The paper considers a stochastic approach to modeling the actions of walking and has focus on the vibration serviceability limit state of footbridges. The use of a stochastic approach is novel but useful as it is more advanced than the quite simplistic deterministic load models seen in many design codes. Using a stochastic approach, however, reqiures a number of decisions to be made (statistical distribution and associated parameters) for walking parameters. These decisions might have an impact on the outcome of serviceability evaluations (bridge acceleration levels), but it is often not a simple matter to foresee their impact. The paper contributes by examining how some of these decisions influence the outcome of serviceability evaluations. The sensitivity study is made focusing on vertical footbridge response to single person loading.