Within the context of the Internet of Things (IoT), the distance-vector IPv6 Routing Protocol for Low-power and Lossy Networks (RPL) is one of the most popular choices for the network routing layer. Within RPL, distance calculation is abstracted by the Objective Function (OF) and two OF implementations have been standardized, namely OF0 and MRHOF. However, these OFs build network topologies where bottleneck nodes may suffer from excessive unbalanced traffic load. The load distribution problem is a major issue for existing OFs defined in RPL because it decreases network performance and the network's lifetime. In this paper, we propose a new OF called the Traffic Aware Objective Function (TAOF), which balances the traffic load that each node processes in order to ensure node lifetime maximization. To implement this OF, we altered the DIO message format, introduced a new RPL metric, named Traffic Rate, and used a new parent selection algorithm. Simulation experiments have been conducted to examine the performance of our proposal. The results show that TAOF achieves enhanced performance in terms of Packet Delivery Ratio (PDR) and that it builds more stable networks with fewer parent changes.