Entry curves

As shown in Exhibit 6-1, the entry curves are the set of one or more curves along the right curb (or edge of pavement) of the entry roadway leading into the circulatory roadway. It should not be confused with the entry path curve, defined by the radius of the fastest vehicular travel path through the entry geometry (R1 on Exhibit 6-12).

The entry radius is an important factor in determining the operation of a roundabout as it has significant impacts on both capacity and safety. The entry radius, in conjunction with the entry width, the circulatory roadway width, and the central island geometry, controls the amount of deflection imposed on a vehicle’s entry path. Larger entry radii produce faster entry speeds and generally result in higher crash rates between entering and circulating vehicles. In contrast, the operational performance of roundabouts benefits from larger entry radii. As described in Chapter 4, British research has found that the capacity of an entry increases as its entry radius is increased (up to 20 m [65 ft], beyond which entry radius has little effect on capacity.

The entry curve is designed curvilinearly tangential to the outside edge of the circulatory roadway. Likewise, the projection of the inside (left) edge of the entry roadway should be curvilinearly tangential to the central island. Exhibit 6-24 shows a typical roundabout entrance geometry.

The primary objective in selecting a radius for the entry curve is to achieve the speed objectives, as described in Section 6.2.1. The entry radius should first produce an appropriate design speed on the fastest vehicular path. Second, it should desirably result in an entry path radius (R1) equal to or less than the circulating path radius (R2) (see Section 6.2.1.5).

Entry curves at single-lane roundabouts

For single-lane roundabouts, it is relatively simple to achieve the entry speed objectives. With a single traffic stream entering and circulating, there is no conflict between traffic in adjacent lanes. Thus, the entry radius can be reduced or increased as necessary to produce the desired entry path radius. Provided sufficient clearance is given for the design vehicle, approaching vehicles will adjust their path accordingly and negotiate through the entry geometry into the circulatory roadway.

Entry radii at urban single-lane roundabouts typically range from 10 to 30 m (33 to 98 ft). Larger radii may be used, but it is important that the radii not be so large as to result in excessive entry speeds. At local street roundabouts, entry radii may be below 10 m (33 ft) if the design vehicle is small.

At rural and suburban locations, consideration should be given to the speed differential between the approaches and entries. If the difference is greater than 20 km/h (12 mph), it is desirable to introduce approach curves or some other speed reduction measures to reduce the speed of approaching traffic prior to the entry curvature. Further details on rural roundabout design are provided in Section 6.5.

Entry curves at double-lane roundabouts

At double-lane roundabouts, the design of the entry curvature is more complicated. Overly small entry radii can result in conflicts between adjacent traffic streams. This conflict usually results in poor lane utilization of one or more lanes and significantly reduces the capacity of the approach. It can also degrade the safety performance as sideswipe crashes may increase. Techniques and guidelines for avoiding conflicts between adjacent entry lanes at double-lane roundabouts are provided in Section 6.4.