A large ring road around a major Spanish city has undergone a significant upgrade to improve traffic flows, open up areas for redevelopment and lessen its environmental impact. Parts of the works include lowering sections of ground level motorway into tunnels. A probabilistic fire fighting access analysis was performed for one part of the tunnels. The main aim of the analysis was to evaluate acceptable distances between fire fighting access points. Two different distances were evaluated; 100 m and 200 m. The acceptable distance depends heavily on the type of fires foreseen for the tunnel and the different technical systems (detection, ventilation, etc.) incorporated into the tunnel. This paper discusses the methodology of the analysis. It describes how event tree analysis and CFD modeling were used. The paper gives a description of how a probabilistic approach can be used to evaluate the fire fighting access to a tunnel. The main conclusion for the scenarios evaluated was that it is acceptable with access points at 200 m centres instead of 100 m centres for this specific section of the tunnels. This was due to the minor consequences associated with increase in distance (from 100 m centres to 200 m centres) between accesses points produced, which amounted to some extra time in smoke free conditions while approaching the fire. Another important conclusion from the fire fighting access evaluation was that the probability of a successful fire fighting operation is much higher if the fire brigade is equipped with infrared equipment. The configuration of the ventilation system is very important; fires will behave differently depending on where in the tunnel they start. When configuring the ventilation system the fire size and the fire location are the two most important factors to keep in mind.