ian bishop

Ian BishopTraffic Engineer

Developing an Approach to Speed Management on the Rural Road Network

The Need for an Approach

• The need for less speed – casualty reduction.– The effects of minor speed reductions on crash reduction. Doeck et al

(2011)– The relationship between free travelling speed and relative crash risk

Kloeden et al (2001)

• The most contested road safety treatment – why?

• Consistency and credibility is essential!

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Factors Influencing Speed

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• Attitudes towards speeding• Motorist’s perception of speed• Motorist’s perception of travel time• Understanding the road rules• Road infrastructure• Environmental factors

Attitudes Towards Speeding

• The broader community’s attitude counts!

• Study of 4100 participants (Lahausse et al, 2009) found:– Urban 50km/h and 60km/h restrictions were appropriate– Reductions to 40km/h and 50km/h considered too low– Rural limits perhaps too high

• Suggested 90km/h for sealed roads• 80km/h for unsealed roads

– Majority of participants admitted to consistently exceeding limit by 5km/h

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Motorist’s Perception of Speed

• Prolonged exposure to speed causes under-estimates (Triggs, 1986)

• Generally, speeds are under-estimated across the 30km/h to 100km/h range (Salvatore, 1968,1969 and Reason, 1974).

• Minor reductions occurred in average speed on SA roads with 110km/h to 100km/h posted speed reductions (Long et al. 2006)– Motorists already selecting reasonably safe speed.

• Visual Cues– Road Geometry– Roadside Objects– Treatments e.g. Transverse Lines

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Travel Time

• The most popular reason for contesting speed reductions

• 110km/h to 100km/h reduction:– T=d/s– 5.5min/100km– 9% increase in journey time

• Studies suggest t=d/s represents upper limit – 4 to 9% may be expected (Dutschke & Woolley, 2010)

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Understanding the Road Rules

• Lack of understanding the default limits!

• 2013 RAA Advocacy Survey – Default Rural Limit?

~ 54% responded 100km/h ~ 34% responded 110km/h ~ 6% responded 80km/h or 90km/h

~ 4% responded 120km/h

• Relationship between speed selection and age– Suggests educational campaigns have not been successful

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Road Infrastructure

• Factors:– Alignment,– Road Surface– Lane geometry– Sight distances– Access points / intersections– Medians

• Factors influenced in new roads with the design speed– Historically 2.5 second reaction time– Allows for margin of error, HOWEVER– Studies have shown motorists travel at the design speed

• Crash Countermeasures Vs Safe Speed

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Environmental Factors

• Urban and rural roads differ considerably in their engineering and community expectations (Lay, 1984)

• Weather conditions– Lahausse et al (2009)– 2002 Survey showed around 80% of participants opted for driving to the

conditions rather than meeting speed limit.

• Presence of wildlife (Rowden et al, 2008)– 66% of crashes occur at dawn or dusk– Country drivers found to slow significantly at dusk

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Developing an Approach

• Road Hierarchy• General approaches to setting speed limits• Austroads Model National Guidelines 2014• Addressing the unsealed network

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Road Hierarchy

• Safety remains paramount however mobility should not be unduly compromised

• Current guidelines offer a reasonably complex structure – simplify for speed!

• Specific zones within hierarchy can be subject to different limits• Netherlands re-classified road network to apply speed limits and

identify infrastructure upgrades (SWOV, 2010)• Re-classification undertaken on the principal of

– Ensuring no ambiguity of the intended road function– Establishing consistent practice– Application of credible and defensible speed

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General Approaches for Setting Limits

• Engineering Approach– Select base speed– Apply speed modification factors

• Expert System Approach– Software mimics expert’s thought process for complex problems– Considers factors including

• Road geometry• Roadside development• Nature and level of road user activity• Crash history• Existing speeds• Environmental factors• Traffic volumes

– USLIMITS2

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Austroads Model National Guidelines

• AP-R455-14 – Model National Guidelines for Setting Speed Limits at High Risk locations (March 2014)

• Considers mobility and function use of road

• Aims to provide consistent application of limits

• Considers key risk factors:– High severity crash rates per 100m VKT– Road use and users– Road features– Existing speed

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Austroads Model National Guidelines 2014• Promotes minimising frequency of changes in limits by adopting

route based approach.• Route based approach:

– Prevents localised reductions for hazards– Favours “speed limit ahead” signs– Recommends speed bands assigned in multiples of 20km/h

• Frequent risks to be documented• Emphasise of speed as a temporary treatment• Signing per AS1742 with addition of advisory plates• RAA generally supportive of document

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Default Limits & Unsealed Network

• Default limits for unsealed roads is 100km/h

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• Island Beach Road– High pedestrian activity in peak

season– Poor visibility– High wildlife activity– Numerous property accesses– Expanding suburb– 80km/h (AS1742:4)– Local roads 60km/h?

• Safe to post speed limits?

• NSW permits 80km/h limit

Blanket Approach!

Summary

• Factors influencing speed is complex – blanket approaches are inappropriate for this reason.

• Balance of safe systems approach and engineering risk based approach is required.

• Adoption of the 2014 Austroads Model Guidelines (exception: speed banding should remain at 10km/h, not 20km/h)

• Unsealed roads need new limits!

• Speed Zones must be consistent & credible!

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Questions

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