So, Traffic Evaporation.

You've heard about it, but don't believe it.

Let's consider how a busy road might cause traffic evaporation. I live near Barlow Moor Road. I must walk, cycle, or drive on it to get anywhere. It's a congested two lane road that's at capacity.

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I personally know, that it's just not worth using the car between 4-7pm due to this, and I will NEVER drive between then. Even if I got a job I couldn't cycle to, I'd ensure there was public transport, or I could work flexibly etc. That right there is traffic evaporation.

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There's talk about how LTNs might cause main roads like this to see more traffic. I posit that it physically can't take anymore traffic. It's already chokka for much of the day, it somehow manages to carry 18,000 motor vehicles a day!

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So if you filter all the side roads and stop them being a pressure release valve for BMR, and force everyone onto BMR, what happens? Well, the number of cars doesn't really go up: the road can't take any more, at least at rush hour

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So what happens? People make their car journey at different times, they walk or cycle instead, they consolidate multiple journeys, or they don't make the journey at all! That's traffic evaporation in action, and as established earlier, many of us already do it to a degree!

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It's the reverse of a well known phenomenon called "induced demand" where traffic will fill a road to its capacity, so if you widen a road and double the capacity, traffic volumes will double.

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The great thing is that this works in reverse, so if you reduce the traffic capacity of an entire area, by filtering side streets so they can't be used as pressure release, you also reduce the amount of motoring in that area.

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Of course the main roads also need upgrades too, eventually. Cycle tracks, crossings, etc. And there needs to be a Clean Air zone, and wider traffic reduction measures like less parking, road pricing etc. But not having those in place right now doesn't make LTN's pointless.

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