Two-way stop-control alternative

The majority of intersections in the U.S. operate under TWSC, and most of those intersections operate with minimal delay. The installation of a roundabout at a TWSC intersection that is operating satisfactorily will be difficult to justify on the basis of performance improvement alone, and one of the previously described selection categories is likely to be more appropriate.

The two most common problems at TWSC intersections are congestion on the minor street caused by a demand that exceeds capacity, and queues that form on the major street because of inadequate capacity for left turning vehicles yielding to opposing traffic. Roundabouts may offer an effective solution to traffic problems at TWSC intersections with heavy left turns from the major route because they provide more favorable treatment to left turns than other control modes. “T” intersections
are especially good candidates in this category because they tend to have higher left turning volumes.

On the other hand, the problems experienced by low-volume cross street traffic at TWSC intersections with heavy through volumes on the major street are very difficult to solve by any traffic control measure. Roundabouts are generally not the solution to this type of problem because they create a significant impediment to the major movements. This situation is typical of a residential street intersection with a major arterial. The solution in most cases is to encourage the residential traffic to enter the arterial at a collector road with an intersection designed to accommodate higher entering volumes. The proportion of traffic on the major street is an important consideration in the comparison of a roundabout with a conventional four-leg intersection operating under TWSC. High proportions of minor street traffic tend to favor roundabouts, while low proportions favor TWSC.

An example of this may be seen in Exhibit 3-8, which shows the AADT capacity for planning purposes as a function of the proportion of traffic on the major street. The assumptions in this exhibit are the same as those that have been described previously in Section 3.3. Constant proportions of 10 percent right turns (which were ignored in roundabout analysis) and 20 percent left turns were used for all movements. As expected, the roundabout offers a much higher capacity at lower proportions of major street traffic. When the major and minor street volumes are equal, the roundabout capacity is approximately double that of the TWSC intersection. It is interesting to note that the two capacity values converge at the point where the minor street proportion becomes negligible. This effect confirms the expectation that a roundabout will have approximately the same capacity as a stop-controlled intersection when there is no cross street traffic.