An approach is provided for detecting lane-level dangerous slowdown events based on probe data and/or sensor data. The approach, for example, involves splitting probe data, sensor data, or a combination thereof into at least one vehicle trajectory, wherein the data is collected from one or more vehicles traveling on a road segment. For each vehicle trajectory of the at least one vehicle trajectory, the approach also involves processing said each vehicle trajectory to detect a slowdown event based on a speed reduction greater than a threshold reduction. The approach further involves classifying a slowdown event type of the slowdown event based on a final driving location, a final driving speed, or a combination thereof of the at least one vehicle trajectory. The approach further involves providing the slowdown event, the slowdown event type, or a combination thereof as an output for the road segment.