Routing metric plays a vital role in the performance of routing protocols for multi-interface WMN. How to capture the traffic interference more precisely and improve both the capacity and real-time of WMN become difficult problems on the design of routing metrics. In this paper, we propose a new interference-delay aware routing metric for multi-interface WMN, called IDA metric. The IDA metric captures the inter-flow load interference and the intra-flow load interference within two hops. Moreover, the IDA metric integrates transmission delay, packet loss ratio and transmission rate. Base on this, we present a novel routing protocol, called IDAR, which uses the IDA metric to select routing paths. The simulation results show that the IDAR protocol outperforms other routing protocols in terms of the average throughput, end-to-end delay and packet deliver ratio.