In 2017 my Department published the The Costs of Congestion - An Analysis of the Greater Dublin Area. The methodology for how these costs were estimated is described in detail in Appendix B of that report, which is available on my Department's website. For ease of reference, I include a link here:
http://www.dttas.ie/corporate/publications/english/cost-congestion-appendix-b-methodology.
The report focuses specifically on the direct impact, in terms of the monetary cost of increased travel time, on road users in the GDA. It does not, therefore, estimate the impacts of congestion on pollution or collision rates. However, as referenced in the report, the cost of congestion study carried out by New Zealand Transport Authority estimated that the value of time impact accounted for 92.5% of the total cost, which included emissions and environmental costs, vehicle operating costs and indirect costs such as schedule delay costs.
Using this rule of thumb, the 2013 costs of congestion would rise from €358m to €387m, and the 2033 costs would rise from €2.08bn to €2.25bn. It is worth noting that this would still not account for collision rates.
There isn't definitive evidence on the impact of congestion on collision rates. On the one hand, increased traffic can lead to increased numbers of incidents. but, conversely, congestion reduces speeds which can reduce the likelihood and severity of collisions.
In 2015 my Department published the Strategic Investment Framework for Land Transport,
which identified addressing urban congestion as one of three key priorities for future investment. The cost of congestion report is a valuable piece of evidence on the extent of the problem in one city region, and it is planned to extend this analysis to Ireland's other cities, in collaboration with the National Transport Authority and its regional modelling system.
