File talk:Revised petrol use urban density.jpg
Is Los Angeles really almost as dense as New York? I find this to be on the edge of plausibility. NYC is geographically large, but much of this is water, and it seems misleading to include it in an "area per capita" calculation. Toppler 18:02, 30 October 2006 (UTC)
All right, I calculated the population density of both cities using information available here on Wikipedia. For both New York City and Los Angeles, I used the "total area" figure, which includes both land and water.
- New York: ~150 m^2 per capita
- Los Angeles: ~315 m^2 per capita
These figures don't seem to match the graph. New York is particularly misrepresented. From what data is the graph derived? Toppler 18:11, 30 October 2006 (UTC)
- The data is derived from a very commonly used study carried out in 1989. The revised graph is a little more difficlut to understand than a previous version uploaded which was taken from a UK Government policy document, and can also be found in many transport planning text books.
The old version:
.
The comments relating to this graph are about the Newman and Kenworthy 1989 study. In this graph their statistical methods have been criticised. To quote the discussion 'The essential problem is examining the relationship between ‘motor spirit use per person’ and ‘persons per hectare’. Because population is in the denominator in one term and the numerator in the other". Reanalysis of the primary data comparing gasoline use per person to area per person shows a more complex relationship and is the revised graph. So the points do not show area, rather they show the area available per person, not density, if that makes any sense. LA is actually fairly dense, where it is populated. I believe from what i remember, that the study did a slightly more complicated sum than area/people. I would think that such a widely accepted and recognised study took into account water and so on.
It needs to be revised or kept, becuase it should not be removed, the fact of the matter is, density and petrol use are linked, and this is an important graph to support the text in the article. for the time being i think the added paragraph 'blah blah, inaccurate' should also be removed, the section is correct even if we discover the graph isn't. Bjrobinson 17:51, 31 October 2006 (UTC)
Yes—the area per person statistic in both versions of the graph seems inaccurate. New York's area per person should be far less than that of Los Angeles, both intuitively and per the calculations above (150 vs. 315 m^2 per capita). The graph shows them being almost equal. Or have I misunderstood? Toppler 04:43, 1 November 2006 (UTC)
- Also, the original graph indicates NY's annual per-capita gasoline use is less than LA's, yet the revised graph appears to indicate the opposite, which is contrary to pretty much everything I've ever seen. How did Newman and Kenworthy obtain their figures (e.g. are they based on metropolitan areas, city limits, or what?) and how was the data obtained for the revised graph? Toppler 04:50, 1 November 2006 (UTC)
- Yeah you have a good point... i honestly don't know. The original one made sense to me, but the new one. Hmm if only my maths was better. Let me try and explain what i think is going on, the first version is very simple x, y. The new version is gasoline use, per person, by density... okay. But NY surely does'nt... Ill try and get to the bottom of it. I guess it will look like work anyway. Bjrobinson 16:58, 1 November 2006 (UTC)
Okay... you ready for all of this? This is from someone who does appear to know what he's talking about... David Pritchard'. Who runs a very interesting blog and has studied these specific figure sin the past. Theres a lot to read:
"I haven't sat down recently and looked at the actual data in depth, or Newman & Kenworthy's various papers. I do believe that the literature commonly talks about Los Angeles - and yes, I believe it does have a higher density than New York City.
The key point here is that we are talking about averages over the entire metropolitan statistical area - and while parts of NYC are extremely high density (i.e., Manhattan), a huge part of the surrounding agglomeration is quite low density. I'm not sure how N&K define New York, but it probably includes large chunks of neighbouring New Jersey. Los Angeles is often found to have a surprisingly high density - a recent paper discussed the "worst of both worlds" situation in Los Angeles, where parts have near-urban densities while only having suburban transportation options and urban design.
One common criticism of studies of urban density is that the results are sensitive to the definition of the city boundary. Where does a city stop? This is particularly hard to edefine for modern American cities with a large "exurban" semi-rural fringe who still commute downtown or to suburban subcentres. If you can't define the city boundary, you have a hard time defining the total population and the total area, and hence have an unreliable estimate of density. I understand that Newman and Kenworthy put a lot of effort into defining a good methodology, but their numbers remain suspect. (By the way, urban area never includes water. There are also two definitions of density, net density and gross density, that exclude/include roadways and other amenities from density calculations.) I would strongly suggest that you find a copy of this book if you want to understand their methods:
Jeffrey Kenworthy, Felix B. Laube, Peter Newman, Paul Barter, Tamim Raad, Chamlong Poboon, and Benedicto Guia Jr. An International Sourcebook of Automobile Dependence in Cities 1960-1990. University Press of Colorado, 2000.
The N&K 1989 paper was *extremely* controversial when it was released.
While many in the profession believe that their conclusions may well
be correct, I'm not sure that their original data and method were
really sufficient to prove what they tried to prove.
If I have a chance, I may go back and revisit their papers and contribute to the Wikipedia discussion directly, but that won't be any time soon.
- David"
I guess this is what WP is all about :) Affirming and making sure sources are correct! Bjrobinson 21:25, 1 November 2006 (UTC)
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Wow, very informative. Thanks for digging up that explanation. :) Toppler 05:25, 4 November 2006 (UTC)
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The units on the graph are abreviated and I don't know what they stand for. GJ units of gasoline? What is that? A Jillion Gallons? And sqm for area. Is that square meters or square miles? Or maybe squirrels queueing mosaics? I realize it doesn't really make any difference to what the graph is trying to illustrate, but the units out to be there, and they shouldn't be abreviated. C. Pergiel. —Preceding unsigned comment added by 71.117.211.16 (talk) 17:10, 13 August 2010 (UTC)
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I'm assuming normal SI usage, in which GJ means "gigajoules" - so, an energy measure rather than a mass or volume measure. That would make it easier to "add" multiple fuel types, for example electric transit systems to petrol and diesel vehicles. Though I haven't gone back to check the source so this might be wrong!
David Pritchard's comments are well made - the boundary has a crucial impact. Would be interesting to take a few non-metropolitan counties and draw the dots for those. — Preceding unsigned comment added by 195.194.62.56 (talk) 12:13, 31 August 2011 (UTC)