A testbed for evaluation of online distributed simulations of transportation system infrastructures is described that includes a modest portion of an urban road network in the midtown region of Atlanta, Georgia. The testbed includes sensors, servers, wireless communications, and mobile transportation simulations configured to model the testbed region. The system architecture for this testbed is described. Results of experiments evaluating wireless communication performance are presented. An implementation of an online traffic simulation based on a commercial simulator was developed, and results comparing the system's predictive accuracy with observed travel times through the simulated region are presented to illustrate a typical use of the testbed and to identify certain requirements for achieving reliable travel time predictions using online simulations.