Monitoring traffic loads is crucial for effective steel bridge maintenance because fatigue damage in steel bridges is caused by the long-term cumulative effect of traffic loads. This article presents a simplified portable bridge weigh-in-motion (spBWIM) system consisting of only accelerometers, which are easy to install because they can be magnetically attached to a painted metal surface without removal of paint. The spBWIM system uses a single accelerometer for all traffic lanes to determine bridge displacement responses attributable to traveling vehicles and two accelerometers for each traffic lane to detect vehicle axles. Field measurements were performed using three different test trucks on an actual in-service bridge to study the effectiveness and accuracy of the proposed spBWIM system. The results show that the spBWIM system can be used to estimate gross vehicle weights (GVWs) to within an error of 615.3% compared with static weighing using a truck scale. Based on the results, the authors believe that the spBWIM system can potentially be developed into a viable alternative to conventional BWIM systems that use strain gauges. (C) 2017 American Society of Civil Engineers.