Footnotes

• Funding This work was funded through the salaries of the authors provided by their institutions.

• Contributors LRS contributed substantially to designing the study and analysing the data, and drafted the article; SL contributed substantially to designing the study and to acquiring, analysing and interpreting the data, and critically revised the article for important intellectual content; CH and EL contributed substantially to analysing and interpreting the data, and critically revised the article for important intellectual content; all authors gave final approval of the version submitted.

• Competing interests None.

• Provenance and peer review Not commissioned; externally peer reviewed.

• This paper is based on a presentation given by LRS at the Transportation Research Board Conference ‘Emerging issues in safe and sustainable mobility for older people’, Washington, DC, 30 August to 1 September 2011.