Urban vehicle tunnels in Australia have proven popular with the commuting public, but have attracted criticism in relation to the in-runnel air quality, particularly in congested and incident traffic conditions. In response to community, political and media criticism, local Health Authorities have tightened regulations on permitted exposure to carbon monoxide. The previously specified CO limit of 87ppm (rolling 15 minute average) remains in place, but there is now an additional, more stringent requirement of 50ppm (rolling 30 minute average). Urban tunnels of around 3km and longer run the risk of having motorist travel times of around 15 minutes, or more, in congested conditions, in the event of a breakdown or other incident during peak traffic periods there is a high probability of motorists (and recovery crews) exceeding the new CO exposure criteria. This is particularly the case for stranded motorists whose overall exposure time to the CO environment can be in excess of 30 minutes in current tunnel configurations. Ventilation solutions alone result in higher construction costs and will not meet the revised high level performance criteria. The paper explores a range of traffic congestion incident scenarios, design CO limits, corresponding tunnel pollutant profiles, breakdown incident probabilities and impact on tunnel construction costs. It also discusses the relative merits of lane number, tunnel gradient, ventilation rate, vehicle recovery options, systems and operational procedures that should be considered in order to establish a design which meets the consumer expectations of providing a safe travelling environment, whilst providing an economic construction cost.
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