LTNs appear not to push traffic on to boundary roads, London study finds

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Low-traffic neighbourhoods significantly reduce the number of motor vehicles within their boundaries without appearing to push traffic on to roads around their edges, the most comprehensive study yet of such schemes in the UK has concluded.

The research, which was based on traffic count data before and after the installation of 46 so-called LTNs in London, found a reduction in motor traffic within the zones of 32.7% when measured as the median, and a 46.9% drop when calculated as the mean.

Of the 413 roads inside the LTNs with before-and-after traffic counts, the percentage experiencing an average of fewer than 1,000 motor vehicles a day, seen as a good shorthand for a street receptive to more cycling and walking, rose from 41% to 66%.

This could mean “a qualitative change in the local environment” on at least some streets because of the LTNs, the researchers said.

LTNs use either physical filters, such as bollards and planters, or traffic cameras to prevent motor vehicles using some smaller residential streets as through-routes, while bicycle and foot traffic is unaffected.

Opponents of the schemes, which have proved controversial in some places, with a handful removed, claim they do not reduce the overall amount of motor traffic but simply move it to other roads.

Graphic for LTN traffic data

While the authors behind the research, from the University of Westminster’s Active Travel Academy (ATA), noted they only had useable data for just under half the 96 LTNs installed in London between March 2020 and May 2021, they said there was significant overall evidence of so-called traffic evaporation.

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Data from the 174 count points on boundary roads showed what the authors said was a more mixed picture, but without apparent evidence that such roads were necessarily seeing more traffic once an LTN was installed.

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Of those monitored, 47% showed a fall in motor traffic and 53% showed an increase. When measured as a median, the overall figure for boundary roads rose by 1.3%, but fell by 1.6% when calculated as a mean (average).

When the totals were adjusted using Transport for London data for wider traffic changes, to account for factors like the Covid-19 pandemic and differing seasons, boundary roads had an overall mean increase of 0.7% in motor traffic, or 82 vehicles a day on average.

Within this, the researchers found what they called “substantial variation in both directions” on boundary roads. They concluded this was mainly due to non-LTN factors such as other works, and said more research could be done on reducing traffic on boundary routes.

The study noted other caveats, including that the majority of counts took place within inner London rather than more distant suburbs, and that the extent and quality of traffic data, provided by councils, was varied, with some not having produced any monitoring at all.

The researchers also pointed to the need for further study on other effects of LTNs, including how to best mitigate their impact on people who particularly need to drive on local journeys, for example some people with disabilities.

The research used data from TfL, was funded by the climate charity Possible, and carried out by the ATA.

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Asa Thomas, a PhD researcher at the academy and the lead author of the study, said the findings pointed to a significant drop in motor traffic within LTNs, along with “little indication of systematic displacement of this traffic to boundary roads”.

Prof Rachel Aldred, the director of the ATA and co-author of the study, said: “The research indicates there has been overall ‘traffic evaporation’ as a result of these schemes, as the mean average reduction in motor traffic on internal roads is around 10 times higher than the mean average increase on boundary roads, adjusting for background trends.

“This suggests that not only do LTNs have substantial benefits inside their boundaries, but they can also contribute to wider traffic reduction goals.”

This article was amended on 20 January 2023 to give the median motor-traffic increase on boundary roads as 1.3% rather than 2.1%.

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