We analyzed the relationship between the temperature and traffic accidents in Seoul-Incheon, Busan, Daegu, Daejeon, and Gwangju by the time-of-day (06:00 to 22:00, divided into segments of 4 h) and age of casualties between 2012 and 2017 for the summer season (June to August). A generalized additive model and meta-analysis were employed to analyze this relationship. We found that the threshold temperatures was observed to be approximately 30 degrees C. Above this temperature, traffic accidents increased in four urban areas, except Busan, which is a popular tourist location. In total, traffic accidents increased by approximately 0.59% (95% confidence interval of 0.41-0.75) per 1 degrees C increase in the daily maximum temperature, with substantial differences between the different areas, ranging from 0.12% (CI = - 0.26-0.50) in Busan to 1.08% (CI = 0.45-1.71) in Gwangju. The morning and evening hours showed a greater increase in traffic accidents than other timeframes. The increase in traffic accidents for young casualties was statistically significant at all times, and that for elderly casualties was observed at 10:00-14:00 and 18:00-22:00. The results of this study could provide information for developing customized traffic accident reduction policies considering time-of-day, age of casualties, and type of city.