Urban road tunnels are a reasonable remedy for the inconvenience due to the congested road traffic. However, they bring specific threats, especially those related with the possibility of fire outbreak. The work is a case study for an urban road tunnel, and uses detailed data on traffic intensity and its generic structure. It was shown that although tunnel fire accidents are very rare events, a single fire can be expected in a relatively short time perspective, up to 10 years. The probability of a catastrophic fire is only serious in longer time horizons (about 50 years). The next important finding is that the hourly distribution of the fire accident probability is uneven, it is significantly higher at daytime, reaching the maximum value at 3 pm. This probability on weekends is significantly lower than for workdays. Such quantitative findings may serve as a basis for decisions on temporary restrictions on heavy vehicle traffic.