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4320Re: [carfree_cities] More pollution from slower traffic

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  • Chris Bradshaw
    Feb 7, 2002
      Andras Toth wrote:

      > Is it true or false that a car trapped in a traffic jam or slowed
      > down by sleeping policemen pollutes more on a given distance than
      > a car doing the same journey at an optimal speed/rpm whatever?
      > If true, how significant is the difference?

      Perhaps the problem is the question. A better one might be, If the top
      speed were held down and more stop-and-go introduced to the driving
      experience, would fewer people choose to drive, and would the use their
      car in a more efficient manner?

      The argument your question implies is part of a
      favour-the-filthiest-mode thinking: whichever road users pollute the
      most, given them a favoured postion on the road. In the name of
      "safety," road engineers also argue that vulnerable road users must
      yield to motorists who, after all, could kill them. In both cases, the
      pedestrian and cyclist are restrained and, surprise-surprise, over time
      their numbers decrease, with many of those trips switching to driving,
      making the congestion-caused delays greater. Please note that there is
      an ecology at work here.

      Speed and delays are a product of conflicts on the roads caused by a
      demands for the road's use. If you could make sure only one person
      could use the road system at a time, voila!, no conflicts. But would
      that be politically acceptable?

      I am glad the wind friction was mentioned, since it suggests that the
      premise itself is wrong.

      And as far as the stop-and-go factor is concerned, in general, there are
      more occasions-per-mile to have to stop or slow in a dense environment
      than a less dense one, partly due to the smaller number of intersections
      in each mile. But lower densities translate into longer distances for
      each trip, more than wiping out any savings in reduced need to use the
      brakes and gas-pedal.

      One final note: speed is a _personal_ benefit to the driver, not the
      community. As speed increases, the distanes between vehicles has to
      increase proportionally, meaning that fewer drivers can be accommodated
      at higher average speeds than at lower ones (and lane widths must be
      higher, too). Also, increased speeds, as you have noted, increase the
      seriousness of collisions, and they increase the marginal utility of
      driving over walking/cycling/transit, not to mention the longer-term
      effect of the increases in modal split cutting at the critical mass the
      other modes need to be viable.

      Chris Bradshaw
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