In this paper, the rapid mobility of MIP/WLAN is emulated on a test-bed. The performance of MIP/WLAN at different moving speeds is evaluated. The result shows that current MIP protocol is not suitable for rapid moving environments. This paper depicts the relationship between the performance and the moving speed and breaks down the handoff latency of MIP/WLAN. A Speed Adaptive MIP extension is proposed and implemented on Hierarchical MIP. The emulation result shows that the Speed Adaptive MIP greatly improves the performance of MIP/WLAN in rapid moving environments.