According to the characteristics of actual traffic stream, an on-road speed estimation model and algorithm based on wireless magnetic sensor networks was researched. In this model, we used 3 sensor nodes working together to estimate the speed of passing vehicle. To achieve long-life of the model, the mode was designed as a hierarchy architecture to reduce the power consumption. Furthermore, we presented a power consumed scheme, Duty-cycling-V, which could take the speed of vehicle queue as parameter for dynamically adjusting the working cycle of the sensor node. And we used it to the key sensor node of the model, which could farther reduce the power consumption. And the results of the emulation and on-road experiments are demonstrated that the vehicle speed captured by the 3 nodes model is more precise and better power efficiency than the 2 nodes detection model.