The use of mobile phones by drivers while stopped at red traffic lights represents a potential threat due to the cognitive residuals from recent phone use. This paper presents the results of a study that investigated the rates, strategies, and activities of phone use in this situation using both video-based naturalistic and online questionnaire data, and analyzed its significant influencing factors. The study found that about 37% of drivers used their phones at traffic lights, and the majority (85%) of phone use was initiated at intersections, suggesting the relevance of phone use to traffic lights. The questionnaire data revealed that the main purpose of phone use at red lights is social media (32%), followed by text messaging (19%) and map navigation (13%), with the phone ringing, boredom, and habit being the main reasons for engaging in such activities. The logistic regression model identified waiting time, time of day, driver's age, vehicle category, vehicle value, driver's license age, and the presence of passengers as predictors for phone use. Policymakers and driver educators can find useful information in this paper to enforce laws and educate drivers to ensure safety and efficiency at signalized intersections, thus avoiding potential accidents.