favourite comment ever. I'm not sure I've ever witness bad or dangerous driving that could be blamed on a pothole."
Reminds me of a comments from someone at an old workplace, when we were having a discussion about injury rates and I pointed out the number of pedestrians injured and killed by drivers compared to by cyclists, and got the gem response (delivered in all seriousness with a straight face) of, "Yes, but how many of those accidents were caused by drivers avoiding cyclists doing something wrong?".
This may be an opportune point to present some figures. Having lived in a 20 zone for many years I was sufficiently curious, when the pollution aspect was raised on here some months ago, to conduct an experiment now that I have a vehicle that gives an instant readout of consumption.
Being an old-fashioned sort of cyclist I am as tight as that oft-quoted part of a duck's anatomy and always keen to maximise my MPG figues. Over the years, with a variety of vehicles, I have always managed to better published road test consumption figurs by a reasonable margin. I know how to drive economically.
Using a quiet stretch of road I compared consumption figures at a steady 20 and 30 mph and repeated the exercise several times to confirm the results using the highest gear the car would comfortably cope with at the speed.
Being the sort of chap I am I have checked my speedo using two speed/distance calculating apps and found that the speedo overreads but the mileometer under reads. In any case an error here would be near enough equally represented at both speeds.
Caveats are that the figures apply only to my vehicle (though I see no reason not to regard it as typical) and that a steady 20 and 30 does not necessarily accurately reflect driving with those speeds as a maximum, though it was the only way to get a replicable comparison, but probably shows the general picture.
The facts? Travelling at a steady 20 used one third more fuel than at 30. Make of that what you will but it does seem to reflect the propaganda of governments of all hues to engage top gear as soon as possible. Initially introduced at the time of the Seventies oil crisis this remained the advice for decades. Whilst it may no longer be the current advice it will have stuck in the minds of many motorists.
An interesting side effect of this advice is that many motorists tend to drift above the thirty as it really needs a lower gear to easily stay within that limit in a modern car. I am lead to believe that motorists elsewhere, particularly Scandinavia, do not have the same fixation with being in top gear all the time.
@Blueth - thanks for this - very interesting and certainly appears to add credence to anti-20mph arguments to which I object. I don't doubt your figures at all, but there is one subjective part to your test which is when you say "using the highest gear the car would comfortably cope with at the speed" - I am absolutely sure you are right in your choice. However - the argument to which I objected was that about rpm at those speeds - I'd be interested to know if the rpm was roughly the same.
The other point that urchaidh and edd1e_h have been making pretty well, is that normal driving up to 30mph in normal traffic involves more acceleration and hence energy use that driving up to 20mph in normal traffic.
Thanks for the report of your tests though - I stand corrected.
I'm no expert on rpms or fuel consumption, but doesn't your testing just confirm the criticism of most models on emissions/fuel consumption? ie that they model/test driving on a quiet stretch of road, rather than the usual stop, go that is the hallmark of most city driving, even at quiet times of the day?
"they model/test driving on a quiet stretch of road, rather than the usual stop, go"
I think the point of Blueth's test is to show that people who say 'my car pollutes less at 30 than 20' are talking complete nonsense.
Factor in the urban stop-start reality, with frequent acceleration, the figures are likely to be even more stark.
@chdot - actually Blueth is arguing that driving at a steady 20 uses more fuel than 30 so it's not showing that they are talking nonsense.
you and SRD is completely correct though - a point I also tried (not very well) to make.
"actually Blueth is arguing that driving at a steady 20 uses more fuel than 30 so it's not showing that they are talking nonsense"
Oh, my brain not believing my eyes (confused by all the previous talk of aerodynamic efficiency etc.)
Does seem strange!
Of course there is also the separate issue of pollution - diesel particles and also the efficiency of catalytic converters - esp whether or not they work over short distances from cold.
Anyway 20mph is more about safety - people suffer less damage if hit at a lower speed AND are less likely to be hit - plus more willing to walk and cross roads.
The answer is of course fewer journeys by motor vehicles, but...
I'm not sure that fuel consumption/pollution is the best argument to further a 20mph limit, because results can be ambiguous as @blueth showed. It would be best to concentrate on the safety aspect of lower speeds.
The target to reduce pollution should be fewer vehicles, and moving the existing ones from diesel to petrol to electric to bikes! Not an insignificant challenge.
I was trying to work out mentally and then on a back of an envelope and then on a spreadsheet what the actual journey time impact of changing a speed limit from 30mph to 20mph on a given stretch of road would actually be. Specifically I had the 1.6mile rat run in Holyrood Park from the Commonwealth Pool gate to Meadowbank gate in mind.
I was of course aware that it's not just a simple 1/3 increase in time as the limits (and having to pass through a number of roundabouts) mean it's not a case of a journey of a constant 20mph vs. a constant 30mph.
Without a car to actually test these things out for myself I got reading and found out something called the "Time Saving Bias" which is a sort of collision of maths and psychology about peoples' perception of journey times when it comes to changing the speed they are driving at.
I read a couple of the papers that I could actually access without a university journal subscription and the Wikipedia article seems largely lifted from these; https://en.wikipedia.org/wiki/Time-saving_bias
Anyway it's quite interesting and there's a formula in there to calculated the actual change in journey time when the speed is changed;
t=cD (1/V1 – 1/V2),
where c is a constant for converting between units, D is the distance, V1 is the original speed and V2 is the new speed.
This formula shows that the relationship between increasing speed and journey time is curvilinear: a similar speed increase would result in more time saved when increasing from a low speed compared to a higher speed.
So to put it simply, the faster you go the less effect on your journey time you are having.
Now one of the consequences of this all is;
Drivers who underestimated the time saved when increasing from a low speed or overestimated the time lost when decreasing from a high speed, overestimated the speed required for arriving on a specific time and chose unduly high speeds, sometimes even exceeding the stated speed limits.
So at lower speeds, people are over-estimating how fast they need to go and end up driving faster than they need to.
Of course, I haven't yet worked out how to realistically model the effect on actual (and not just straight line, theoretical) journey times, accounting for slowing down for roundabouts and acceleration time again.
But the answer isn't 96 seconds different as the simple formula suggests.
As the recent emissions scandal shows - modern exhaust systems with Particulate filters , standard catalytic converters, NoX trap catalytic converters, and Urea ("blue") catalytic converters and various other things are incredibly complicated. In theory the modern versions shouldn't be performing regeneration phases at 20 or 30. VW got in trouble for altering the regeneration phases when it detected it was on a test rig, reducing the emissions. In reality all manufacturers do that as you don't want your emissions to be high in low speed and hence probably built-up environments. Also as you rightly say - some elements of the exhaust system only work when (very) hot.
Older diesels less complicated, and in general kick out the most particulates when the turbo is spooling up so requested versus actual boost is insufficient leading to the cloud of black smoke you sometimes see behind vans when they accelerate.
The irony for me is that an "efficient" diesel engine in the sense that it accelerates and drives well, smokes like a chimney - look at motorsport diesels. In order to properly reduce the particulates and NoX emissions on a diesel you have to stop it working "efficiently" - i.e. reduce the heat from the turbo, then increase it. You get less NoX when you have more particulates, and you can use the particulate emissions to reduce the NoX in the NoX trap catalytic converters. You'll never get a diesel which uses little fuel and produces low emissions - in order to get one you have to get rid of the other.
In my opinion diesels are just an outdated form of engine, however ingenious the companies get at hiding their emissions. This coming from someone who owns a diesel campervan which is practically part of the family.
@algo, The trouble with diesels is that they are currently cheaper than petrols on a per mile basis which is all the majority of consumers care about. There is a large amount of education needed before an average car buyer is aware that there is more to emissions than CO2.
Unfortunately unless the government increases the tax on diesels to reflect the higher emissions there is unlikely to be any change and given the amount of the "transport" lobby have invested in diesel trucks that is unfortunately going to be a very hard thing for a politician to do.
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