Discussion on Influencing Factors of Free-flow Travel Time in Road Traffic Impedance Function

Road traffic impedance is an important part of traffic assignment and has a direct impact on the urban transportation planning, especially on the optimization of urban road network. The BPR function has been the classical traffic impedance since 1960s, which pays more attention on the link level. This paper aims to construct an improved BPR function based on the influencing factors of road traffic impedance, such as the density of intersections, the density of bus stops, non-motor vehicles and saturation. These factors can reflect the difference between road and link. The real data collection from different roads in Dalian are used to calculate the road traffic impedance function by the SPSS software. The results show that the density of intersections, the density of bus stops and saturation have positive effects on unit travel time in Dalian roads. These results not only point out the necessary of influencing factors, but also validates the correctness of the proposed impedance function. In summary, this function has a simple structure and high flexibility so that it is expected to be transplanted to other cities.