Is this correct?

September 26th, 2012 at 6:15 am by David Farrar

Stuff reports:

Wellington road policing manager Inspector Pete Baird said most buses trundled along the Golden Mile at between 13kmh and 17kmh but getting hit by a 12-tonne bus “is like getting hit by a car at 140kmh”.

Is it? Instinctively I’d choose a bus at 17 km/hr over a car at 140 km/hr? I’ve been run over by a bus, but never hit by one, so I can’t judge off experience.

Someone with a better memory of physics want to calculate the force in each case?

Incidentally the article is on lowering the speed limit in the golden mile from 30 km/hr to 20 km/hr, which I’m fine with.

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51 Responses to “Is this correct?”

  1. East Wellington Superhero (1,139 comments) says:

    Kinetic energy = 1/2*mass*velocity squared. So the faster the vehicle, the exponentially higher the energy it has. But I’m not sure if this is best explanation. A bus would possibly hit you more square when as a car kinda clips, flips, and throws you. So in one sense a bus might be able transfer more energy onto you that a car’s more glancing blow. However, at 140kph a car would probably disintegrate you, whereas is can’t imagiine a bus at 17kph doing that.

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  2. MT_Tinman (3,322 comments) says:

    Slowing traffic down to faster than it currently travels at because Wellington’s pedestrians are too stupid to safely cross the road.

    Fantastic stuff proving once again why newspapers are the best form of entertainment.

    I particularly, given the fact that most vehicles travel below 20KPH now, loved the “We don’t want a racetrack” comment”.

    Priceless!

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  3. Pete George (23,833 comments) says:

    A knowledge of physics isn’t necessary – there’s no time to calculate relative risk when playing on roads.

    Avoid getting hit by any metal brick on wheels.

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  4. Manolo (14,179 comments) says:

    Incidentally the article is on lowering the speed limit in the golden mile from 30 km/hr to 20 km/hr, which I’m fine with.

    Bollocks. Why not 10 or even 1 km/hr instead?

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  5. RRM (10,104 comments) says:

    Kinetic energy of an object is proportional to the mass of the object, and it’s also proportional to the velocity squared of the object.

    But it’s kinda moot IMHO as the Wellington buses are huge, and they’re painted bright yellow, and surely our mothers taught us all at a young age to ALWAYS look both ways before crossing the street?

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  6. Keeping Stock (9,384 comments) says:

    Will Wellington’s Green Mayor introduce a rule requiring buses to have a man with a flag walking in front of them to warn unsuspecting pedestrians, and to avoid scaring the horses?

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  7. Doc (81 comments) says:

    Well, the momentum of a 12 ton bus travelling at 17kph is roughly 56,000 kg m/s

    The momentum of a 1.5 ton car travelling at 140 kph is roughly 58,000 kg m/s

    …so, in pure Newtonian terms, the comparison is valid. (Except that the real world has many other variables)

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  8. RRM (10,104 comments) says:

    Bollocks. Why not 10 or even 1 km/hr instead?

    Don’t be a fuckwit – there’s a proposal and it’s not 1, it’s not 10, it’s 20.

    If you’re in a hurry to get somewhere you wouldn’t be taking Lambton Quay, most traffic along Lambton can’t even do 30 at the best of times so no-one’s freedom is being affected in any significant way.

    But I agree with MT Tinman, the “We don’t need a racetrack” comment in the stuff article is the most retarded thing I’ve read so far today. Pedestrians already friggen rule this town as it is.

    Oh and cyclists who complain about traffic not giving them enough room, and then in the next breath run a red light. Those fukkers are part of the problem too, when are the cops going to start doing something about them?

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  9. Griff (8,419 comments) says:

    You would bounce off a bus at 17kph I have seen someone fall 7 m flat they bounced and walked away
    Hit by a car at 140kph there would be a smear of bits for many meters the acceleration would rip you to pieces

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  10. Rab McDowell (6 comments) says:

    You could do the physics but here is a simpler way of considering it. Imagine the car or bus is stationary and you are moving. You could do it by riding a motorbike and jumping off into the front of the stationary vehicle. OK, do it at 17 km/hr for the bus. Now do it a 140 km/hr for the car. Which one is going to make the bigger mess of you even though neither of them is moving relative to the ground the stand on?
    These trafiic guys who make these idiotic comments in the name of road safety do more harm than good.
    No brainer, in fact you will have no brain left if you hit the car. Most guys here are considering the kinetic energy of the vehicle but their are two parties involved.

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  11. wreck1080 (4,001 comments) says:

    If a fully loaded 747 jumbo jet hit you at 5kph you’d not be hurt at all. Despite the fact that the jumbos total kinetic energy is higher than a car going at 140kph.

    The mass of a person relative to a car or a bus is miniscule in either case so the main factor would be velocity.

    Given the choice, I’d choose a bus @17kph over a car @140kph.

    Sounds like one for mythbusters :)

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  12. Whaleoil (650 comments) says:

    So let me get this correct…the Plod wants to lower the speed limit…to an amount that is higher than the average speed already recorded by the buses…this will achieve what precisely?..other than revenue for sign makers having to change over the signs.

    The fact that people in Wellington are too stupid to see a fucking great big bus and constantly walk out in front of them should be of more concern.

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  13. nasska (12,111 comments) says:

    RRM

    You’re displaying your child like trust in the omniscient authority of the state again.

    Should Wellington’s civic masters grant Mr Baird the legislation he desires he will be back requesting further speed restrictions in a couple of years time….it’s the nature of the beast. The safety of pedestrians, milked properly, will see an expanded Gestapo to rule the wayward Wellington commuter (& Mr Baird’s fiefdom increase).

    I would suggest that this is the start of a five year plan to reduce city traffic speeds to >10kph.

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  14. Peter (1,695 comments) says:

    Get rid of the buses. Send them around the outside and pedestrianise much of the golden mile, as Bob Jones proposed.

    The speed limit argument is a red herring. They shouldn’t even be there to start with.

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  15. fisho (19 comments) says:

    The calculations of kinetic energy are correct, but its far complex than just comparing the two raw values. Neither vehicle is going to transfer all of its KE into a person, or even another vehicle. The coppers poor understanding of basic physics is sensationalizing his argument.

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  16. Simon Arnold (102 comments) says:

    The fallacy here is that all the energy in the moving vehicle is transferred into the stationary pedestrian.

    I suspect (without thinking too hard about it) that because the mass of the car or bus is so much higher than the pedestrian (meaning limited resistance from that source) it is neither here nor there which hits you (although the ability of the car’s breaks to absorb the energy and stop faster from a given speed will be a factor), and it will the acceleration of the pedestrian that does the damage. This is basically determined by the speed of vehicle.

    However if you are hit by another pedestrian (or car hits bus) the resistive forces become important and the mass will make a difference.

    I think.

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  17. Mark (1,502 comments) says:

    having been run down by a car doing 50 kph and ending up with a broken leg amongst other injuries 140 kph is not an option I would like to try. I think you would have more chance of survival with the 17kph bus

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  18. MD (62 comments) says:

    Thanks Simon, I was just writing the same thing. The physics is rubbish and is an indication of the quality of thought going into this proposal.
    You are not stopping the bus or the car, which would involve all that energy. In general terms you will bounce so the energy involved is that necessary to accelerate your body mass to 17 kph over some relatively short time frame. This is considerably less than trying to accelerate the same body to 140 kph over a shorter time period. Think whacking a balloon, despite all the force you may put into your arm swing, very little of that force transfers into the balloon as it easily accelerates to the requisite velocity.
    They are suggesting that most buses are already travelling below that speed so reducing the speed limit won’t make any difference. The question then is do these accidents happen during off peak hours (overnight) when you may actually be able to do the current speed limit. If not the proposals are irrelevant and just another example of stupid bureaucrats leading blind politicians.

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  19. tvb (4,560 comments) says:

    Bus drivers are pigs. They think they can drive any way they like. They power through streets scattering pedestrians everywhere. I saw a bus driver full of passengers jump a red light. They pull out without regard for ongoing traffic. I bet half of them are drunk when they turn up for work after a hard night drinking. They should be drug tested regularly, and tested for alcohol especially after payday. They are a bunch of unprofessional pigs endangering the safety of the public.

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  20. loonybonkersmad (27 comments) says:

    Witnessed an almost fatality today – crossing at the entrance of the bus station (near The Thistle). Green light for buses, woman walks straight out into the path of a speeding bus … which was doing way more than 17 kph. Loud toots, screaming tires, running pedestrian (who was laughing when she made it).

    Bottom line … pedestrians in Wellington are dumb and impatient creatures. This creates a nasty mix when there are long waiting times at crossings for no apparent reason.

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  21. Doc (81 comments) says:

    Yes… for the ‘physics’ argument to be valid, you would need for the pedestrian to be standing with their backs up against a large, immovable object, and for the two vehicles to be fitted with a battering ram (so as to focus it’s total energy into a pedestrian sized point) and drive directly into the person at their respective speeds…

    But never let the facts get in the way of sensationalising a road-safety message ;-P

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  22. davidp (3,585 comments) says:

    I think Pete Baird would be correct if the person weighed far more than the bus and didn’t move much on impact. Take a house for instance… the damage caused by being hit by a 17km/hr bus is going to be much more than the damage caused by a car traveling at the same speed. However, a normal sized person being hit by either a bus or a car traveling at 17km/hr will accelerate from 0 to 17km/hr in a fraction of a second and the damage will essentially be the same (ignoring the slight deceleration of the vehicle caused by the pedestrian, and any differences in the shapes of the vehicle).

    To summarise: Pete Beard is an idiot. No doubt we’ll see his made-up physics featured on a series of television advertisements any day now.

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  23. UrbanNeocolonialist (315 comments) says:

    Better comparison:

    hit by 17km/hr Bus is about 5m/s, about the same as falling from 2.5m onto ground. Painful, but generally not fatal.

    140km/hr is about 39m/s, about the same as falling from 80m up onto a car. Dead almost every time.

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  24. East Wellington Superhero (1,139 comments) says:

    “(who was laughing when she made it).” What a fool.
    What is wrong with these people?

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  25. the deity formerly known as nigel6888 (717 comments) says:

    Are we allowed to laugh or cry about this.

    So how much did the Council waste on its much vaunted bus lane down willis St and Manners Mall?

    Was the justification for the expense the shortening of the time that it took to get buses through town (trip time was the major benefit i seem to recall being quoted).

    Obviously killing and injuring more pedestrians may have had an impact on the BCR. Wonder if all the shops closing on the desert that is now Manners St has had any impact on the town centre economics either.

    But now to add insult to injury (sic!) the council is going to slow vehicles down further.

    Winning strategy that. Anyone else think that having a mad greenie as mayor was a good move?

    For those still undecided, are you enjoying the success of Wellington’s “improved” recycling system?

    How are you finding the new earthquake standards? I hear that inner city apartment owners are deeply impressed, as are owners of older buildings all over the CBD. Just as well Ms Wade-Brown is on the job isnt it?

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  26. scrubone (3,097 comments) says:

    Reminds me of the story of the guy who always carried a fake bomb with him on flights, since the chances of two bombs being on board were astronomicilly small.

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  27. Matthew Hooton (135 comments) says:

    It might be better for the council to encourage residents to look both ways before crossing the road than consider even lower speed limits.

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  28. scrubone (3,097 comments) says:

    The Kinetic energy will tell you about momentum, but it would be the impact speed that counts surley- not how far the vehicle would go after it hit.

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  29. Griff (8,419 comments) says:

    Shit lets get technical
    acceleration is not instantaneous say 0.1m deflection at 5m/s =0.02s
    mass is 80kg
    energy =80kg 0.02s=4000kg/s
    over surface area 1.80 X0.30= 5400cmsquare
    4000kg/s over 5400sqcm=.74kgsqcm ouch not death 100mm defalection is probably broken shoulder pelvis maybe leg or arm

    now do the same with acceleration @40ms
    energy =80kg 0.0025s =32000kg/s
    5,9kgcmsq sledge hammer compared to normal hammer shattered body bits

    happy if you can do the equation better i left school 35 years ago @15 so may be rusty ;lol:

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  30. rouppe (984 comments) says:

    I’m not fine with it. It’s a stupid idea.

    With all the stops that busses do, and holdups at lights and wotnot, I can get from Central Wellington to the end of Courtney Place on foot about 5 minutes after a bus as it is, and if you factor in not waiting for a bus to arrive it gets pretty close.

    Slowing them down to 20kph is dumb. 30kph was borderline.

    All that has to happen is people look before they leap.

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  31. Falafulu Fisi (2,141 comments) says:

    Doc…

    Well, the momentum of a 12 ton bus travelling at 17kph is roughly 56,000 kg m/s

    The momentum of a 1.5 ton car travelling at 140 kph is roughly 58,000 kg m/s

    Doc’s calculation of momentum above is correct. It is the momentum that needs to be calculated for comparison rather than velocity comparison.

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  32. Griff (8,419 comments) says:

    No the transfer of force is not instantaneous the deflection of the body slows it down its the force of the acceleration that does the damage
    I have seen a train hit a tractor at slow speed that was a massive transfer of force and No deflection. When you hit a body it deflects the force is not applied instantaneously. transfer of energy over time and distance=acceleration

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  33. Griff (8,419 comments) says:

    Sorry not acceleration its force is kg m s there is no way the body absorbs all the momentum its the momentum of the body that results that is the force of the impact felt if you transferred 56000kg of force to a 80kg body instantaneously it would go ballistic people don’t they take time to absorb the impact

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  34. Brian Smaller (3,966 comments) says:

    Having now seen three people run over by buses, the bus going 17kph still knocks the person over and that sap then gets run over by a huge wheel with tonnes of weight above it.

    This proposal is ridiculous. The bus lanes were changed to speed up trips through town. I would hazard a guess that trips are already taking longer? Anyone able to comment on that?

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  35. meh (165 comments) says:

    I call bullshit on his initial premise – they have installed the speed displays outside the Telecom building on Willis street and every day I see buses exceeding the 30kph limit, most stick roughly to the limit but I can’t say I’ve ever seen one going 17 or 13..

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  36. Griff (8,419 comments) says:

    put it a simple way how much energy does it take to accelerate a 80kg body from 0 to 5ms in .02 sec ie 100mm deflection so the force is over time
    and a 80hg body from 0 to 40ms in .0025 sec see the difference in energy transfer not all the momentum is transferred the bus or car still moves forward less the energy absorbed by the accelerating body

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  37. Falafulu Fisi (2,141 comments) says:

    Griff…

    No the transfer of force

    I’m not sure what you’re trying to say here, but an object (car) that travels at constant speed (say 140km/hr) has zero force. The acceleration is zero (for constant velocity/speed). You may be thinking of the rate of change in the impulse of the momentum, which is not quite the same thing. The impulse (rate of change in momentum) will be different for different objects’ collisions (be it elastic or non-elastic).

    So, the transfer you’re talking about is momentum and not the force. It is momentum that should be used for comparison and Doc has already done that in his comment above.

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  38. Griff (8,419 comments) says:

    No his equation gives the momentum of the bus that is not the total foce absorb by the body else the bus would stop and the body fly of faster then the original bus speed the force on the body is its acceleration 80kg accelerating to 5ms in .02s if you subtract that energy from the bus you get its resulting speed you know you are thinking pure physics= instantaneous its not it acts over time the body takes time to adsorb the force so its acceleration kg/m/s/s
    Tell you what I will jump in front of a bus at 10kph and you do the same to a car at 60 who will live? according to your physics you will :lol:

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  39. nasska (12,111 comments) says:

    I still prefer Brian Smaller’s more pragmatic view…..having a tyre supporting it’s part of a twelve tonne bus run over you tends to turn speed at point of impact into a relatively academic discussion.

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  40. Griff (8,419 comments) says:

    agreed but its fun to argue with fff he is a phd and forgets reality at times. splat

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  41. ross69 (3,652 comments) says:

    I don’t agree with a reduction to 20kmh. The public may become even more relaxed and blase, thinking that buses aren’t going to be a problem. But they will continue to be a problem if people walk in front of them!

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  42. ross69 (3,652 comments) says:

    The bus that hit Tim Brown was doing about 20km/h. It caused significant injury. He admitted he looked left when he should’ve looked right.

    http://tvnz.co.nz/national-news/bus-collision-victim-praises-luck-5017678

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  43. Steve (North Shore) (4,538 comments) says:

    Who the hell needs Physics? Any mental capacity should be used for avoiding hard things.

    Human flesh is soft, steel is hard. If the human flesh with no brain wants to take on the hard steel thing, is is going to get demolished. Don’t blame the hard steel thing. Maybe you omoebas can now work that out

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  44. Tauhei Notts (1,688 comments) says:

    The problem is one of Wellingtonians.
    A species of mankind that could vote in a queer manner and deprive us of an M.P. with Stephen Franks’ ability and intellectual vigour. What kind of species is that?
    The majority of Wellingtonians are there to bludge off the producing areas of New Zealand. It is that “bludge” mentality that brings to their deluded minds the set
    “the roads belong to ME and every other person should watch where I’m going”
    An old friend said that when he was reincarnated he wanted to come back as “head office”. Every activity he had ever been involved in sent money to head office and the wallahs there developed a mentality where the workers and producers of N.Z. owed them a living.

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  45. orewa1 (410 comments) says:

    Classic case of a big solution to solve a small problem.

    Wellington’s burning safety issue is the new bus lanes. In the old days trams intermingled with pedestrians. Trams were noisy so the danger was lessened. Nonetheless trams were equipped with distinctive gongs which they rang regularly when there were pedestrians nearby.

    So why not do the same for Wellington’s bus fleet – a gong or electric bell with a pleasant but distinctive sound, mounted on the front of the bus, which operates when they are in malls or other confined, narrow or high risk zones?

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  46. tristanb (1,127 comments) says:

    Get those stinking, hulking, downright ugly buses out of what should be a pedestrian mall.

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  47. PaulL (5,450 comments) says:

    Melbourne has trams in their shared pedestrian/tram malls. People don’t seem to die very often. Maybe Australians are smarter? But seems there’d be a fair number of tourists there too, including NZers.

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  48. RRM (10,104 comments) says:

    Interesting suggestion Orewa1.

    Alternatively, tards could just look before they cross the street to see if there are any giant yellow / orange / bright green buses coming?

    I know it’s a radical idea…

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  49. MH (830 comments) says:

    all bodies should travel in a straight line -to the morgue,unless they are actors about the forces of evil. It’s a New Torian law as is Men in glasses shouldn’t make passes at suburbian buses.

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  50. Anthony (768 comments) says:

    Why not bring back the law that all vehicles have to have a man waving a flag walking six paces in front?

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  51. R3 (3 comments) says:

    A bit later than the rest, but some of the physiscs thinking above is good, and some isn’t. Overall the conclusions are broadly right, a 12 tonne bus at 17 km/hr is a much better bet than a 1.5 tonne car at 140 km/hr. And a fully loaded jumbo at 5 km/hr is better than the bus by a significant margin.

    While the mometum of the bus and car is about the same, the energy in the car is about 8.5 times greater than the energy in the bus. In a collision that energy goes into heat, sound and buggering up both objects.

    After a collision, both momentum and energy have to be conserved. If you do the sums on a simple collision scenario, the person will bounce of either vehicle at a little less than twice the speed the vehicle was going. With the car at 140 km/hr the g forces associated with going from 0-280 km/hr in a fraction of a second will be enough on their own to bugger you. Even worse, the more similar the weights of the object the greater proportion of energy gets transferred. For the bus, about 3% of the energy is transferred, with the car its about 19%.

    So 8.5 times the energy at a conversion rate 7 time higher means the car is transferring 60 times more energy to you than the bus. Plus there’s the g-forces, the fact that you are way softer than the car, mitigated only by the fact that as someone pointed out you might be “lucky” enough to be flipped over the car rather than having a square on collision.

    Give me the bus over the car any day.

    As for the jumbo jet at 5 km/hr its a lot heavier at 400,000 kg loaded up. It has about 3 times more energy than the bus at 5 km/hr, but because its significantly heavier than the person a lower proportion of energy is transferred (0.1%). So even in the worst case where you manage to get yourself in a position to be hit by the plane nose on, its much better than even the bus. You also have the satisfaction of slowing the jumbo down by a whole 0.002 km/hr.

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