Disseminating warning information by vehicular ad hoc networks (VANETs) is of great significance to alleviate traffic problems in future intelligent transportation systems. However, in the urban expressway environment, it is critically challenging to design efficient dissemination mechanisms with strict quality-of-service (QoS) requirements due to complex road structures. In this paper, we propose the fast and reliable warning message dissemination mechanism (FRWD), where emergency messages can be delivered to a specific area (e.g., the entrance/exit of the expressway) in time and keep alive for a while. Hence, drivers can be informed to detour timely and avoid getting stuck on the expressway. In this scheme, two types of forwarders, relay vehicles and backup vehicles, are selected by balancing the forward progress and packet reception probability. Furthermore, a vehicle in the specific area is designed to periodically broadcast with a reasonable time interval. Through this, message dissemination with low latency and high reliability can be achieved. Theoretical analysis and simulation results demonstrate the effectiveness of our mechanism.