Posted on: May 10, 2018 at 9:53 AM    

The recently issued GPS on Land Transport identifies accelerated implementation of the Speed Management Guide as a key strategy for reducing death and serious injury on our roads. This priority draws on a substantial evidence that proves small reductions in travelling speed can generate large reductions in crashes, both by number and severity of outcome. Having speed management at the core of a forgiving road transport system seems inherently sensible; yet there remains considerable public resistance to proposals that reduce speed limits. 

Whilst many transport issues and proposed changes generate public debate, discussions on speed seem to escalate to another level. Road users use their own experience to form opinions about what a safe speed is. They travel as fast as they feel comfortable. Consequently, when we as practitioners suggest a slower speed, the road user presumes this means a less efficient use of their time: the additional delay is perceived to be disproportionate to any potential benefit.

As industry experts, we know that crashes are predominantly due to system failures (~55% for fatal, ~90% for non-fatal) as opposed to extreme behaviours.  Whilst multiple factors often contribute to any crash, roads and roadsides are the most strongly associated with fatal crash outcomes.

Figure 1: AP-R560-18 Austroads Safe System Infrastructure, A compendium of Knowledge, Figure 2.2

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So why are the public so resistant to lower speed limits?

  1. They feel under time pressure to get to their destination, compounded by congestion.
  2. Going slower is a less efficient use of their time.
  3. They have lived with “high” speed limits for many decades and crashes are the fault of others.
  4. In the aim of accommodating vehicle volumes, we’ve created massive infrastructure that encourages speed – “efficiency = speed”.
  5. We live in an increasingly fast paced society – so doing anything slower seems like a step backwards.
  6. Research into the effects of reducing speed limit on higher speed roads (80-110km/h) demonstrate increases in travel time are proportionately less than the decreases in speed limit. Logically this relationship would be even less in CBD areas where traffic signals “add delay” to higher speed traffic.

In reality, the time implications of a slower speed limit in urban areas is likely minimal, especially during the day where average speeds are typically well below 40km/h already.

The problem lies in how to encourage consistently lower speeds rather than highly fluctuating start-stop speeds that underlie the lower average speed.  Applying self-explaining road principles (an environment that elicits safe behaviour simply by design) to high volume roads will be challenging for corridors which are necessarily wide to accommodate current traffic volumes.  Without substantial changes to the environment, experience has shown changing speed limits alone are unlikely to make a difference.

Figure 2: AM peak period travel speeds in Auckland

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Does that mean they shouldn’t be changed? With the continued application of safe and appropriate speed limits (in line with the Speed Management Guide) we, as practitioners, are trying to send road users a consistent message – that speed limits reflect the safe and appropriate maximum travelling speed. All corridors need to have environments that reflect their purpose – where place is more important than movement, then yes slower speeds should be encouraged and consequential reductions in (motorised) capacity may need to be an accepted consequence. Conversely where movement is a priority, speed limits will need to be comparatively higher, provided the characteristics of the road can safely accommodate those speeds.

Utilising a multimodal Network Operating Plan to identify place and movement priorities for different modes at different times of the day will help both stakeholders and road users to understand the overall approach to creating an effective transport system. It would build on the Roads and Streets Framework, demonstrating that by reducing speeds at these first/last leg locations, it will provide a more reliable journey time and enables reallocation of space to people and alternative modes.

As custodians of the transport network, we have a responsibility to continually, and often subliminally, educate the public in how to drive. Historically the public have heard the speed limit is the maximum safe speed to travel under optimum conditions and they have enjoyed the freedom to travel at speed, to the main part without incident. Now we must better manage that expectation around both the network’s capacity and choice of speed.  This can be achieved through more consistent and trusted speed limit setting that is reflective of the holistic road design and use,  that optimises safety and access to economic and social opportunities.

[Table 2.2 AP-R560-18 Austroads Safe System Infrastructure, A compendium of Knowledge [[1]] Extrapolation for Wundersitz, Baldock et al. (2011) and Stigson, Krafft et al. (2008)]

Blog written by Colin MacArthur, Associate Director, Abley

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