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Huge errors found in forecasts for small road improvements

02 April 2015
 

Forecasts for the time saving and accident benefits of sub-£10m improvements to England’s strategic road network are usually wildly inaccurate, according to a new report. 

The post-opening project evaluation (POPE) report into local network management schemes by consultant Atkins reviews actual against predicted performance for hundreds of schemes costing less than £10m, such as the addition of new lanes at junctions and traffic signal improvements. The evaluations were conducted a minimum of one year after scheme opening.

The Atkins report summarises the findings of 11 years of such evaluations, covering 756 of the 2,000 projects delivered. Of the schemes evaluated, 689 were safety or economy schemes, and 161 had journey time impacts evaluated. 

“The results at an individual scheme level show that the forecasting of benefits is poor,” says Atkins.

“Only 12% of schemes evaluated are within 25% of the forecast accident benefit and 23% within the forecast journey time benefit. Cost accuracy for individual schemes is more encouraging than benefit forecasts, with 58% of schemes within 25% of the forecast.”

 A spokesman for Highways England said that, despite the errors in individual scheme forecasts, the report by Atkins demonstrated that the investments offered excellent value for money.  

“Overall the schemes evaluated were found to have delivered savings of 3.7 million vehicles hours in their opening year and on top of that the safety benefits were 16% higher than expected,” he said. “Indeed, the average scheme is expected to pay for itself 17 times over during the course of its life.” 

Atkins says the data “clearly shows that, as scheme cost increases, the accuracy of both journey time and accident benefit forecasts improves”.

In aggregate, the outturn journey time benefits of £55m from the 161 schemes compared with predicted benefits of £191m, a difference of £136m.

Says Atkins: “Of the 161 schemes considered in this analysis, 58 achieved journey time savings greater than predicted, totalling an additional £27m worth of benefits. The remaining schemes delivered journey times savings worse than predicted, equating to £163m of unrealised journey time benefits.

“This means that two-thirds of schemes with forecast journey time benefits are set too high, and points towards a potential issue with the process of forecasting journey time benefits,” says the consultant. It points out, however, that the aggregate results were partly shaped by one scheme that delivered a journey time benefit £84m lower than forecast. Explaining this error, Atkins says: “The project appraisal report for this scheme predicted journey time savings in the wrong units (in seconds rather than minutes), leading to a large over prediction.”

Atkins says that £94.4m of the £163m of forecast time saving benefits that did not materialise came from the 19 schemes that cost below £100,000.

“Improving the accuracy of predictions for these schemes alone would reduce the scale of the overprediction by 70%,” it says. “It is rare for less than £100k schemes to impact journey times, and so this needs to be reflected in forecasts in future project appraisal reports.” The consultant says forecasts for such low cost schemes should predict either no or only very small time saving impacts.

The impact of schemes on average journey times and journey time reliability was assessed using anonymised TomTom GPS-probe data.

POPE of local network management schemes annual report 2014 is available at http://tinyurl.com/q6hf2e6

Atkins will be presenting details from these findings at Modelling World 2015

TomTom will be explaining how they deliver this journey time and journey time reliability data while ensuring compliance with EU data protection and privacy legislation at Modelling World 2015

Specialist Transport Services Manager
Warrington Borough Council
Warrington
GRADE 13 (£58,797 - £63,735)
Specialist Transport Services Manager
Warrington Borough Council
Warrington
GRADE 13 (£58,797 - £63,735)
Specialist Transport Services Manager
Warrington Borough Council
Warrington
GRADE 13 (£58,797 - £63,735)
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