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evaluating LTNs

A guest piece by Simon Munk, Senior Infrastructure Campaigner, London Cycling Campaign

Should there be baseline measurement and assessment of LTNs? Hell to the yeah. Will there be? Likely, yes. What should we assess? Overall motor traffic volumes, traffic volumes on peripheral main roads, traffic volumes in adjacent areas, ideally pollution also (but traffic volumes will likely be an incredibly close correlation if not), walking & cycling rates, resident feedback and more. But, wait.... when should we measure? This is really important. IMO measurement of the post-scheme steady state should not happen until at least six months after all schemes are in fully.

We know that LTNs, from all the results elsewhere, can take months to bed in. Most transport experts suggest giving it six months at least. Even then, that's only for the basics of "traffic evaporation" to happen on nearby roads. That means, to be explicit, ratrunning drivers who've been displaced out of the LTN and are now sitting in a temporarily longer queue on nearby main roads give up and either switch time of day of journey, or don't make the journey, or switch mode, or drive out of the area on a very different route etc. Only when this has happened can you start to make a guess at the long term impact of the scheme. Before that, you might as well say "well we had some roadworks and the roads got a bit messed up". So you have to wait a bit.

Of course, even at 6 months, what hasn't generally happened much at that point according to the data, is "behaviour change" other than the stuff forced on drivers by removing routes for them. So the ratrunners won't be able to ratrun any more but other longer term impacts won't have necessarily manifested. It takes over a year for the impact inside the LTN to really start to show - kids playing out, people walking and cycling more, ditching, even selling, their car, community links increase etc. And similarly, the drivers inside and outside the LTN take time to get that bike out of the shed, get it maintained, buy a new one, get their kids on tagalongs, get comfy on the quiet roads etc. Prof Rachel Aldred's longitudinal study on Waltham Forest LTNs is demonstrating this slow, but sizeable shift. But ignoring the long term impacts, six+ months is really minimum for assessment IMO, with three key caveats...

Caveat: I cannot find, and I've looked, a scheme that shows worse main roads near it years after implementation. So yes, traffic can and sometimes does go up temporarily on nearby main roads to LTNs, but temporarily only. It would be a very unusual thing to find a long term impact - so all assessments should be in that framework. Indeed, read this on traffic volumes at a city level: https://www.wired.com/2014/06/wuwt-traffic-induced-demand/ That's from 2014. It shows you build more roads, you get more traffic; you get rid of roads, you get less traffic. That is, in a nutshell what LTNs do. So you care about air quality - read that carefully.

Caveat: if the pollution or congestion levels on a nearby main road *are* significantly higher than the levels on similar nearby main roads, does that mean the scheme should come out? No. Most likely it means some traffic lights need rephasing or some other tweaks are needed or the main road needs improvement etc. If we accept the general idea that climate change is real and at crisis levels, that poor air quality isn't OK, that inactivity levels are severely impacting our lives and the NHS, if road danger and kids cooped up isn't acceptable, we have a duty to do more - not oppose this scheme here, now, while holding out for some magic unicorn perfect scheme that's 30 years away and a load of people will still oppose then anyway. Do more, don't make perfect the enemy of good, monitor and fix. Indeed if issues are emerging in a scheme in month 3, it's IMO quite right for the council to tweak the scheme, add to it, improve it, and then reset the clock. We desperately need far more bold, rapid action on the issue of car use, not less action.

Final caveat: What are people on social media on about roads being bunged up by the LTN? Correlation does not equal causation. Traffic patterns are very chaotic across the country currently. So be very wary of making claims that LTNs have caused congestion until you have good data. Similarly, any measurement happening now has to be in the context of what similar roads nearby, but not near an LTN, are experiencing. Lots of London boroughs doing LTNs are looking at the baseline volumes and comparing patterns through lockdowns to the roads near LTNs and finding main roads significantly down in volumes compared to other main roads nearby. Be very wary of assuming that what you see from the windshield is down to LTNs.

Hope that helps clarify monitoring issues.


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