Mr. Derek Brady:

I thank the Chair and members of the committee for the invitation to attend today. I was appointed as head of strategic and transport planning in TII in 2019. I have a degree in electronic engineering, a masters of science in applied maths and computing and I have worked in the transport sector since 2000. I am joined by my colleague, Ms Helen Hughes, director of professional services in TII since 2015. Ms Hughes oversees several technical areas, including road safety, research and standards, environmental policy, and strategic and transport planning. I have been invited to discuss a TII report published in March entitled "Impact of National Road Speed Limit Reductions on Greenhouse Gas Emissions".

In 2020, the Department of Transport requested analytical assistance to investigate measures to reduce carbon emissions associated with the national road network. A range of speed limit reduction scenarios for national roads was assessed by TII on behalf of the Department. A key measure under consideration was the reduction of speed limits on the motorway network. Before explaining the analysis in more detail, I will provide an overview of the relationship between speed and carbon emissions in fossil fuel-powered vehicles in Ireland. Generally, emissions are highest at low speeds while the optimum speed range for petrol and diesel vehicles for limiting greenhouse gas emissions is 60 km/h to 70 km/h, depending on vehicle type. Emissions increase again above these speeds albeit more gradually. The greenhouse gas emission characteristics of the entire national vehicle fleet are a key data set in TII’s road emissions model. A detailed representation of travel patterns and traffic conditions on the network is included in TII’s national transport model, which is a multi-modal, primarily inter-urban model that includes data sets such as place of work, school or college - census of anonymised records, CSO POWSCAR, as well as other data sources. Both the transport and emissions models were used in this assessment.

In addition to detailed information on trip origins and destinations, the transport model holds road network information including speed limits for all national and regional road links. The model predicts the route drivers choose when faced with a choice of route options by estimating the generalised cost of competing route options. Generalised cost accounts for journey time, fuel and toll costs along different routes. The traffic prediction mechanism in the model is validated through comparison of the model predictions of traffic levels with actual records of traffic levels from 350 automatic traffic counters nationwide.

With a fully validated national model correctly predicting driver route choice, TII can test the impacts associated with network changes such as changes in speed limits.

TII's road emissions model integrates data on the current vehicle fleet with emission rates. The fleet information is based on the Irish car stock model version 2.1 provided by the MaREI energy and climate research team in UCC. The emission rates were sourced from the UK Department of Environment, Food and Rural Affairs emissions factor toolkit, which in turn was based on European COPERT emission rates.

TII analysis shows that the application of speed limit reductions to the motorway network alone would reduce the time advantage that motorways offer over alternative parallel single carriageway routes. This would result in traffic diversion from the motorway on to alternative national primary and regional routes, which also have speed limits of 100 km/h over large sections. Some of these parallel single carriageway road networks were formerly interurban national roads, for example, the former N8 between Durrow and Cork is now the R639 regional road passing through the towns of Urlingford and Johnstown, while the former N6 between Kinnegad and Galway is now the R446 regional road passing through towns such as Moate and Kilbeggan. With diversion, traffic would also be diverting back through bypassed towns and villages.

With the scenario involving a 10 km/h speed limit reduction applied to motorways only, the analysis shows that there would be a national decrease of 0.62% in carbon emissions from transport. This rises to 0.79% when a 10 km/h reduction is applied to all national roads. This is not an insignificant reduction. However, diversion of traffic to routes where motorists drive through towns and villages with stop-start driving conditions will limit the carbon emission reductions. Greater carbon emission reductions would be likely to be achieved when there is no traffic diversion. This could be achieved through broader application of speed limit reductions across the entire road network, not just on national roads.

The Department also requested that TII carry out a high-level strategic analysis on the economic and safety impacts of any national road speed limit interventions. TII used its national transport model to compute the journey time impacts of speed limit reductions. Standard values of time were used along with the journey time information to give insight into the economic impact. These are listed in appendix G of the report. For safety impacts, TII applied a standardised injury collision rate to the rerouted traffic. For the scenario where a 10 km/h speed limit reduction was applied to motorways only, there was a forecasted annual increase in road fatalities of five persons. With a 10 km/h reduction applied to all national roads, there was no forecasted change. With higher speed limit reductions applied to national roads only, the forecasted fatalities increased.

This negative safety impact is because of increased traffic diversion to less safe roads. The alternative routes available to motorway drivers include single carriageway former national primary roads between the five cities. They mostly have speed limits of 100 km/h and have low traffic volumes which facilitates driving close to the speed limit. There are accesses to many properties, there are at-grade junctions and the alternative routes pass through towns and villages. As with emissions, a wider speed limit intervention beyond national roads is not likely to cause traffic diversion and consequent safety issues.

These scenarios tested and referred to in the report were the first set of possible speed limit interventions requested to be studied by the Department. TII is working with the Department and the NTA to carry out further analysis on the impact of speed limits changes across the entire road network having regard to road safety, emissions and the implications for active travel. This work is also in the context of a wider speed limit review being carried out as action 6 in the road safety strategy.

I refer to appendix A in the report, which shows that by far the greatest carbon emissions from any vehicle with an internal combustion engine occur when the vehicle is travelling at low speeds. Concentrated, slow-moving traffic in congested urban areas results in higher amounts of emissions per vehicle kilometre travelled. This is a particular issue for freight vehicles on busy national roads. TII considers that the application of traffic management and modal shift measures in urban areas could yield significant reductions in emissions. We will endeavour to answer any questions the committee may have. If we cannot provide a comprehensive answer today, we will provide a response as soon as practicable.