Tunnel accidents with transports of combustible liquefied gases may lead to explosions. Depending on the substance involved this can be a Boiling Liquid Expanding Vapour Explosion (BLEVE), a Gas Expansion Explosion (GEE) or a gas explosion. Quantification of the risk of these scenarios is important to take informed decisions on tunnel design and routing of dangerous goods. In a risk analysis scenarios have to be taken into account each with their own physical effects (overpressure and fire), consequences (damage to the tunnel, lethality, injury), and probability of occurrence. The current paper focusses on the gas explosion scenarios. First the gas explosion mechanism is summarized to introduce the key parameters for the explosion load in a tunnel. Then an integrated model for the dispersion and probability of ignition is presented and applied. Case studies with an instantaneous LPG release show that the cloud is initially too fuel-rich to be ignited. Subsequently the gas concentration either remains mostly fuel-rich by the time the cloud reaches the end of the tunnel or falls within the flammability limits before the tunnel exit, depending on the initial amount of fuel released. A case study with a continuous LPG release shows that depending on the release rate and the ventilation speed, the tunnel may be completely filled with a combustible fuel-air mixture. The simulations provide the overpressure for different scenarios linked with their probability of occurrence.