An important element of achieving quality in a road network is the control of vehicle vibration due to pavement roughness and road irregularities. Scientific literature and international standards suggest that we evaluate these phenomena by measuring the whole body vibration (WBV) on the road user, but, for the practical aims of road engineering, this expression has to be related to road unevenness indexes, especially the most common one (the international roughness index, IRI). This index, in turn, is obtained from measured pavement geometric data using a conventional model of a mechanical system representing part of a vehicle. To better investigate the problem of user comfort, more complex models and analyses are needed. In this paper, a model of a real and common vehicle is presented and used, after a calibration process, to perform many dynamic simulations. The obtained results, in terms of weighted vertical acceleration (aWZ, that is, the WBV index, according to ISO standard), show good correlations (R2 from 0.75 to 0.93, depending on vehicle speed) with the IRI values of the examined road sections. On the basis of this correlation, authors propose threshold values for both vibration and roughness indexes; these thresholds could be used for road users' comfort evaluation and adopted in technical standards. © 2010 ASCE.