In a roundabout way

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           SOFTWARE review

        In a roundabout way
        Let's take a trip around a roundabout to explore the sort of detail that you will find in any Bentley
        Civil application

                   hether you like them or hate
                   them, whether you think they
                   obstruct a perfectly good bit of
        road and we are building too many of
        them anyway, or that they are a valid
        device for improving road safety, the
        humble roundabout is here to stay. If it’s
        any consolation the civil engineers who
        have to design and construct them
        probably feel the same way, as the
        challenge of laying out a roundabout is
        just about as difficult as it gets when it
        comes to laying out roads.
          Just think about it - you have more than
        two lines of traffic heading for a common
        point, often on different gradients and
        tangents to the roundabout, and the
        designer has to ensure that all of the
        cambers and the curved approaches are
        sufficiently planned to slow the
        approaching traffic down at just the right
        rate, so that the driver can take the
        roundabout at a speed that will not
        unduly disrupt the flow of their journey!     roundabout to replace - the software          allows us to use standard CAD tools,
        Overdo it, and you will be cursed. Who        works out for itself how many                 together with the reference tree, to
        remembers the bewildering satellite           approaches it needs and places the            modify the geometry - to soften the curve
        roundabouts of High Wycombe in the            roundabout, configuring the geometry to       of the deflection area and change the
        1980s?                                        the approach angles, curves and               flare length - designed to change the
                                                      transitions. You can always add another       performance of the roundabout. Text
        BENTLEY CIVIL                                 approach as easily later on, anyway.          fields that define element properties can
        Alternatively you could create a                                                            also be used to change the element.
        roundabout almost automatically, using        MODIFYING THE GEOMETRY                           If you are too free with your changes,
        any of the Bentley civil design               The Navigation task bar provides access       the software will flash up a warning sign
        applications - MXROAD, InRoads or             to other roundabout tools - an Element        to tell you that you have infringed one or
        PowerCivil. You start the whole process       Information option, for instance - and        more of the basic design rules for
        by selecting Roundabout in Civil Tasks,       Entry Path Curvature Analysis. Selecting      roundabouts. There is a Civil Message
        and pick a roundabout type from the           Element Information brings up detailed        Centre that you can bring up to list
        library of standard objects - a left hand     data on each feature of the roundabout,       warnings, allowing you to return to the
        lane, English style roundabout with dual-     in a tree view, as you select it.             geometry and re-modify it, until all of the
        carriageway approach, for instance.             The mouse can be used to either             warnings are cleared.
          It will probably come up with four          identify elements by hovering over them,        If the roundabout that you have created
        approaches, but you needn't worry about       or right clicking to select them for          is part of a larger project, or you are
        these, as after selecting the roundabout      modification. Selecting the approach and      working for a local authority that uses a
        and then clicking on and snapping to the      its properties, for instance, brings up the   number of similar roundabouts, it can be
        centre of the reference features - the        entry and exit lanes, the deflection area,    saved and placed in the library for re-use
        junction of roads that you want the           flare and island. Zooming in on the island    at another junction.

        32 CAD User   November/December 2008                                                                  Free copies at

           SOFTWARE review

                                              Analysing entry and exit path curvature

        ANALYSING THE ROUNDABOUT                       once more, whether you are                      carriageway - or vice-versa! To do that,
        If you make any changes, then you will         overstepping your boundaries. The               though, you may have to increase the
        have to analyse the performance of the         analysis tool may prompt the designer to        size of the roundabout. This is simple to
        roundabout. This is the crux of the            make further modifications, perhaps             achieve once you have selected the
        matter. A roundabout is designed to            including an adjustment of the centre           radius of the central island and changed
        allow motorists to approach at an              point of the roundabout, or to change           its dimensions by typing in a new value.
        optimum speed consistent with the local        the length of the flare or the exit width, in     Changes in the alignments of the
        traffic conditions - and with safety, of       each case.                                      approaches are accommodated in the
        course. Too sharp an entry path                  After modifications, the software not         same way - using design tools to offset
        curvature, and the motorist will have to       only changes the geometry of the feature        the position of the approach road from
        take evasive action to avoid the island        being modified, but synchronises it with        the centre of the island, merely by
        before he enters the roundabout. The           the whole roundabout - effectively              dragging it to a new position and
        software looks at the roundabout and           rebuilding it to incorporate the changes.       allowing the software to make the
        determines the critical points for the                                                         necessary adjustments.
        entry path curve, and calculates the           ADDITIONS TO THE ROUNDABOUT
        shortest and straightest curve through         Major changes are handled just as               MAGIC ROUNDABOUTS?
        the roundabout.                                easily. Adding another approach to the          It almost looks like a paper exercise -
          This can either be based on spline           roundabout entails going back to the            but, as I am sure you are aware, the
        geometry, or back-to-back arcs - the           main alignment or road design area of           roundabout has to be placed on real
        user selects which. It also depends on a       the application, which allows you to            terrain, with local concerns about
        number of other factors, all of which are      either design or place the additional           visibility of approaches, and the normal
        available for modification by the user,        centre line for the new approach.               problems of building roads with cuts,
        and which include things like vehicle          Returning to the roundabout tool, simply        infills, cambers and so on. The
        width, design speed, offset from the           select the roundabout type, the place           integration of the roundabout tool within
        curve paths, minimum and maximum               approach icon and identify the new              Bentley Civil road design applications
        entry radii, crossfalls and side friction      centreline. The software will incorporate       ensure that terrain specific conditions
        factors - all impacting on the final result.   the new approach, adjusting the flares          are incorporated within the design.
          If you are not happy with the result, it     and approaches automatically, and                 So the next time you are held up at a
        can be improved by changing, for               integrate it with the rest of the               roundabout, spare a thought for the extra
        instance, the maximum entry radius or          roundabout, with all geometry healed.           thought that had to go into its creation -
        the entry path curvature - and re-               You can even change one of the                all for the comfort of the motorist!
        analysed. A warning signal will tell you,      approaches to become a dual-          

        34 CAD User   November/December 2008                                                                     Free copies at