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20 November 2020, 16:57 | Updated: 21 November 2020, 08:08
On 30 October, something changed in England. Right across the country, people started to go out.
Data on movement trends reveals the scale of the shift.
That day the Citymapper app showed travel in London was at 50% of pre-pandemic levels. By 4 November, the day England's second coronavirus lockdown came into force, that number was up to 60%.
In just five days, travel in the capital had jumped ten percentage points, reaching its highest level since 16 March, while the number of journeys being made across the rest of England also jumped.
What happened? Sky News has analysed the latest travel data to work out where and why so many people were travelling before the latest lockdown in England came into force, and what this means for future COVID-19 restrictions.
It is impossible to say for sure, but we don't need to look very far for an inciting event.
Late on 30 October, news of an English lockdown was leaked to the newspapers. The next day, the prime minister and his scientific advisers made it official.
Facing four weeks in confinement, people took advantage of their last gasp of freedom to hit the streets.
News of an event that was designed to restrain movement, instead caused it to surge to unexpected heights.
The strength of the trend is striking.
Citymapper only covers London and Manchester (which had a 12% jump in those five days), but Google tracks movement right across the country - and in the days leading up to 4 November, its data shows clear rises everywhere.
From Milton Keynes to Middlesbrough, Wokingham to Worcestershire, Bedford to Blackburn with Darwen, people were out and about in the days before lockdown.
This chart shows Google movement data from Cambridge, Southampton and Kingston upon Hull - three very different cities, which all experienced the same distinct surge.
England might be divided in many ways, but in this regard, it appeared to act as one.
We don't just know that people were moving, we also have a sense where they were going, because Google breaks down its data by destination.
During those five days, the data shows that trips to parks and workplaces remained stable.
The jump was in "retail and recreation" - that is, social activity, exactly the kind of behaviour which ends up spreading the virus.
You might think not - after all, rushing out ahead of a deadline seems like an ingrained human instinct.
But thanks to its devolved governments, the United Kingdom is effectively conducting a series of natural experiments in the management of the pandemic, which suggests that there is nothing predetermined about movement of this kind.
On 7 October, Nicola Sturgeon announced tough new restrictions across Scotland's central belt.
In Glasgow, one of the cities affected, the reaction was barely visible. Movement in the city flickered but did not spike.
By comparison, Leeds, an English city of roughly the same size, saw a big spike in the days after the lockdown leak.
The comparison between England and Scotland is not exact.
Cafes and restaurants in Glasgow weren't about to be closed completely, and the restrictions took two days to come in instead of five.
Yet communication experts argue that the difference could be explained by the way restrictions were announced - or, in the case of England, leaked.
"Leaking really does cause problems, because you end up with badly-timed messaging from unclear sources and that just reduces trust and confidence in the government," says Liz Stokoe, professor of social interaction at Loughborough University.
"We do know that confidence in the UK government's handling of the pandemic has fallen.
"On 5 November, confidence levels remained really high in Scotland and Wales and lowest in England."
It is not possible to say for certain that this surge in movement translated into a rise in infections.
There is no obvious regional link, for instance, nor an obvious connection between dates of increased activity and later cases.
But, given the number of infections, researchers have their suspicions.
The chair of one of the most authoritative surveys of the outbreak in England said recently that rumours of lockdown might have increased the number of infections as early as 2 November.
"There was a lot of speculation on Friday," said Professor Paul Elliot of Imperial College London, who leads the influential REACT study. "It's very tight timing, but something happened."
This would mean that England went into lockdown with the outbreak growing at a faster rate.
In the days before 30 October, there were signs that infections had slowed following the school half-term.
If that trend had continued, it might have been possible to end lockdown on 2 December.
Now, it looks as if some controls will have to be extended.
This response might have effects beyond the current lockdown.
Public health experts fear that if future restrictions are preceded by surges in movement, then they will become harder - perhaps even impossible - to impose.
"It raises concerns that if we're going to impose lockdown, what happens is that people increase sociability and increase transmission, undermining some of the value of lockdown," says Professor Paul Hunter of the University of East Anglia in Norwich, one of the many cities where people went out in larger numbers before lockdown.
"It could make us think twice about future lockdowns if it makes us increase the spread of the disease before lockdown."
With the Christmas holiday approaching, many in government fear this theory could be put to the test.
After the first nationwide lockdown was lifted, movement took some time to return to normal.
Will the same thing happen again, or will the easing of restrictions produce an immediate burst of social activity?
Will the holiday proceed quietly and locally, or will it result in mass movement on a scale comparable to the days when students went to university, the episode largely responsible for raising infections to the present level?
These questions prey on the minds of the government and its scientific advisers.
At this week's data briefing, I asked Professor Dame Angela McLean, chief scientific adviser to the Ministry of Defence, if the surge in movement before this lockdown was a reason to feel anxious.
"These things are all a worry to us," she replied.
"You can imagine why people would want to maybe get some shopping in before the shops shut... but we are concerned about how we can have a safe run-up to Christmas so that we can have some kind of a good family Christmas."