That Ealing LTN discussion yesterday which showed an increase of 52% of trip length comparing the before & after driven journey lengths between each cell and a series of boundary road destinations. Notwithstanding the fact that it's a simplistic analysis, it's been bugging me 1/
I've run the numbers with another assumption that once someone has driven to the destination point (which are all short journeys) they may well actually be driving further and therefore, the percentage increase between the no LTN state and the post LTN state must reduce with 2/
the overall distance traveled. I think I am applying the same logic, so feel free to call me out on my mathematics at the end of this thread. 3/
In the original analysis, we have 6 traffic cells set up in the LTN and 9 "destinations" on boundary roads. For the with/ without scenarios, the shortest distance between the centroid of each traffic cell and each destination is calculated made up of the distance from the 4/
centroid to the boundary road plus the distance traveled on the boundary road to get to the destination. Each combination is added up to give a total mileage in the before and after state. I've actually found a couple of minor rounding errors which takes us from 52% 5/
in the original paper and 53.4% with my initial check of the figures - this may just be in how things were measured and rounded up - the tolerance of 0.01 mile is 16 metres or about 20 of my lanky paces. It's of no consequence to the model 6/
The model is a 6 (traffic cells) by 9 (destinations) matrix or 54 possible combinations. So if we assume for each of these trips, once a person gets to the destination, they actually drive another mile. We will then need to add one mile to each trip which I've applied to the 7/
destination calculation (I can't apply this to both the first mileage in the LTN and second on boundary roads because that's double counting). So with this extra mile, the percentage increase created by the LTN falls from 53.4% to 20%. 8/
If we add 3 miles then this increase drops again to 9%. Given that this is a perfectly cyclable distance, the LTN will have no impact on that. At 5 miles, we're at 6% which is cycable but I can see people wanting to drive or get a taxi. 9/
At 10 miles, we're at 3% and if we're driving out to see Auntie Doris who lives 50 miles away in Milton Keynes, then we are at an increase of 0.63%. I'm simply applying the same logic to the original model which I contended only supports those wanting to drive short trips 10/
The model does not consider car ownership/ access, mode split, cost, journey time, behaviour and all of the other interesting variables which are thrown into the mix when we look at how and why people travel. I think I am right, please take me to task! 11/END
BTW, here's my table - I've added the "within LTN" and boundary roads into single before and after tables to fit in a reasonable image 
