Map matching results are essential input to safety-critical and liability critical applications such as autonomous driving and vehicle tolling. However, GNSS positioning errors can significantly impact the reliability of these services. While aviation integrity systems and map matching techniques provide error detection and correction, they often fail to offer meet the accuracy requirements for lane-level navigation in challenging environments and provide no alert, causing loss of credibility. This study introduces a novel method for assessing GNSS credibility by integrating GNSS measurements with map information. By incorporating GNSS result covariance into the map-matching process, the approach generates a credibility probability for the trajectory, offering a comprehensive reliability assessment for safety alert. Validated with real vehicle data, the proposed method demonstrates significant potential for practical applications in safety-critical services.