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Maths error casts doubts on Greater Cambridge Partnership’s road-charge consultation document

Doubt has been cast on data used by the Greater Cambridge Partnership (GCP) to justify its road-charging proposals after a maths blunder.

The GCP claimed that Greater Cambridge’s population will grow by 28 per cent by 2031.

Traffic congestion on East Road in Cambridge Picture: Keith Heppell
Traffic congestion on East Road in Cambridge Picture: Keith Heppell

It used the figure in its Making Connections consultation to explain why it was putting forward plans to fund a radically improved bus network by charging car drivers £5 – or more for larger vehicles – to travel in Cambridge each weekday from 7am to 7pm.

Prof Kathy Liddell, founding director of the Cambridge Centre for Law, Medicine and Life Sciences, challenged the GCP using the Freedom of Information Act to explain how it arrived at the figure.

“I am seeking information that will enable me or other citizens to verify that the executive board’s decisions – about the need for a new bus network, the need for a road user charge, and the need to reduce congestion – have been, and continue to be, reasonable and rational,” she said.

The GCP said in response: “The summary statistic referred [to] was drawn from a Cambridgeshire Insights report which reports on the forecast population growth by 2031.”

It adds that an evidence base document for the Local Plan provides further information.

And then the response notes that the latest Census statistics showed the Cambridge population had increased “since 2011 by 17.6 per cent” while in South Cambridgeshire it had “increased by 8.9 per cent”, adding “so for Greater Cambs this is 26.5 per cent”.

The GCP added: “These all show an upward trend of population growth and corroborates the 28 per cent growth.”

Prof Liddell responded: “If I’m not mistaken, it is incorrect to add the percentages. They need to be averaged to assess the growth rate for Greater Cambs. In other words, if ONS data shows that the Cambridge City population grew 17.6 per cent and South Cambs grew 8.9 per cent then the growth overall for Greater Cambs is 13.25 per cent (assuming both districts are weighted equally). Would you agree that the figure of 26.5 per cent mentioned in your response is incorrect?”

Cambridge political commentator Phil Rodgers – the Cambridge Independent’s new columnist – said: “As anyone with GCSE maths can tell you, this is complete nonsense. It doesn’t exactly fill you with confidence about the quality of the analysis behind the STZ [Sustainable Travel Zone] plans.”

The GCP’s response also pointed to the independently conducted Cambridgeshire & Peterborough Independent Economic Review (CPIER), which published an “assessment of population and economic growth trends” in it 2018 report. Examining this report shows CPIER said Cambridge’s population is forecast to grow by 27.5 per cent by 2031 to 151,800 people – but this was compared to the 2009 population figure (119,100).

Cambridge political commentator Phil Rodgers Picture: Keith Heppell
Cambridge political commentator Phil Rodgers Picture: Keith Heppell

Similarly, the CPIER report suggests South Cambridgeshire’s population will rise 26.7 per cent by 2031 to 181,900, compared to the 2009 level (143,600).

The latest Census data, from 2021, does show that Cambridge and South Cambridgeshire have grown fast, but does not suggest 28 per cent growth in the coming decade. In Cambridge, the population rose 17.6 per cent from just under 123,900 in 2011 to around 145,700 in 2021. And South Cambridgeshire’s population rose by 8.9 per cent from around 148,800 in 2011 to around 162,100 in 2021.

Cambridge accountant Chris Howell warned the figures “did not look great” for the GCP.

“The narrative in the FOI response is nonsense,” he said. “There is a basic maths error, and it attempts to use 2021 Census data for 10 years growth to justify the 28 per cent rate presented – no prediction is close to that high over 10 years.”

He suggested you could “only get close to 28 per cent growth” by rounding up the Cambridge city growth figure from 2009 to 2031 in the CPIER report and claiming it applied to Greater Cambridge.

“But as well as rounding wrongly and not using the correct area, it would of course have been hugely misleading of the GCP to use a 2009 baseline and 10-year-old out-of-date analysis in a 2022 report to show ‘growth to 2031’,” he pointed out.

Examining the other evidence cited by the GCP – a consultants’ report from 2020 – shows it predicted 24.5 per cent growth, but this was from 2020 to 2041.

“This again shows that ‘28 per cent growth to 2031’ looks to be a wild overstatement,” said Chris.

He described the figures as “highly misleading” and added: “Where did the numbers come from? Cynically, did they falsely inflate the growth numbers to support the proposals? I remain sceptical of the modelling behind their ‘50 per cent traffic reduction’ and ‘increase public transport use by 40 per cent’ claims.

“Councillors should be all over this. Greater Cambridge deserves so much better – it is too important for the local and national economy.”

How the 28 per cent population growth estimate was presented in the GCP’s Making Connections consultation
How the 28 per cent population growth estimate was presented in the GCP’s Making Connections consultation

On Tuesday, the GCP acknowledged the error in its response to Prof Lidell but suggested the consultation was right to point to fast population growth – something the latest house-building proposals in the emerging Local Plan would certainly support. But it has yet to explain how it arrived at the 28 per cent figure for growth by 2031.

A GCP spokesperson said: “Census 2021 showed that there has been substantial – and faster than expected – growth in Greater Cambridge over the last 10 years. Accordingly, Local Plan population forecasts for Greater Cambridge to 2041 have very recently been updated to 360,800 (approximately 32 per cent growth over the 2011 population).

“Unfortunately, the 26.5 per cent figure for growth from 2011 to 2021 that we provided in a response to a Freedom of Information request which has since been published online was wrongly calculated. We apologise for this error in the FOI response, and we will contact the individual who made the FOI request but to confirm this does not impact the recent Making Connections consultation.”

About 24,000 consultation responses were received.

Additional reporting by Paul Brackley

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