This paper studied the effect of weight limit signs on restricting truck drivers' behavior based on the results from a questionnaire survey conducted among truck drivers in a province in China. Based on the statistic properties of the field measured traffic flows, models for random traffic flows were developed by using the Monte-Carlo method while taking into consideration the effect of the weight limit signs on restricting the drivers' behavior. Taking a weight limit of 55 t for example, the load effect for the midspan bending moment and end shear for simply-supported beam bridges under the generated random traffic flows were studied by using the method for analyzing the load effect provided in the Unified Standard for Reliability Design of Highway Engineering Structures. A parametric study was also conducted to study the effect of the average daily vehicle traffic, conditional restriction level, and bridge span length. The calculated load effect under the generated random traffic flow was compared with that calculated by using the design load given in the current Chinese bridge design code. The results show that: 1) the weight limit sign can effectively reduce the maximum vehicle load effect and insure the bridge safety and reliability only when the truck drivers strictly obey the weight limit regulation; and 2) the design vehicle load in the current Chinese bridge code may have underestimated the real traffic loads, especially for short-span bridges, to which special attention should be given.