A study of interference caused by incumbent radios to UWB devices is presented. Through an extensive set of measured spectral activities in the low-band IEEE802.15.4 UWB operating channels, we explore the outage probabilities of a UWB system when it is equipped with an intelligent detector for combating interferers and compare them against those of a naive system that has no interference avoidance capability. Our results reveal that a UWB system with an effective interference avoidance capability may lead to a few orders of magnitude improvement in its outage probabilities. The measured outage probabilities are confirmed through measuring the frame error rate of a simulated filter-bank UWB transceiver system when impacted by the interferences obtained through our experiments.