The response of pedestrians to lively bridges is one of the problems that has to be addressed in the scope of analysing serviceability. The excitation process by walking and/or running and the sensitivity of the human receiver both have to be understood as random variables. The present paper analyses the expected scatter of the response of pedestrians based on today's available information. A discussion is then presented on a method of correlating observed reactions of pedestrians and measured vertical vibration amplitudes, which allow a serviceability criterion to be developed. A tentative limit value for serviceability is proposed based on the observations so far.