 |
 |
> Andrew Smith: > So a model that explains 80% of the traffic flows in Central London > (before congestion charging), based on an axial map, will predict no > changes after such road pricing is introduced, as the axial map does > not change. Similarly with bus lanes, traffic light phasing, traffic > calming: all the standard tools of urban traffic management. But > flows have changed significantly ... and a useful transport model > would predict those changes reliably. > [AP] I'd love to get at the data on the changes pre/post congestion charging, Andrew (hint hint ;-) Can you tell me whether there have been substantial changes in relative flows between different streets within the zone? I can imagine several reasons why there could be and I believe that some of these might be predictable in principle from syntax analysis. For example, I would anticipate a reduction in the proportion of through trips across the zone, and so a reduction in radius of integration of the explanatory variables in the regression. I would also hope that there would be a reduction in congestion (that was the idea wasn't it), and so a reduction in the proportion explained by road width. But just to come back on the matter in hand. The policy of congestion charging was presumably subject of a substantial traffic modeling exercise. Now, following its implementation, how good were the predictions? Can we establish a best practice 'standard error of estimate' for the pre/post changes at the individual link level? Also of course at the overall area average level - how much did the price point of the charge affect demand, and how good were the traffic models at predicting this? > > As traffic (or more usefully, general transport) models use > origin/destination matrices, I can't see how one would implement Alan > Penn's suggestion of: "using space syntax measures ... in the matrix > estimation phase of the construction of traffic models", but am > willing to be enlightened. I'm often agnostic as to the usefulness of > models based on such O/D matrices, but they do have lots of > advantages in terms of being able to inform policy decisions on land > use and transport. > [AP] Well, I have to come clean on this - I am no expert on matrix estimation either - the idea was suggested to me some years ago by Pilo Willumsen who is. I believe that the process would be that you would use our regression equation as a proxy for flows on the network, insert that in the model and use it to estimate what the O-D matrix must be to give rise to those flows. The point here is that although our regression is based on a sample of flows, the configurational analysis allows a good estimate for ALL links. This might offer a substantial improvement on current methods of estimating O-D matrices. But as I say I have never done this and am no expert.... Alan