| Topic: | Re:Re:Re:Re:Re:Re:Re:Re:Re:Re:Chiswick Curve or Chiswick Curse? | |
| Posted by: | Michael Robinson | |
| Date/Time: | 08/01/16 13:24:00 |
I've skimmed document 00479697 containing Appendix 7.5 with the air quality modelling and a few things stood out. "there is evidence that the current ‘official’ emissions factors published by Defra may over- predicted the rate at which road traffic emissions of nitrogen oxides will fall in the future" In other words, because of the practices of VW and other manufacturers, predicted improvements in air quality as a result of "cleaner" engines may not happen (or at least may not happen to the extent the government believed they would) They've predicted a 1.074 growth factor of traffic by 2020 but haven't given any derivation of this figure. Is it made just made up from an increase in "through traffic" or have they also factored in the cumulative effect of other developments such as the new stadium development? Other major schemes like the new Brentford stadium will have predicted traffic figures so it would be interesting to know if their figures are consistent. "Traffic speeds have been estimated based upon professional judgement" In other words, it looks like they haven't done any computer modelling of the impact of this traffic upon Chiswick roundabout and surrounding roads (computer modelling of complex junctions is costly and time consuming and is by no means perfect but at least should provide an indication of capacity thresholds for a junction.) Traffic flows are non-linear in that they can increase, reach a threshold then traffic slows down dramatically or grid-locks. I know the Brentford stadium development did do computer modelling of the impact of traffic on Chiswick roundabout as the model results were in the consultation documents. Computer models can be useful tools but it is garbage-in, garbage-out so if the traffic volumes and speeds assumed are questionable, the pollution predictions will be questionable, not to mention the governments faith that engines will get cleaner in the real world. |