Number of Entry Lanes

A basic question that needs to be answered is how many entry lanes a roundabout would require to serve the traffic demand. The capacity of a roundabout is clearly a critical parameter and one that should be checked at the outset of any feasibility study. Chapter 4 offers a detailed capacity computation procedure, mostly based on experiences in other countries. Some assumptions and approximations have been necessary in this chapter to produce a planning-level approach for deciding whether or not capacity is sufficient.

Since this is the first of several planning procedures to be suggested in this chapter, some discussion of the assumptions and approximations is appropriate. First, traffic volumes are generally represented for planning purposes in terms of Average Daily Traffic (ADT), or Average Annual Daily Traffic (AADT). Traffic operational analyses must be carried out at the design hour level. This requires an assumption of a K factor and a D factor to indicate, respectively, the proportion of the AADT assigned to the design hour, and the proportion of the two-way traffic that is assigned to the peak direction. All of the planning-level procedures offered in this chapter were based on reasonably typical assumed values for K of 0.1 and D of 0.58.

There are two site-specific parameters that must be taken into account in all computations. The first is the proportion of traffic on the major street. For roundabout planning purposes, this value was assumed to lie between 0.5 and 0.67. All analyses assumed a four-leg intersection. The proportion of left turns must also be considered, since left turns affect all traffic control modes adversely. For the purposes of this chapter, a reasonably typical range of left turns were examined. Right turns were assumed to be 10 percent in all cases. Right turns are included in approach volumes and require capacity, but are not included in the circulating volumes downstream because they exit before the next entrance.

The capacity evaluation is based on values of entering and circulating traffic volumes as described in Chapter 4. The AADT that can be accommodated is conservatively estimated as a function of the proportion of left turns, for cross-street volume proportions of 50 percent and 67 percent. For acceptable roundabout operation, many sources advise that the volume-to-capacity ratio on any leg of a roundabout not exceed 0.85 (1, 2). This assumption was used in deriving the AADT maximum service volume relationship.