1 Introduction  
2 Related Surveys  
3 Background  
3.1 Intelligent Transportation Systems (ITS)  
3.2 General Introduction to Graphs and Graph Neural Networks  
3.3 Graph Neural Networks (GNNs)  
4 The Applications of GNNs in ITS  
4.1 Traffic Forecasting  
4.2 Vehicle Control System  
4.3 Traffic Signal Control  
4.4 Transportation Safety  
4.5 Demand Prediction  
4.6 Parking Management  
5 Challenges and Future Directions  
5.1 Research Challenges  
5.2 Future Directions  
6 Conclusion