Operational improvement

A roundabout may be considered as a logical choice if its estimated performance is better than alternative control modes, usually either stop or signal control. The performance evaluation models presented in the next chapter provide a sound basis for comparison, but their application may require more effort and resources than an agency is prepared to devote in the planning stage. To simplify the selection process, the following assumptions are proposed for a planning-level comparison of control modes:

1. A roundabout will always provide a higher capacity and lower delays than AWSC operating with the same traffic volumes and right-of-way limitations.
2. A roundabout is unlikely to offer better performance in terms of lower overall delays than TWSC at intersections with minor movements (including cross street entry and major street left turns) that are not experiencing, nor predicted to experience, operational problems under TWSC.
3. A single-lane roundabout may be assumed to operate within its capacity at any intersection that does not exceed the peak-hour volume warrant for signals.
4. A roundabout that operates within its capacity will generally produce lower delays than a signalized intersection operating with the same traffic volumes and right-of-way limitations.

The above assumptions are documented in the literature (7) or explained by the analyses in Section 3.5. Collectively, they provide a good starting point for further analysis using procedures in Chapter 4. Although a roundabout may be the optimal control type from a vehicular operation standpoint, the relative performance of this control alternative for other modes should also be taken into consideration, as explained in Chapter 4.

Roundabout performance at flow thresholds for peak hour signal warrants

There are no warrants for roundabouts included in the Manual of Uniform Traffic Control Devices (MUTCD) (8), and it may be that roundabouts are not amenable to a warranting procedure. In other words, each roundabout should be justified on its own merits as the most appropriate intersection treatment alternative. It is, however, useful to consider the case in which the traffic volumes just meet the MUTCD warrant thresholds for traffic signals. For purposes of this discussion, the MUTCD peak hour warrant will be applied with a peak hour factor (PHF) of 0.9. Thus, the evaluation will reflect the performance in the heaviest 15 minutes of the peak hour.

Roundabout delays were compared with the corresponding values for TWSC, AWSC, and signals. A single-lane roundabout was assumed because the capacity of a single lane roundabout was adequate for all cases at the MUTCD volume warrant thresholds. SIDRA analysis software was used to estimate the delay for the various control alternatives because SIDRA was the only program readily available at the time this guide was developed that modeled all of the control alternatives (9).

The MUTCD warrant thresholds are given in terms of the heaviest minor street volume and sum of the major street volumes. Individual movement volumes may be obtained from the thresholds by assuming a directional factor, D, and left turn proportions. A “D” factor of 0.58 was applied to this example. Left turns on all approaches were assumed to be 10 to 50 percent of the total approach volume. In determining the MUTCD threshold volumes, two lanes were assumed on the major street and one lane on the minor street.

Based on these assumptions, the average delays per vehicle for signals and roundabouts are presented in Exhibit 3-7. These values represent the approach delay as perceived by the motorist. They do not include the geometric delay incurred within the roundabout. It is clear from this figure that roundabout control delays are substantially lower than signal delays, but in neither case are the delays excessive.

Similar comparisons are not presented for TWSC, because the capacity for minor street vehicles entering the major street was exceeded in all cases at the signal warrant thresholds. AWSC was found to be feasible only under a limited range of conditions: a maximum of 20 percent left turns can be accommodated when the major street volume is low and only 10 percent can be accommodated when the major street volume is high. Note that the minor street volume decreases as the major street volume increases at the signal warrant threshold.

This analysis of alternative intersection performance at the MUTCD peak hour volume signal warrant thresholds indicates that the single-lane roundabout is very competitive with all other forms of intersection control.