Urban VANETs will be an important part of intelligent transportation in the future as it provides an effective solution to solve the city traffic problem. Most of the existing routing schemes have low performance on efficiency and reliability, because they can't solve the problems of high speed mobility, complex city traffic environment and different traffic density. Focusing on the urban environment of signal propagation and traffic pattern with red-light stops and fixed and cyclic bus route, this article proposes a layered urban vehicle ad-hoc network routing protocol model on mobility support in public transportation. The model transfers the complex and uncertain routing protocol model between the vehicles to collaborative and definitive three layered routing, taking advantages of the fixed bus route to optimize communication between ordinary vehicles and buses with the cluster head strategy. Mechanism of single-hop selection is provided according to the signal propagation attenuation model and vehicle mobility model, and mechanism of multi-hop forwarding is given based on delay probability. The resulted design can ensure the reliability and efficiency on bus-assistant forwarding. A cluster trigger strategy capable of adapting to the vehicle density is also presented. The simulation results show that the whole scheme can achieve good data delivery and low delay with sufficient hops.