Most Algorithms for detecting incidents have been developed under the premise that the congestion must happen whenever an incident occurs. For the reason, the performance of algorithm could not be guaranteed in case the congestion does not happen due to the traffic operations of low flows despite the occurrence of an incident. This paper is to propose a new diagram that can reliably detect the incident under the various conditions of traffic operations including a low volume state. Compared to the McMaster Algorithm, the newly proposed diagram is tested with three different cases that the incident occurs in the traffic operations with the low volume state, the relatively high volume state, and the recurrent congestion section. Evidently, the new diagram shows a capability to identify the flow characteristics of incident for all the three cases.