Abstract: As the rapid development of the transportation industry, the steadily growing traffic load is gradually becoming a significant factor impacting the service safety of existing bridges. To investigate the influence of the actual traffic growth on the safety of long-span bridges, a methodology was presented for extrapolation of traffic load effect on long-span bridges considering an interval growing traffic model. An advanced Rice formula was developed considering a ‘time-disperses’ and ‘probability-in-series’ model to capture the time-variant characteristics of extreme traffic load due to traffic volume growth. Two verification examples demonstrated the accuracy and effectiveness of the proposed method in capturing the extrapolation of time-variant probabilities. Stochastic traffic load models were simulated based on monitored traffic load data. The maximum deflection of a suspension bridge was evaluated accounting for traffic volume growth in the bridge lifetime. The numerical results show that the time-variant characteristic of the extreme effect due to traffic volume growth is significant. The typical extreme value theory and the Rice's formula based on the assumption of independent and identically distribution are poor in capturing the tail data of the extreme value. However, the advanced Rice's formula is able to fit and extrapolate the extreme value. When the annual traffic volume growth ratio is in the range between 1% and 3%, the amplification factors of the extrapolated deflection of a suspension bridge in lifetime are in the range between 1.30 and 1.98. If the annual traffic volume growth ratio is greater than 3%, the maximum deflection in a return period of 1000 years of the bridge will be greater than the standard design value.