A quantitative analysis is developed for the biases that arise when trip frequencies for a general population are measured through an online survey instrument. Data from a national official survey in Italy, concerning both mobility behaviors and skills in using computers and the Internet, were deployed to assess differences in mobility levels between those who can answer a computer-Internet survey and those who cannot. Positive correlations were found between ability in using information and communications technologies (ICT) tools and trip frequencies. These latter correlations are about 15% to 150% higher for the ICT literate, according to the travel means under consideration. A Heckman sample selection model showed that these biases have different explanations. People who know how to use the Internet are different from others in their car-driving behavior because of a range of self-related factors. Conversely, public transport patterns of use are more similar between the two groups: the observed bias is mainly because of the fact of using the Internet itself, which could, for example, lead to a more active life-style. Such a distinction is of practical interest because it can help to define a method to correct these biases. According to the results of this study, the overestimation of public transport frequency of use by an Internet survey could be corrected by looking at the Internet diffusion in the population, whereas corrections for car-driving frequencies are more complex and should be based on differences in attitudinal and personal characteristics between Internet survey respondents and the remainder of the population.