Improved theory for the design of high-speed roundabouts to suit - - PowerPoint PPT Presentation

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Improved theory for the design of high-speed roundabouts to suit - - PowerPoint PPT Presentation

Jan 25, 2023 277 likes •564 views

Improved theory for the design of high-speed roundabouts to suit heavy vehicles Mark Tomarchio 1 Motivation & Problem Statement Being a road designer, I wanted a topic relevant and useful to the transport sector The problem in

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Improved theory for the design

  • f high-speed roundabouts to

suit heavy vehicles

Mark Tomarchio

1

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Motivation & Problem Statement

  • Being a road designer, I wanted a topic relevant and useful to the transport

sector

  • The problem in general is that there is insufficient clear and comprehensive

guidance in the current standards regarding the specific geometric requirements for safe operation of high-speed roundabouts by heavy vehicles.

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Motivation & Problem Statement

  • Articulated Heavy Vehicles – Austroads Classes 6 - 12

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Aims & Objectives

  • Analyse and evaluate the commonly available standards and guidelines for high-

speed roundabouts

  • Identify if and where improvement in the standards may be required
  • Develop a document that clearly and comprehensively details the design

standards and guidelines specifically catering to heavy vehicles on high speed roundabouts, in the format of a supplement to current Austroads Guide to Road Design (AGRD) standards.

  • Case Study and analysis of existing roundabout to confirm findings and test

recommendations

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Literature Review

  • Roundabout safety performance is largely based on its speed reduction properties

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Literature Review

  • All roundabouts are essentially a series of reverse-curves

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Literature Review

  • The speed reductions work on the “Limiting Curve Speed” theory, ie a driver will visually assess a

curve, and decelerate to the maximum speed considered to be comfortably tolerable

  • The indicator of this comfort level is the amount of coefficient of side friction factor “f” generated

during travel of the curve

  • Side friction factor is a measure of the frictional force between the pavement and the vehicle tyre,

required to keep the vehicle on the road 7 Project Conference 2019 - Toowoomba – Mark Tomarchio 7 Horizontal Curve Equation:

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Literature Review

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Literature Review

  • Car maximum comfortable f = 0.35 (at low speeds)
  • Current Australian standards assume vehicles will negotiate roundabouts at side

friction factors up to f = 0.5, and this is what many of the central island radii, approach curve geometry, speed prediction models etc are based on

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Literature Review

  • Trucks however, will generally become unstable and roll at side friction factors of 0.3

and up, with the least stable vehicles having a roll over threshold as low as f = 0.24 (Ervin et al 1986)

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Literature Review

  • Furthermore, car drivers are likely to enter the curve at a speed that is high for the

curve, and continue to decelerate within the curve, commonly within the first 80m.

  • Truck drivers will generally decelerate to what they consider the appropriate speed

before the start of the curve, because of the dangers associated with trucks braking on curves.

  • Therefore, trucks require a sufficient “tangent” length, ie length of straight between

successive reverse curves, to facilitate braking

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Methodology

  • Analysis of current central island radii and circulating carriageway widths

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Methodology

  • Reverse Curve Approach Geometry Analysis – Austroads (2015a)

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Methodology

  • Reverse Curve Approach Geometry Analysis – Main Roads Western Australia

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Methodology

  • Reverse Curve Approach Geometry Analysis – Florida Department of Transport

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Methodology – RB1 Case Study

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Results & Discussion

  • Recommended approach reverse-curve geometry

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Results & Discussion

  • RB1 – ARNDT Modelling
  • Recommended geometry tested in ARNDT
  • ARNDT predicts vehicle speeds and crash rates
  • Compared existing versus recommended crash

rates

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Results & Discussion

  • Input roundabout geometry

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Results & Discussion

  • Input initial approach design speed & traffic flow parameters

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Results & Discussion

  • Crash Predictions – Preliminary reduction of overall crashes per year by approx. 5.2%

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Original: Modified:

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Results & Discussion

  • Unique linemarking treatment on the approach allows truck drivers to drive a

larger radius (ie tracking over the chevron markings) and the edge lines guide the cars through a smaller radius to restrict their speed

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Results & Discussion

  • Crash Predictions – Final reduction of overall crashes per year by approx. 11.7%

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Original: Recommended:

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Conclusions

  • Current standards essentially contain all necessary information, however it is

spread across multiple chapters/sources, and doesn’t show how to specifically apply heavy vehicle requirements to roundabouts

  • Curve radii specified in current examples are insufficiently sized for heavy

vehicles, resulting in side friction factors that induce instability when driven at intended speeds

  • Insufficient lengths of straights are specified in the current examples, which does

not allow heavy vehicles to decelerate to appropriate speeds to safely negotiate the curves

  • Insufficient advanced warning is provided to heavy vehicle drivers to warn them

that a potential hazard (the reverse-curves) are ahead

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Recommendations

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Further Work

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  • Additional case studies (ie have only looked at one roundabout in detail thus far,

studies like Arndt’s looked at 100’s over 10 – 20 years)

  • Supplement to be reviewed by a road authority (eg TMR E&T or Austroads)
  • Trials (build using recommendations, monitor and evaluate)
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Questions?

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