The natural vehicle path
As discussed in Section 6.2.1, the fastest path through the roundabout is drawn to ensure the geometry imposes sufficient curvature to achieve a safe design speed. This path is drawn assuming the roundabout is vacant of all other traffic and the vehicle cuts across adjacent travel lanes, ignoring all lane markings. In addition to evaluating the fastest path, at double-lane roundabouts the designer must also evaluate the natural vehicle paths. This is the path an approaching vehicle will naturally take, assuming there is traffic in all approach lanes, through the roundabout geometry.
As two traffic streams approach the roundabout in adjacent lanes, they will be forced to stay in their lanes up to the yield line. At the yield point, vehicles will continue along their natural trajectory into the circulatory roadway, then curve around the central island, and curve again into the opposite exit roadway. The speed and orientation of the vehicle at the yield line determines its natural path. If the natural path of one lane interferes or overlaps with the natural path of the adjacent lane, the roundabout will not operate as safely or efficiently as possible.
The key principle in drawing the natural path is to remember that drivers cannot change the direction of their vehicle instantaneously. Neither can they change their speed instantaneously. This means that the natural path does not have sudden changes in curvature; it has transitions between tangents and curves and between consecutive reversing curves. Secondly, it means that consecutive curves should be of similar radius. If a second curve has a significantly smaller radius than the first curve, the driver will be traveling too fast to negotiate the turn and may lose control of the vehicle. If the radius of one curve is drawn significantly smaller than the radius of the previous curve, the path should be adjusted.
To identify the natural path of a given design, it may be advisable to sketch the natural paths over the geometric layout, rather than use a computer drafting program or manual drafting equipment. In sketching the path, the designer will naturally draw transitions between consecutive curves and tangents, similar to the way a driver would negotiate an automobile. Freehand sketching also enables the designer to feel how changes in one curve affect the radius and orientation of the next curve. In general, the sketch technique allows the designer to quickly obtain a smooth, natural path through the geometry that may be more difficult to obtain using a computer.
Exhibit 6-44 illustrates a sketched natural path of a vehicle through a typical double lane roundabout.