[1] Univ Cambridge, Sch Clin Med, MRC Epidemiol Unit, Cambridge CB2 1TN, England
[2] Univ Cambridge, Sch Clin Med, UKCRC Ctr Diet & Activ Res CEDAR, Cambridge CB2 1TN, England
[3] Univ Florida, Dept Urban & Reg Planning, Gainesville, FL USA
[4] Univ Twente, Fac Engn Technol, Ctr Transport Studies, NL-7500 AE Enschede, Netherlands