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Current WiFi systems based on the IEEE 802.11 standard apply carrier sense multiple access with collision avoidance (CSMA/CA) to prevent users from transmitting simultaneously on the same channel. However, due to its random backoff nature, CSMA/CA does not perform well if the number of users is large. In this paper, we first measure traffic statistics, i.e., average ON/OFF periods and duty cycle, on the IEEE 802.11(ac) WLAN channel. Then we exploit the estimated duty cycle to design two functions, i.e., arctangent and Weibull functions, to adapt backoff parameters in CSMA/CA. For these two proposed functions, the backoff parameters initially increase as the estimated duty cycle increases, but they finally saturate and decrease respectively. The simulations show our proposed backoff functions outperform the conventional scheme using constant backoff parameters in terms of throughput performance.