38
Technical Paper
Transportation P rofes s ion al
S eptem ber 2009
| Reprinted by permission. All rights reserved. From Transportation Professional, magazine of the Institution of Highways & Transportation, September 2009. Website: www.iht.org |
Dynamic roundabout design: Research and development Software has been developed to speed up the often long winded, iterative method of roundabout design. Daniel Shihundu describes the research and development behind development of TORUS. Introduction Few motorists will appreciate the process that underpins the operational and safety benefits inherent in good roundabout design. High quality design promotes safer driving with improved traffic flow. It is a delicate balance between traffic capacity and physical geometry. This paper summarises the typical roundabout design process with regard to the interrelationship between layout and traffic speed. Field research conducted in the development of a new dynamic and integrated approach is described with the work flow for one practical modern solution.
Traditional methods Roundabout design is generally iterative, requiring many interrelated parameters to be checked simultaneously. Traditionally, this design process involves laying out a roundabout based on typical values specified in the established highways standards, ie, in the UK: TD16/07 of the Design Manual for Roads & Bridges. Design vehicle paths and speeds are then checked against this preliminary layout. If the vehicle paths and speeds do not meet the design requirements, the roundabout geometry is revised and the checks repeated until the design criteria are satisfied. This can be a monotonous, costly and time consuming process. Integrating vehicle paths and speeds at the earliest possible stage of roundabout design enhances the design process and reduces labour intensive iteration.
Development of TORUS – field study Design tools are available with work flow processes that incorporate these features, such as Transoft Solutions’ TORUS software. This is a CAD based roundabout design package featuring design vehicle characteristics and leveraging on the widely used AutoTURN vehicle swept path algorithm.
TORUS instantly updates the entire roundabout when changes to parameters are made. It was developed over a period of two years, through a process including GPS field studies on vehicle paths and speed profiles (patterns) to refine the software algorithm. The GPS field study for vehicle driving paths and speeds was conducted in collaboration with the German University of Bundeswehr München. The objective was to examine the driving speeds and natural path patterns, related to acceleration, deceleration and vehicle positioning for approximately 300 runs through different roundabout configurations. A typical passenger car, bus and articulated lorry were used in these tests. Data was reproduced in CAD and the graphical summaries produced and
studied for the various runs. These findings provided insights into the innovative concept of designing roundabouts based on idealised paths. The speed profiles indicating the acceleration and deceleration zones provided feedback into the comfortable driving speeds for the different roundabout sizes.
Design process and work flow The TORUS design process can be simplified into two main steps. Step 1 lays out the roundabout for single or dual carriageway while considering the design vehicle movements, fastest path and required visibility. The vehicles are configured initially in the relevant design guideline and are assumed to travel on idealised paths composed of a series of curves. These
38 Septem ber 2 0 0 9 Transportation Technical Paper
Profes s ion al
Transportation P rofes s ion Technical al S eptem ber 2009 39 Paper
Dynamic roundabout design: Research and development Software has been developed to speed up the often long winded, iterative method of roundabout design. Daniel Shihundu describes the research and development behind development of TORUS. A bus with GPS units manoeuvres within a roundabout configuration with a central island diameter of 35m. A car and an articulated lorry were also tested on this configuration
Introduction
Few motorists will appreciate the process that underpins the operational and safety benefits inherent in good roundabout design. High quality design promotes safer driving with improved traffic flow. It is a delicate balance between traffic capacity and physical geometry. This paper summarises the typical roundabout design process with regard to the interrelationship between layout and traffic speed. Field research conducted in the development of a new dynamic and integrated approach is described with the work flow for one practical modern solution.
Daniel Shihundu (MIHT) is a senior civil engineer at Transoft Solutions in Canada. He was instrumental in the research and development of key algorithms for Transoft’s TORUS roundabout software; research that has provided insights into the driver behaviour pertaining to vehicle Sample Drive Path (top) and acceleration, deceleration, speed and Speed Profile fastest paths. Daniel has recently (bottom) of a provided technical advice to the North test run from the American AASHTO committee on GPS Field Test. The plan view of geometric design based on vehicle the path shows kinematics and turning templates. only 1 of 10 runs Daniel is a graduate of the University Traditional methods of Nairobi, Kenya and holds an MBA Roundabout design is generally iterative, degree from the School of Management pre-determined paths make generation of tostopping sight distance on approach, requiring many interrelated parameters at the University of the Free State in vehicle straightforward by be movements checked simultaneously. Traditionally, forward visibility at entry, visibility to the South Africa. He is a registered simply the entry and exit thisidentifying design process involves laying out a right, circulatory visibility and pedestrian professional engineer both in the carriageway. roundabout based on typical values crossing visibility. Also in Step 1, other province of British Columbia, Canada and In specified addition, in offsets from the swept paths important aspects of the roundabout are the established highways South Africa. Daniel is a member of the provide an initial of theof the generated: extent of the over run area, standards, ie,approximation in the UK: TD16/07 South African Institution of Civil roundabout layout. Using this & initial Design Manual for Roads Bridges. effective dimensions of entry and exit Engineering as well as IHT. His main design,Design fastest vehicle paths and visibility relatedare width, entry angle, approach half width, paths and speeds interest liesfor in the issues (landscaping or rightthis of way TORUS instantly updates the entire then checked against preliminary studied the development various runs.ofThese circulatory width, all based on the design software solutions for solving complex encroachments) evaluated roundabout when changes to parameters layout. If theare vehicle pathsaccordingly. and speeds do vehicle findings provided insights into the swept paths. problems. The fastest are dynamically are2 made. It was developed over a period engineering not meetpaths the design requirements, the innovative concept of designing Step approximates the geometric calculated with the entry path (the radius of two years, through a process roundabout geometry is revised and the elements roundabouts based on idealised paths. typically representing curbsincluding or traditionally employed some design of path at the vehicle’suntil centre) GPSlines. fieldTORUS studies features on vehicle paths checks repeated the design criteria painted The speed profiles by indicating the tools thatand engineers, but is easily integrated within approximated. This occurs as athe profiles (patterns) to refine the are satisfied. This can be monotonous, assistspeed acceleration and deceleration zones in approximating the entry and exit the manual process; enhancing roundabout parameters are modified. For costly and time consuming process. provided feedback into the design comfortable curb software radius foralgorithm. the initial design. The and productivity. example, the roundabout is moved, The GPS study for vehicle Integrating vehicle centre paths and speeds at fastest path driving speeds for the different can field also be verified on thedriving quality The opportunity for improved junction deflection in thepossible arms is varied and the and speeds was conducted in the earliest stage of roundabout roundabout sizes. final paths geometric elements. design with further use of roundabouts is inscribed circle diameter Speed collaboration the German University design enhances the increased. design process and Design standardswith provide guidance for a Design process and work flow well documented. The pressure is consistency entering, Bundeswehr München.based The objective reduceschecks labourfor intensive iteration. wideof range of key parameters on a The TORUS design process can be for increased quality and circulating, exiting and turning paths are to examine the speeds givenwas roundabout size ordriving type. Step 2 and mounting Development of TORUS – field simplified two main steps. Step 1 lays productivity in into the roundabout design carried out and updated at the samestudy time. natural path patterns, relatedthe to value allows the engineer to determine Design tools areisavailable with out the forprocesses single or dual process. Newroundabout methods and Sight line analysis important forwork flow within acceleration, deceleration and the range that would best fit vehicle the processes that constraints. incorporate TORUS these features,design whileresistance. considering the design meet with some However, determining safety positioning approximately situation. for Though similar to 300 runs oftencarriageway such as Transoft vehicle movements, pathitand history has proven many fastest times that is calculates and checksSolutions’ minimumTORUS stopping through different roundabout traditional design methods, the crucial a CAD based roundaboutdifference visibility. only required through the development of sightsoftware. distancesThis andisthrough graphical configurations. is that the initial geometry is design package design vehicle laid outAfrom The solutions vehicles are configured initially in innovative that design processes representation, helpsfeaturing identify hazard typical passenger car, bus and valid, accurate design vehicle characteristics leveraging on the the relevant design guideline and are simplified and improved. zones and effects ofand obscuring features. lorry were used in these tests. become pathsarticulated and at inception meets fastest path, widely checks used AutoTURN vehicle swept assumed to travel on idealised paths Visibility are also calculated Data was reproduced in CADcriteria. and the vehicle movements and visibility path algorithm. composed of a series of curves. 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