<p>Bus transportation offers an effective alternative to private vehicles, promoting a more sustainable service and playing a crucial role in urban mobility. However, different factors can affect bus reliability in service operations. The difference between actual and scheduled arrival times at each stop is used to assess reliability. It accounts for both bus delays, when the vehicle runs behind schedule, and early arrivals, when the vehicle runs ahead of schedule. This paper proposes an approach to assess bus service reliability based on Global Navigation Satellite System (GNSS) data from connected vehicles. The initial step includes estimating bus arrival time at each bus stop. The second step consists of comparing the actual bus arrival time with the expected scheduled time. The study area is located at the main campus of the University of Campinas in Brazil. GNSS data were obtained from a monitoring system called Main Module IoT (MMIoT), which is responsible for providing bus trajectory information. In addition, occupancy data from a bus passenger counting (BPC) system installed in the same buses are used to evaluate the impact of bus occupancy on travel time reliability. Results from input data analysis revealed significant bus delays, especially during peak hours. On-time performance (OTP) is used as an indicator of bus service reliability. Moreover, Pearson correlation coefficients are used to examine the association between service reliability and bus occupancy on campus. Finally, the results provide insights into improving bus service reliability and efficiency on campus, thereby potentially increasing passenger use and satisfaction.</p>