Tehran is capital of Iran in wide metropolitan area. Large-scale disasters such as earthquakes in such a city cause many casualties and economical damage. One of the most effective risk mitigation actions in such disasters is emergency evacuation. This paper evaluates the dynamics of the vehicular traffic of Tehran under an evacuation condition. The main arterials of the city of Tehran are simulated by a macroscopic traffic flow simulation model (FastLane). The dynamics of traffic in the network are assessed by means of the Network Fundamental Diagram under different loading durations. Simulation results confirm the effect of loading duration and spatial spread of density on network performance. As was previously suggested in the literature, increasing loading duration leads to higher average network flow in the recovery phase compared to maximum network flow in the loading phase. Furthermore, simulation results demonstrate a clear decrease of average velocity by increasing the spatial spread of the densities in the network. These results can be used when making decisions regarding emergency evacuation plans for the city of Tehran.