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Chapter 8 - System Considerations 8.1 Proximity to Other Traffic Control Devices................................................................... 140 Queuing at Roundabouts..................................................................................................................... 140 Unsignalized intersections .................................................................................................................. 140 Signalized intersections....................................................................................................................... 141 Rail crossings ................................................................................................................................... 141 Parking ....................................................................................................................................142 8.2 Access Management.......................................................................................................... 143 Access into the Roundabout................................................................................................................ 143 Access near the Roundabout ................................................................................................................144 8.3 Roundabouts in Series ...................................................................................................... 146 Chapter 8 – System Considerations Kansas Roundabout Guide Page 140 October 2003 8.1 Proximity to Other Traffic Control Devices Due to the close spacing of intersections in many urban and suburban areas, roundabouts are often considered for locations near other activities such as unsignalized intersections, signalized intersections, railroad crossings, and parking areas. One of the principle measures to identify this spacing is an estimate of queuing. This section provides a brief discussion of queuing, followed by a discussion of locating roundabouts in each of these locations. Queuing at Roundabouts Queuing is an important measure of the performance of a roundabout and how other intersections perform in close proximity to it. A downstream queue that extends into a roundabout impedes circulating flow. As circulating flow is impeded, exits upstream of the impeded exit become blocked, further increasing the queuing within the circulatory roadway. In theory, an entire roundabout could become jammed if an exit is blocked for a sufficient period of time. In addition, queue spillback into a roundabout reduces the overall capacity of each approach that is blocked. Therefore, it is generally preferred to avoid having downstream queues back up into a roundabout for any significant period of time. The principal measure to determine how close a roundabout should be located to a stop- controlled intersection is the amount of queuing expected at each intersection. The HCM 2000 provides procedures for estimating queues at stop-controlled intersections and should be used to make this assessment. For roundabouts, either the estimation procedures in Chapter 4 of this guide, FHWA’s Roundabouts: An Informational Guide, or estimates provided by software should be used. In general, it is desirable for the 95th-percentile queue to be completely accommodated within the space between the two intersections. The 95th-percentile queue at an unsignalized intersection represents the percent of time during the peak period being analyzed (typically the peak fifteen minutes of the peak hour) that the queue will be equal to or less than the percentile estimate. For example, a 95th-percentile queue of 8 vehicles during the peak period means that during the peak period the queue will exceed 8 vehicles only 5 percent of the time. Queuing estimates for a signalized intersection appear to be similar to those for unsignalized intersections but are actually quite different. As noted above, a 95th-percentile queue estimate for an unsignalized intersection represents a queue that will not be exceeded for 95 percent of the time period being studied. A 95th-percentile queue estimate for a signalized intersection typically represents a queue length that will not be exceeded for 95 percent of the signal cycles during the time period being studied. Because the maximum back of queue at a signalized intersection only occurs once each signal cycle and because the 95th-percentile queue occurs for only a small fraction of those cycles, it has less of an impact than the 95th-percentile estimate for unsignalized intersections. Unsignalized intersections This section discusses the case where the roadway approaching or departing the roundabout is interrupted by a stop sign. A common case is an all-way stop-controlled intersection, although it Kansas Roundabout Guide Chapter 8 – System Considerations October 2003 Page 141 could be a two-way stop-controlled intersection with a higher-level roadway. Other cases involving two-way stop-controlled intersections where the major roadway is uninterrupted or cases involving other roundabouts are discussed in subsequent sections. Stop-controlled intersections near a roundabout primarily influence a roundabout through queuing effects. In general, it is best for the 95th-percentile queue between a roundabout and a stop-controlled intersection to be completely contained between the two intersections. It is also best for the 95th-percentile queue from a stop-controlled intersection to end short of the crosswalk area of a roundabout to avoid creating additional potential driver distractions that may compromise pedestrian safety. Signalized intersections Signalized intersections can influence a roundabout in several ways: • Queuing effects. For signalized intersections, it is best for the 95th-percentile queue from a signalized intersection to not back up into the roundabout. However, because such backups are infrequent and momentary (as discussed above), it may be acceptable in highly constrained locations to allow these momentary backups into the roundabout. This should only be done in areas where the downstream signal is operating below capacity and can reliably flush out any queue that builds within the signal cycle and where the unblocked capacity of the roundabout is sufficient to accommodate the loss of capacity during these blocked periods. • Platooned arrival patterns. Signalized intersections create platooned arrival patterns at a roundabout. As noted in Section 8.5.1 of Roundabouts: An Informational Guide, the platooned arrivals from a nearby signal at a roundabout can increase a roundabout’s capacity due to a regular pattern of gaps in traffic that can be used efficiently. The isolated analyses discussed in Chapter 4 will therefore be conservative; simulation could be used to more precisely estimate the effect. • Signal preemption and priority. A roundabout cannot be preempted or give priority to certain vehicles. If a traffic signal is preempted frequently, queues from the signal backing towards the roundabout may be larger than estimated using the above procedures. Rail crossings This section discusses the situation where rails cross one of the approaches to a roundabout (see Exhibit 8-2 of Roundabouts: An Informational Guide) and crossing gates are warranted. In such cases, the rail crossing itself is typically gated. The question is whether the crossing gates on the affected approach are sufficient (Case (a) in FHWA Exhibit 8-2) or whether each approach to the roundabout needs to be gated (Case (b) in FHWA Exhibit 8-2). The primary issue that affects this question is one of safety: what is the likelihood of a queue from the roundabout extending over the tracks, and what ability is there to clear the tracks before an oncoming train arrives? The secondary issue is one of operations: what is the likelihood that a queue at the rail crossing will back up into the roundabout? The following items provide guidance on this analysis: Chapter 8 – System Considerations Kansas Roundabout Guide Page 142 October 2003 • If the 95th-percentile queues between the rail crossing and roundabout can be completely contained between the two facilities, only the rail crossing needs to be gated. This can occur where the rail crossing is located far enough from the roundabout or where traffic volumes on the affected roadway are low. • If the 95th-percentile queue on the affected approach entering the roundabout enters the rail crossing area, then the entire roundabout should