The link quality based Collection Tree Protocol (CTP) has been demonstrated an efficient and reliable routing protocol for data collection networks that deploy a clustered tree topology. However, CTP measures the link quality by packet throughput rather than transmission latency, which is not appropriate for networks supporting sensor events with transmission time deadlines. In this paper, we propose an average transmission time (ATT) based routing protocol under which real-time events are transferred along the routes with the shortest transmission time expectation, rather than the ones with highest throughput. Simulation experiment on TinyOS platform demonstrates ATT protocol outperforms existing ETX approach by reducing the packet transmission delay up to 17.7%, while maintains same level of transmission reliability. The proposed ATT protocol has shown to be a promising core feature in construction of a real-time transport protocol for data collection networks.