The 70-page study was prepared by the Dutch research and consultancy organisation TNO together with experts at the International Transport Forum and the OECD Environment Directorate. The safety impact analysis was prepared by the Dutch Institute for Road Safety Research (SWOV).

The report assesses the three scrappage schemes, which were the largest ones introduced primarily to stimulate consumer spending on cars in the wake of the 2008 economic crisis, and investigates the impact on CO2 and NOx emissions of the resultant 2.8 million transactions.

It claims that in the US, there were positive results from targeted incentives with regard to fuel economy. However, these incentives were not optimally designed to achieve improvements in fuel consumption or pollutant emissions.

With the German scheme, a greater number of lighter and smaller vehicles were traded in for medium-sized vehicles which reduced its effectiveness.

Finally, the French scheme benefited from imposing a type-approval CO2 limit for new cars and retiring very old gross-emitters. However, this led to a very high share of new diesel vehicles which strongly limits lifetime NOx benefits.

With respect to road safety, the car renewal schemes are estimated to avoid around 40 fatalities and 2,800 serious injuries in the US. For Germany, the estimated impact is 60 deaths and 6,100 serious injuries avoided. France is somewhat lower at 330 fewer serious injuries, 20 of which would be fatalities.

The report concludes that it is vital to consider the objectives of the schemes very carefully when setting conditions and incentives on the traded and new vehicles to deliver on objectives such as reducing pollutant emissions. Further, the study suggests that seeking CO2 reduction ahead of pollution or safety improvements in the design of the schemes leads to decreased cost-effectiveness and lower overall societal benefit.

'Subsidies for car renewal can bring real benefits only if they are carefully designed,' said Jack Short, secretary general of the ITF. 'Here a best practices approach is key. We hope that comparative studies like this one will help countries pondering similar schemes to find the right solutions for them.'