Reducing speed limits on motorways was identified as a possible longer term transport mitigation measure in the National Mitigation Plan in 2017 (Measure T24) because energy use increases at higher travelling speeds. During the drafting of the Plan, my Department officials estimated that the impact of reducing maximum speed limits on motorways from 120km/h to 110km/h for cars and vans and from 90km/h to 80km/h for heavy goods vehicles compared to current speed limits would result in a potential cumulative greenhouse gas emission reduction saving of 1,881 tkCO2 over the period 2017-2030. Further analysis carried out by my Department during the development of the Climate Action Plan 2019 suggested that under the same speed limit reductions approximately 1,700 tkCO2 of transport emissions could be abated between 2020 and 2030. Assuming annual transport emissions remained relatively stable over this time period (c. 12 MtCO2 per annum) it could be expected that reducing speed limits could yield a 1.4% annual reduction in transport emissions.
There are possible co-benefits to reducing driving speeds, in relation to road safety and air quality; however, negative economic impacts, reduced fuel tax revenue, and longer travel times are also noteworthy concerns. Furthermore, it is possible that a proportion of the motorway traffic would divert on to secondary networks as a result of a reduced speed limit with associated unintended consequences. Reviews to the design speed for existing infrastructure and speed enforcement procedures would be required in light of any proposal to lower speed limits. It should also be highlighted that National Strategic Outcome 2 (Enhanced Regional Accessibility) in the National Planning Framework commits to improving average journey times for connectivity reasons, targeting an average inter-urban speed of 90kph; a reduced motorway speed limit may be in conflict with this aim.
Moreover, actual fuel consumption benefits of lower speed limits for motorway drivers is dependent on a number of external factors such as vehicle types, driving patterns, the frequency of speeding, road load patterns and congestion levels. I am given to understand that carbon savings from lower speed can also vary markedly depending on engine characteristics, vehicle weight, distances travelled and supplementary equipment such as rear mounted storage racks or even taxi signs. In addition, the vehicles we drive and the fuels that power them are steadily becoming less carbon intensive; new vehicles must comply with increasingly more stringent standards and the penetration of renewable biofuels as a share of road transport energy has increased significantly over recent years. In 2018, it is estimated that biofuel use saved over 500 tkCO2, over 4% of transport emissions. Such developments will erode emissions reduction benefits of reducing speed limits.
As we contemplate the extent of the climate challenge which we now face, it is imperative that we continue to consider all measures which may deliver emissions savings. The Climate Action Plan is to be reviewed annually, and I expect that my Department will examine once again the issue of decreasing speed limits as a potential emission-saving measure at this time.