Unveiling the Impact of Speed Reduction Trials on SH6

Originally published 8 March 2024.

State Highway 6 (SH6) was the subject of speed reduction trials between Blenheim and Nelson as part  of New Zealand's Road to Zero program. The initiative rolled out in December 2020 aimed to reduce the number of fatalities and serious injuries on the nation's roads. Speed limits were dropped from 100kph to 90kph along much of the route, with some sections reduced to 80kph. 

SH6 Blenheim to Nelson speed review new speed limits map

Dramatic Safety Improvements

Stuff's recent report revealed a staggering 75% reduction in deaths and serious injuries during the trial period. In the ten years prior to the speed reduction trials, there were 19 deaths and 86 serious injuries, as opposed to one death and four serious injuries in the two years following the speed reduction trials. This equates to two people per year who safely return to their whanau and 6-7 people per year who 
avoid serious injury and trauma. This shows the profound impact of the speed reduction measures on reducing road trauma.

Analysing Travel Time Implications

The reduction in speed limits is often perceived to be at the expense of increased travel times and  subsequent inefficiencies in the operation of the transport network. Abley have analysed the impact of the trial on travel times, using TomTom's historical travel time and speed analytics data. We compared  data from the year before the speed limit reductions with data from the year after the speed limit reductions (allowing three months for driver behaviour to change).

The findings revealed that the change in median speed was a modest three km/h, meaning the average journey from Blenheim to Nelson increased by approximately three minutes after the reductions, a journey that normally takes one hour and 19 minutes. This 3.9% increase in travel time is very small in the context of a 75% reduction in deaths and serious injuries along the corridor.

Safety benefits versus increased travel times

Note that this is not a full economic appraisal of the speed reduction, as the implementation costs and other cost/benefit categories are not included, but it is expected to capture the primary social and user benefits/disbenefits of the speed reduction.

NZ Transport Agency Waka Kotahi Monetised Benefits and Costs manual (MBCM) includes procedures for practitioners to calculate the tangible benefits associated with changes in travel times and road safety outcomes as part of a wider suite of evaluation metrics. The additional travel time cost (or disbenefit) associated with the slower speeds along the SH6 corridor are estimated to be $3.0 million annually. The benefit of reducing the number of deaths or serious injuries by 75% equates to 8 fewer road users being killed or seriously injured. The safety benefits are a whopping $27.3 million annually.

To put it plainly, the safety benefits on this corridor outweigh the additional travel time in terms of economic and social cost by a ratio of 9:1.

We understand that traffic management and closures occurred during 4-5 months of this two-year period due to the impacts of the August 2022 storm event which affected the region. When this period is removed from the analysis the number of deaths and serious injuries increases from 2.0 to 3.2 per annum. This still strongly demonstrates the safety benefits outweigh the travel time disbenefits by a factor of approximately eight to one.

The speed reductions on the SH6 corridor were greater for faster vehicles along the corridor as evidenced by the 85th percentile travel times extracted from the TomTom data. The reduction from 87kph to 82kph corresponds to a 6.3% reduction in speed. Similarly, the slower vehicles in the fleet also reduced speed from 69kph to 67kph because of the trial. 

Our analysis of the TomTom analytics data demonstrates that the implementation of speed reductions on SH6 is a resounding success story. The substantial reduction in road trauma is not offset by the slightly increased travel times along the corridor. The SH6 trial results provide tangible evidence of the benefits of implementing speed reductions across higher risk corridors on our transport network.

How Abley can Help

Get in touch with Regan to learn more about how Abley’s transportation planning experts, and TomTom analytics data can provide you with insights and evidence that support your decision making.