Microeconometric studies increasingly utilize travel times to markets as a determinant of economic behaviour. These studies typically use self-reported measures from surveys, often characterized by measurement error. This paper is the first validation study of access to markets data. Unique data from Peru allow comparison of self-reported variables with scientifically calculated variables. We investigate the determinants of the deviation between imputed and self-reported data and show that it is non-classical and dependent on observable socio-economic variables. Our results suggest that studies using self-reported measures of access may be estimating biased effects. © Blackwell Publishing Ltd and the Department of Economics, University of Oxford, 2008.