Bringing bim to Roundabout Design Saves Time and Money and Produces Better Designs

Basic design

Currently the typical roundabout design workflow starts with a roadway design engineer proposing conceptual geometry for a roundabout based upon the appropriate guidelines. Roundabouts require multiple and complex, interconnecting geometric curves which often take many project hours to both derive and adjust. At this stage, the design team will be basing their design on past experience and simply making sure that the geometry complies with the guidelines.

The next stage is to confirm that the proposed geometry can provide the required traffic capacity. Typically for this, a traffic engineer will measure some critical values from the drawing and create an analysis model in a tool such as ARCADY, Rodel or similar. In an ideal world the results of the analysis would show that the roundabout design was adequate in all design years but as we all know we don't live in an ideal world. In practice, the geometry will probably need some adjustment to provide capacity. Of course, in making these adjustments the road engineer has to be careful not to contravene other safety criteria such as entry path radius and vehicle speed, which would decrease safety and increase the likelihood of accidents. This design and analysis cycle will probably be repeated several times and it is not uncommon for a single design change to require several project hours or even days.

Once the roundabout geometry has been agreed the designer must check that visibility sightlines satisfy the standard used and that design vehicles can negotiate the roundabout. Sight analysis at this stage is usually performed on a 2D model, looking for obstructions that are located within the required fields of vision; such as the required stopping sight distances. If visibility is not acceptable at this stage then the geometry must be re-adjusted.

Design vehicles are normally specified in guidelines or provided by the reviewing organization, so at this point the engineer would probably use vehicle turn path prediction software like AutoTrack. If the roundabout has been designed by an experienced designer, the design vehicles may be able to reasonably negotiate all routes through the intersection. If not, it's back to the drawing board once again to adjust the geometry, re-analyze capacity, recheck visibility and then also review the new vehicle paths as these would all change. Once one aspect of design influences the geometry, all other analysis becomes out of date and needs to be redone!

Once all the horizontal geometry has been performed by the various different design professionals or offices and been accepted, a complete 2D roundabout design has been produced. It could take weeks and often months of iteration and analysis to arrive at this point during a typical roundabout design project.