There are two different approaches for the evaluation of tunnel risks.
Relative evaluation approach
Confirming that the required safety level is met by carrying out a relative comparison with a reference tunnel
For carrying out a relative risk evaluation by means of a relative comparison, the reference risk profile of a comparable reference tunnel has to be determined. The reference tunnel must completely fulfil the minimum safety requirements stipulated in the Austrian road tunnel safety law (STSG) and thus exhibit a tolerable risk level.
The minimum safety level is considered to be fulfilled, if the risk of the analysed tunnel corresponds to that of the reference tunnel or is lower. If the tunnel does not fulfil the minimum safety requirements according to STSG, additional risk mitigation measures are required.
Risk of the tunnel under investigation with special characteristics
Risk of the reference tunnel fulfilling the minimum safety requirements
Modified risk of the investigated tunnel (including risk reduction measures)
Absolute evaluation approach
Determining the standard-related level of equipment by assigning the tunnel to a risk class on the basis of the absolute risk value
In the absolute evaluation approach the analysed tunnel is assigned to one of four risk classes based on its expected risk value. The risk class determines which level of equipment has to be chosen.
Assessment of Measures
Additional measures are required if the minimum safety level cannot be reached. These can be measures relating to traffic, operation and construction as well as to tunnel equipment, including combinations of different measures.
The effectiveness of the chosen measures in minimising risk has to be evaluated in a quantitative manner by inserting them into the risk analysis model; measure combinations have to be inserted together into the risk analysis model (i.e. mutual influence of combined measures on their effectiveness). If individual measures cannot be evaluated in a quantitative manner, their effectiveness has to be represented in a qualitative manner. If the risk reference value is only slightly exceeded, a qualitative representation of the effectiveness of an appropriate measure can be taken as compensation.
Risk mitigation measures should be applied to the extent possible for those areas that have been identified by the risk analysis as being problematic (e.g. measures aimed at minimising the risk of fire). In choosing appropriate measures, cost-effectiveness according to the ALARP principle (As Low As Reasonably Practicable) can be taken into consideration (avoiding costs that are disproportionally high in relation to the achievable benefit).