This paper mainly studies on the speed-flow relationships of the major and minor urban roadways, based on the characteristic of interrupted traffic flow on the roadways. Greenshields speed-flow model is used and calibrated by using the surveying data. Due to the fact that Greenshields model can’t fit the collected data very well for the congested situation, G-LN speed-flow model is then proposed by using piecewise modeling method. Greenshields model and G-LN model are compared by criterion R2. The result is that G-LN model is better model to describe the relationship. In the end, the values of speed are calculated and the values of capacity are obtained by section iteration method. It can be seen from the calculation results that the minimum volume is between 8km/h and 10km/h, which is different from traditional concept. When volume reaches to 60% of the capacity, the traffic flow will change from uncongested state to congested state.

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