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  • The furore in the cabbies' case was mainly aimed at the introduction of these schemes without consultation. Obviously, they'll also be unhappy about what exactly has been done, but this was ostensibly about procedure. Highway authorities have every right to introduce traffic orders, and wide leeway as to what they can contain, but it has to be said, and I say this as a supporter of modal filtering, that many of the schemes are terrible and not on the right principles. The Greenwich one is a rare example of a well-conceived scheme, but most contain the usual problems with filtering--filtering on the edges of cells, making cells too large, leaving loops, failing to filter cells properly, etc. Also, some of the measures aimed at widening footways essentially created conditions comparable to ones you find at roadworks, which are known to be more hazardous than ordinary conditions, and generally prevented people from crossing the street.

    Anyway, I didn't expect this case to go the cabbies' way, and I was surprised when the first judgement did, but it's certainly not the last word on this.

  • most contain the usual problems with filtering--filtering on the edges of cells, making cells too large

    Interesting you mention this, the proposed Bruce Grove LTN has filters on the edge of cells and some large cells.

    What is the downside of filtering on the edge of cells? It seems much more intuitive than filters in the middle of cells.

  • In no particular order:

    • Filtering inside has the same effect on through motor traffic, but you move the difficult stuff away from the main/cell boundary street junctions.

    • Fewer drivers will drive fast through the cell from the other side because the distance they have to drive inside it is increased completely unnecessarily. You need to make the distances that have to be driven inside cells from all boundaries roughly even. My standard example where this has not been done well is in Lower Clapton north of Powerscroft Road--filtered on the north, east, and south sides, so all drivers have to come in from and go out via the west side (Clapton Pond). It's a pretty long cell east-west.

    • At its worst, placing filters on the cell boundary can cause drivers accessing a part of the cell they have to spend a long time driving through the cell to behave like rat-runners at the (unfiltered) entry junctions, too--turning fast and injudiciously. If you filter inside instead, you get rid of crashes at the junctions almost completely. It's the bad turning manoeuvres that cause most of them, including the vast majority of crashes involving cyclists (left hooks, right turns across the path of a cyclist on the cell boundary street).

    • Normal-looking junctions with the cell boundary streets that mean drivers on the boundary streets have to drive more cautiously.

    • More active side street junctions with turning movements (no banned turns at all) mean less motor traffic capacity and less severance (easier to cross) on cell boundary streets. Edge filtering treats the cell boundary as the enemy. It isn't, and the interaction with side streets is vital for good functioning of boundary streets. Traffic engineers are usually instructed to at least maintain motor traffic capacity, and as having fewer active junctions does that, they often favour edge filtering, but this generally results in schemes of poor quality. It can kill a lot of natural functions of street corners stone dead, e.g. behind the guard railing that's often been placed there in the past. The junction dies and all that happens there is through traffic flowing past.

    • More routes into and out of the cell, greater chance of achieving 'same way in, same way out'.

    I did a quick filtering pattern without edge filtering here in the area between Stratford and Leyton:

    https://www.lfgss.com/comments/15505024/

    It's a point that's surprisingly difficult to get across, and even harder to get people to implement non-edge filtering.

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