Posted on: January 8, 2020 at 1:59 PM    

Reducing speed limits to improve road safety are often opposed by people who blame crashes on poor driving ability rather than speed. They claim that well-trained drivers are infallible and can handle high speeds safely. But anyone can make a mistake while driving and due to the laws of physics and human body vulnerability, mistakes at high speed can result in vehicle occupants and other road users being killed or seriously injured. The design and operating speeds of our roads needs to be more forgiving of the mistakes we make.

In some situations, a forgiving road environment can be achieved through infrastructure alone. Freeways and motorways are examples of where the road infrastructure supports higher vehicle speeds (100km/h and above).  The crash risk on expressways with at-grade intersections can also be reduced by using safety barriers and roundabouts.

Because of high costs, infrastructure improvements are not feasible for many roads. Much of the rural road network carries low traffic volumes and in urban areas it can be difficult to physically separate vulnerable road users from vehicle traffic, for example with pedestrian overbridges and off-road cycle paths. On these roads, reducing speed is an effective step towards achieving a road system that is more forgiving of human error.

In 2016, the New Zealand government reviewed every road in the country to determine their ‘safe and appropriate’ speed. The ‘safe and appropriate’ speed is defined as the travel speed that is appropriate to a road’s function, design, safety and use, based on criteria set out in the New Zealand Speed Management Guide. The safe and appropriate speed, crash metrics and a proactive infrastructure risk metric were analysed for every road and mapped in the NZ Transport Agency’s Safer Journeys Risk Assessment Tool. The output showed that most (about 95 percent) of the New Zealand road network has a speed limit that does not align with its safe and appropriate speed. 

Local authorities use the Safer Journeys Risk Assessment Tool to identify treatment strategies targeting the top ten percent of the road network which have the greatest potential for death and serious injury reduction. These strategies include “engineering up” to make the road safe at the current speed limit, reducing the speed limit where the road is “self-explaining” (where the design of the road supports a lower speed), or reducing speed limits where operating speeds may not support it. The third option is labelled “challenging conversations”, as getting public support for speed limit reduction on these roads is difficult, and other infrastructure changes may be required to reinforce the lower limit.  

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Engagement with the community, road users and other stakeholders is vital in this process. The New Zealand government provides resources to support conversations about road safety risk and the benefits of speed management. Local authorities then work with their communities to develop options to reduce road safety risk, which could include a combination of infrastructure improvements and speed management options. Once the community understand the crash risks and sees the options and costs, then they better understand why speed limit reductions are appropriate for many roads.

Shane will be presenting on the New Zealand Speed Management Guide and its application at two sessions at the TRB annual meeting in Washington DC in January 2020 (workshop on Sunday 12 Jan, and podium session on Monday 13th Jan). 

For case study examples please refer to the following Austroads Report:

Blog written by Shane Turner, Technical Director, Road Safety

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