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

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 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. Project Conference 2019 - Toowoomba – Mark Tomarchio 2

Motivation & Problem Statement Articulated Heavy Vehicles – Austroads Classes 6 - 12 • Project Conference 2019 - Toowoomba – Mark Tomarchio 3

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 Project Conference 2019 - Toowoomba – Mark Tomarchio 4 4

Literature Review Roundabout safety performance is largely based on its speed reduction properties • Project Conference 2019 - Toowoomba – Mark Tomarchio 5 5

Literature Review All roundabouts are essentially a series of reverse-curves • Project Conference 2019 - Toowoomba – Mark Tomarchio 6 6

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 Horizontal Curve Equation: Project Conference 2019 - Toowoomba – Mark Tomarchio 7 7

Literature Review Project Conference 2019 - Toowoomba – Mark Tomarchio 8 8

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 Project Conference 2019 - Toowoomba – Mark Tomarchio 9999699 9

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) Project Conference 2019 - Toowoomba – Mark Tomarchio 1010101061010 10

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 Project Conference 2019 - Toowoomba – Mark Tomarchio 11 11

Methodology Analysis of current central island radii and circulating carriageway widths • Project Conference 2019 - Toowoomba – Mark Tomarchio 12 12

Methodology Reverse Curve Approach Geometry Analysis – Austroads (2015a) • Project Conference 2019 - Toowoomba – Mark Tomarchio 13 13

Methodology Reverse Curve Approach Geometry Analysis – Main Roads Western Australia • Project Conference 2019 - Toowoomba – Mark Tomarchio 14 14

Methodology Reverse Curve Approach Geometry Analysis – Florida Department of Transport • Project Conference 2019 - Toowoomba – Mark Tomarchio 15 15

Methodology – RB1 Case Study Project Conference 2019 - Toowoomba – Mark Tomarchio 16 16

Results & Discussion Recommended approach reverse-curve geometry • Project Conference 2019 - Toowoomba – Mark Tomarchio 17 17

Results & Discussion RB1 – ARNDT Modelling • Recommended geometry tested in ARNDT • ARNDT predicts vehicle speeds and crash rates • Compared existing versus recommended crash • rates Project Conference 2019 - Toowoomba – Mark Tomarchio 18 18

Results & Discussion Input roundabout geometry • Project Conference 2019 - Toowoomba – Mark Tomarchio 19 19

Results & Discussion Input initial approach design speed & traffic flow parameters • Project Conference 2019 - Toowoomba – Mark Tomarchio 20 20

Results & Discussion Crash Predictions – Preliminary reduction of overall crashes per year by approx. 5.2% • Original: Modified: Project Conference 2019 - Toowoomba – Mark Tomarchio 21 21

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 Project Conference 2019 - Toowoomba – Mark Tomarchio 22 22

Results & Discussion Crash Predictions – Final reduction of overall crashes per year by approx. 11.7% • Original: Recommended: Project Conference 2019 - Toowoomba – Mark Tomarchio 23 23

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 Project Conference 2019 - Toowoomba – Mark Tomarchio 24 24

Recommendations Project Conference 2019 - Toowoomba – Mark Tomarchio 25 25

Further Work 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) • Project Conference 2019 - Toowoomba – Mark Tomarchio 26 26

Questions? Project Conference 2019 - Toowoomba – Mark Tomarchio 27 27