The methods of analyzing roundabouts described in the chapter are primarily based on the HCM 7th Edition. As such, this chapter highlights motorized vehicle analysis while multimodal considerations are limited to level of traffic stress and LOS. Furthermore, this chapter only briefly reviews safety analysis of roundabouts. For more information on safety analysis of roundabouts, see

Chapter 5 and Chapter 6

. This chapter provides roundabout-specific guidance on Microsimulation (Microscopic) analysis, which can be used to verify results of the empirical method found in the HCM 7th Edition. For general guidance on microsimulation (microscopic) analysis, see

Chapter 13

.

The Intersection Control Evaluation (ICE) process as described in

Chapter 10

and

Chapter 11

, provides a framework of quantifiable and qualitative measures to evaluate intersection control alternatives so that public agencies, engineers, and planners can make evidence-based decisions. TxDOT employs ICE to determine intersection control type including roundabout feasibility. Roundabout analysis is a part of the ICE process. Roundabout attributes are described in the following sections.

The reduction in vehicle speeds, conflict angles, and conflict points in a roundabout enhances vehicular and pedestrian safety. Roundabouts provide additional safety for bicyclists due to the slower vehicular speeds, which result in a smaller speed differential between bicyclists and motor vehicles. See

Chapter 5, Chapter 6, Chapter 10, and Chapter 11

for more information on roundabout safety analysis.

Roundabouts are an increasingly common form of intersection control that are typically circular, use yield control, and have geometric features that promote desirable vehicular speeds and provide accessibility to all roadway users. Common geometric features of a roundabout include a central island, traversable apron, splitter islands (channelizing islands), a circulatory roadway, entrance lines, pedestrian crossings, and landscape strips. There are three types of modern roundabouts:

Roundabouts vary in size, with mini-roundabouts and compact roundabouts ranging from 70 feet to 110 feet in inscribed circle diameter (ICD). Single lane and multilane roundabouts range from 110-feet to 200-feet ICD (refer to

RDM Chapter 14

). Geometric elements of roundabouts, e.g., are only partially addressed in part with the available operational analysis models. Number of entry lanes is dominant key predictive variable of roundabout capacity.

Roundabouts can handle a wide range of vehicular volumes and provide operational and safety benefits in many circumstances.

Figure 12-1

shows the typical mini-roundabout features,

Figure 12-2

shows the typical single-lane roundabout features, and

Figure 12-3

shows the typical multilane roundabout features. For information regarding other roundabout design principles, refer to

RDM Chapter 14

and NCHRP 1043, Chapter 2. For information regarding roundabout interchanges, see

Chapter 11

in this manual.

Roundabout analysis begins with determining whether a roundabout is an appropriate intersection control method for a particular location. See

Chapter 10

for guidance on undertaking the ICE process. The prescribed methods in this chapter focuses on roundabout analysis for motorized vehicles, pedestrians, and bicycles. Various methods of analysis are available for the two stages of project development and are discussed in this chapter. See

Table 12-1

for the selection of an appropriate analysis tool according to the project planning and schematic development phases of the project development process