Up until now the estimated cost of natural hazard events has tended to focus on the direct physical cost, but the new study shows that the indirect costs can be as significant, if not even more. By coupling hazard data with a transport network model for Scotland, the researchers identify which transport corridors are most vulnerable across Scotland and the spatial distribution of population affected by each event. Ultimately they hope that this kind of model can produce more comprehensive hazard risk assessments, and will help planners decide which roads to prioritize for safeguard measures.

Benjamin Postance from the University of Loughborough, UK, and his colleagues assessed the indirect cost of landslides on Scottish roads using a national-scale traffic model. This model simulates the journeys of commuters around the UK and works out how many minutes they would be delayed when any one portion of road was blocked. This travel delay is then converted into a cost using nationally averaged values of travel time. By combining this model with landslide susceptibility data for Scotland, Postance and his colleagues were able to calculate the indirect economic losses associated with landslides on Scottish roads.

Their model shows that there are at least 152 sections of road in Scotland that are prone to landslide, which could result in indirect economic losses of more than £35,000 per day for every day of closure.

Rural regions were shown to be most exposed to travel disruption and landslide loss. "This is primarily due to the fact that landslides occur in rural areas," said Postance, "but also as the rural road network is sparse (road sections are very long and there are few alternative routes of similar length) and a large proportion of the rural population tends to make very long trips to work, to regional centres and cities, for example."

According to the new model the six most economically vulnerable sections of road were the A830 Fort William to Glenfinnan, the A9 Dalwhinnie to Calvine, the A90 Queens Ferry Road, the A82 Corran to Fort William, the A82 Achallader to Glencoe, and the A82 Fort William to Inverlochy.

But the model also showed that in some cases economic losses can be overestimated. For example, the estimated cost of a landslide on the A9 near Dunkeld comes in at £150,000 when only one diversion route is considered, but drops to £65,000 when a broader range of diversions are included. "Assuming one prescribed diversion results in an overestimate of cost, because in reality there are other roads and shorter routes available that could and likely would be used," said Postance.

Postance and his colleagues, who reported their work in Environmental Research Letters (ERL), now hope that their model will be used to produce more comprehensive hazard risk assessments. It isn't just landslides that it can be applied to; any hazard that results in disruption to road transport, including earthquakes, flooding, terrorism and road traffic accidents, could be modelled and assessed in this way.

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