Thread by @MichaelDnes1

This weekend, Britain may be about to have its worst traffic jam ever. Up to five hours of delay for 100,000 drivers or more.

Would you believe me if I told you the reason is because Britain's most hated motorway is too good?

Or that someone had a semi-secret plan to fix that🧵

From Friday to Sunday, National Highways will close the M25 (London’s ring road) from junction 10 to 11

It's unavoidable. For the first time in 40 years we're rebuilding one of its junctions – which means demolition work that can’t be done in a night

But it means that traffic currently ramming an eight lane motorway will drop onto this high street in Byfleet. It won’t be pretty.

I used to lead the UK government’s road planning team, and this bit of the M25 - the South West Quadrant (SWQ) - is perhaps the worst problem we had

And I had a mantra that we shouldn't say any problem was too big to think about. So we asked - ‘can we fix this road?’

The road isn’t just congested. It's stuffed.

Most roads have two peaks of traffic – morning and evening. The SWQ doesn’t. The traffic starts at about 6am and goes constantly until about 6pm.

And because it’s stuffed, when things go wrong there's no way to fix it.

The stereotype of highways planners is that they always say ‘add another lane’.

Not here.

And because someone (not me) put a … revealing slide pack onto , I get to tell you about it.gov.uk

In fact, the worst thing you could do to this bit of the M25 is add another lane. And this weekend’s traffic jams will show you why.

The reason this section of the M25 is so busy isn’t just about the M25 – it’s about all the things that aren’t the M25.

When the M25 disappears, what’s your alternative?

Half a mile of dual carriageway, then 8.5 miles of random suburban roads

We did a site visit once, and the local council made a point of making us drive the diversion route. The M25 took c.10 minutes; the red route took c.30.

There’s no parallel public transport worth talking about. Virtually all the trains are designed to run into central London.

The bus network doesn’t cover these kinds of journeys.

Most travel distances aren’t viable by bike or foot.

So even though the M25 is grim, jammed and soul-destroying, it is much better at the business of getting you from A to B than any of the alternatives. Any rational route plan will take you that way and (crucially) no other.

So everyone does that.

78% of the traffic on this bit of the M25 is traveling to or from somewhere fairly local.

Only 2% is staying on the M25 all the way without turning off.

If you’ve got a road that’s taking all the traffic because it’s so much better than the alternatives, the one thing you absolutely _must_not_do_ is make that road wider.

If you do, you’re making your problem worse – because the motorway’s advantage becomes even greater.

So what _can_ you do?

We came up with a ladder of options. Start with the stuff everyone can agree with; and then step by step get to the stuff where you’re going to make a lot of new enemies. And see where along that journey you think you've made a reasonable difference.

Step 1 – Reduce demand. Optimise the existing roads. Make public and sustainable transport better.

No one objects to that. But it also is much easier to say than do – especially at the scale needed to allow meaningful change

For example, there already is a rail line parallel to the motorway. It carries about 500 seats of capacity an hour. Even if you can summon the passengers to fill them (which it currently doesn’t), that’s enough for <5% of the people on the M25.

Step 2 – make the local roads work better.

In our philosophy, making the local roads work better takes the pressure off the motorway.

Lovely theory, but coming up with a list of options was surprisingly hard. This map was what local councils suggested to us. Some bold ideas, but you might notice not a lot of it is actually near the M25.

And that left #3 – building a major new road to take local traffic. Not a motorway – that would just perpetuate the problem – but a sizeable road that would be a useful alternative to a motorway.

This is the bit where you lose friends.

Because this isn’t any old place to build a road. You are looking at the NIMBYest place in England

This map shows you all the protected sites near Woking. If it hasn’t got a roof on it, it’s got a protection order

And four of the local MPs were in the Cabinet.

Once upon a time, road builders didn’t stop for this. Now, we take greater care.

We decided you’d either have to stick to improving roads that were already there or building most of what was new underground. Which took the price north of £10bn – probably not happening

Inside of the M25, the idea of building anything above ground was even harder to believe. There are virtually no good existing roads to work with south of Heathrow - so everything beyond would either destroy the last green patches left, or need to go underground.

So we dreamed up a plan for another tunnel linking Heathrow to the A3, putting all that traffic under the earth where it would bother no one; and get the London traffic away from the motorway permanently.

And … then we stopped. Because any good transport planner, when they’re starting up ten billion pounds worth of infrastructure, needs to ask ‘do people really want this?’

No one was screaming ‘yes’.

People wanted a better M25. But at the expense of digging up large chunks of London or Surrey? No.

Could we make a meaningful improvement with anything less? Very unlikely.

Is the M25 intolerable? Absolutely … until you look at the alternatives.

So instead, we did something roads planners allegedly don't do – we got the government to commit to _not_ widen the motorway. And began the long wait to see if anyone cared enough to make it worth developing the alternatives.

Which, to this day, they have not.

Everything in this thread comes from publicly available information at assets.publishing.service.gov.uk/media/5aabe222…