dtscook’s Recent Comments

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  Re: gobbledygook

  Thanks for the follow-up to my post Laurence. Unfortunately, I don't have all of the necessary data to calculate a true energy life cycle for driving. That's why I did not subtract from the walking calculations anything to do with driving. Instead, I thought it would be most helpful to leave the walking data accurate and wait for someone to come up with accurate driving data.

  In response to the points you made,

  a) Simply subtracting the driver's caloric expenditure  from that walker's is misleading in that driving is much faster than walking. The walking is 3.3 kcal/kg/hr at 3.0 mph. If you change the speed it also changes. Driving on the other hand, we can assume takes the same amount of caloric energy no matter the speed. The maximum driving speed recommended by the EPA for best fuel efficiency is 60 mph. In this case, it takes only one minute to drive a mile, which amounts to 2.0 kcal/kg/hr / 2.2 kg/lbs = 0.91 kcal/lbs/hr. For the person I was considering, we then have 0.91 kcal/lbs/hr * 168 lbs * 1 min / 60 min/hr =
  2.55 kcal burned while driving a mile.
  Subtracting the driver's caloric expenditure  from that walker's would then give 84 kcal/mi - 2.55 kcal/mi =
  81.45 kcal/mi for walking as compared to driving.

  b) This should definitely be included in the calculation, and would bring the real walking efficiency to

  36.37 miles per gge (for the median American man between 20 and 29 walking 3.0 mph)

  c) 135,000 lifetime miles for a car is based on 1983 model-year cars. To be consistent, either the 1983 manufacturing cost should be calculated, or the average lifetime of 2006 model-year cars should be used (I don't know what this is, but I would hope it has improved -- my car has 140k, and I don't want it to die!). I think a good compromise would be to use all data exclusively from the ilea. This found that the fuel cost is about 73% of the energy used.

  d) If we really wanted a comparison between walking and driving any car on the road, we would now increase the efficiency of walking to 81.45 * 7.3 kcal/mi * 3.966 btu/kcal / 114,100 btu/gge = 0.02666 gge/mi =

  57.38 mpg for walking (compared to EPA automotive MPG calculations).

  This would be accurate up to including the fuel, fuel cycle, service, insurance, and manufacture of automobiles. Unfortunately it would neglect, among other things, the cost of infrastructure for automotive travel.

  Since the automotive energy life cycle is still incomplete, I think the most useful (and accurate) thing to do is to note that the median American man between 20 and 29, when all is said and done, gets about 36.37 miles per gge when walking at a moderate pace.

  As far as automotive fuel efficiencies go, they are certainly much less than what the EPA lists if the entire energy necessity is considered.

  At this point, it is quite reasonable to assume that walking is more efficient than driving, but how much I couldn't say.On Walking tall tale posted 3 years, 3 months ago 22 Responses

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  Complete calculations for walking

  Here's what I get when I do the math, please follow up and/or correct:

  At 3 mph, it takes 3.3 kcal/kg/hr / 2.2 lbs/kg = 1.5 kcal/lbs/hr

  For an American man between 20 and 29, the median weight is 168 lbs here.
  So, the median American man then, consumes 1.5 kcal/lbs/hr * 168 lbs = 252 kcal/hr while walking 3 mph.
  Walking at that pace, it takes 20 minutes to walk a mile, so the median American man consumes
  252 kcal/hr * .333 hr/mi = 84 kcal/mi

  Converting this to gasoline gallon equivalents, this would be 84 kcal /mi * 3.966 btu/kcal = 333.144 btu/mi / 114,100 btu/gge = 0.00291975 gge/mi

  = 342.5 miles per gge. Remember this for later.

  Although that number looks great, there is a range of estimates for how much life-cycle energy is required to produce each unit of we in the U.S. consume, but I will use the estimate of Heller and Keoleian (page 42 here: 7.3 units required for 1 unit consumed.

  This means that in eating those 84 kcal to walk a mile, we actually consumed 84 * 7.3 = 613.2 kcal
  Converting this to gasoline gallon equivalents, we have 613.2 kcal /mi * 3.966 btu/kcal = 2431.95 btu/mi / 114,100 btu/gge = 0.02131 gge/mi

  So the average American man between 20 and 29, walking 3.0 miles per hour, uses 46.9 miles per gge.

  However, we can't really compare this to posted automotive fuel efficiencies. Remember the 342.5 miles per gge that I calculated at first? That figure came about from neglecting the total life cycle of the energy used for walking. To make a meaningful comparison, we ought to also be looking at the total life cycle of driving. And when you consider manufacturing, insurance, fuel, fuel cycle, and especially infrastructure, the 33 miles per gallon that a 2006 Honda Civic gets is similarly misleading. I couldn't find a good automotive life cycle analysis, so if anyone knows of one, it would be great if they could post it.

  TysonOn Walking tall tale posted 3 years, 3 months ago 22 Responses