Back in August 2020, there was a report claiming that a Low Traffic Neighbourhood (LTN) in Ealing showed an increase in driven trip miles over 50%. In the last few days, we have had more of this on a scheme in Islington and I thought it worth publishing a thread of my own analysis from the Ealing scheme and to offer a few other observations on the current "analysis".
The following was originally posted on Twitter on 8th August 2020 as a result of me trying to reproduce the results of a report undertaken by R. Witt for the Midhurst and Leighton LTN which was covered at the time by the "Ealing Today" website.
The general thrust was that the LTN increased traffic on the "boundary roads" and the analysis looked at distances from destinations on the boundary roads to locations in the centre of each "cell" of the LTN. The "before" data was the basic direct distances between the centres and destinations and the "after" data was the new routes that people would take. What R Witt had done was to create a spreadsheet traffic model.
The original thread follows below (with some typos corrected) and I will pick things up again after the table at the end:
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.
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 the overall distance travelled. I think I am applying the same logic, so feel free to call me out on my mathematics at the end of this thread.
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 and made up of the distance from the centroid to the boundary road plus the distance travelled 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% in the original paper to 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.
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 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%.
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 cyclable but I can see people wanting to drive or get a taxi.
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 contend only supports those wanting to drive short trips.
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.
OK, that's a social media thread, but I also wrote a post about some of the wider issues, including that those of us who support LTNs need to acknowledge some of the issues raised by those with concerns - that post also referenced the report linked above.
So, why have I dug all of this out again? Well, in the Times over the last few days, a well known opponent of LTNs has used his platform in the paper to have a whine about the Mildmay LTN in Islington. If you can stand it, the piece is HERE, with the usual suspects quoted.
The thrust is that the LTN will generate 1.4 million extra miles driven to get around the LTN each year. Let's pause a second and consider that in 2024, Islington had 225 million miles driven in the borough. The actual existing main roads are on the border of the LTN are in Hackney, and that borough had 274 million miles driven in the borough in the same year.
The "analysis" basically takes Islington's traffic data for King Henry's Walk which is unclassified and not a "main road" as asserted in the piece. The data apparently shows between 6,800 and 12,000 vehicles use the street very day, but we're not given any more detail.
There is then a calculation which takes the lower flow quoted and asserts the traffic will need to go an extra 0.55 miles as a result of the LTN by drivers having to use classified roads; the A104 and A10 to be precise. These are main roads managed for long distance through-traffic.
The 1.4 million miles comes from the volumes assumed to be diverting, multiplied by the diversion distance and then multiplied by 365 days - 1.37 million miles. That assumes the traffic volumes are the same 7 days a week, which isn't usually the case, but we told get told any more.
As with the Ealing report, this is a simplistic approach. Given the person's long opposition to LTNs and their role as a journalist, it would be reasonable to suggest that they know full well how simplistic this is. There is no discussion about the origin and destination of trips, or where traffic might reassign at the network level.
There is, however, a concession to evaporation being possible, at 15%, which takes the diverting traffic miles down to 1.1 million miles, or about 0.5% of the annual driven mileage in Islington or 0.4% for Hackney. I don't know what the 15% figure is based on, but on face value, we are not actually talking about a large number of extra miles, especially give how that just before Covid in 2019, there were 242 and 286 million miles driven in each borough respectively - overall flows are dropping and have been for years.
The piece whines that because of the other LTNs in the wider area, the main road is essentially the only route that can be used. Well, that's actually the point isn't it. Longer distance traffic using the routes designed, maintained and managed for those flows (for better or worse).
It's actually a non-story and while I think this bleating should generally be ignored, it is still occasionally worth explaining what the numbers quoted by the professional anti-LTN folks represent, because they really don't stand up to basic scrutiny. As those of us battle-hardened in the area will know, there is no amount of data, modelling or logic that will ever satisfy the hard-core anti.
No comments:
Post a Comment