Several overarching principles guide the development of all roundabout designs:

Varying characteristics will determine the selection of a roundabout’s size, lane configuration, and feasibility. Refer to , Chapter 3, Chapter 8, and Chapter 9 for roundabout performance-based planning and design information and roundabout design principles and objectives.

It is beneficial to confirm that a roundabout is a feasible solution with respect to site-specific constraints early in the planning process. Advance coordination with adjacent project stakeholders (railroad representatives, neighboring jurisdictions, or local elected officials), review of boundary survey information, and a site visit to observe existing conditions are important steps in validating a potential roundabout installation early in the analysis and design process. Common site-specific constraints consist of:

A performance-based mindset affords the creation of functional designs with the most flexibility. Due consideration of the intersection--specific physical and traffic conditions throughout the life cycle of a subject intersection will be one of the key contributors to the success of a roundabout installation. TxDOT has several tools and resources available for the collection and analysis of traffic data to prepare an operations analysis, including the data sources outlined in the , Chapters 10, 11 and 12.

Within the planning and conceptual process of a proposed roundabout, a documentation of roundabout parameters is encouraged so the engineer-of-record and corresponding reviewing agencies can agree to the parameters selected for a roundabout’s design. Determination of the following parameters after an operational analysis and prior to the concept roundabout sketch can reduce re-sketching and future requests for changes to a roundabout design:

The performance-based geometric design of a roundabout is an iterative process, as outlined in , Exhibit 9.1. It may take multiple revisions to the size of the ICD and the location of the ICD before an optimized solution is arrived at for the subject intersection.

Considerations to the ICD size and placement of a roundabout consist of:

Mini-roundabouts are not common on State routes, due to the higher speed context and presence of added large trucks. Locating mini-roundabouts in suburban areas with moderate speed context is feasible with added approach treatment of reverse curves and longer splitter islands.

Compact and single-lane roundabout design is commonly influenced by design vehicle swept path requirements. Rural design requires provision of a visibility package of approach geometry (frequently using reverse curves), signs, illumination, and markings.

The following design parameters and their associated values shall be documented and agreed upon by all relevant parties prior to a conceptual or schematic roundabout design. Advance determination of these parameters and values is beneficial to reduce the potential for future re-work of the roundabout design.

, Section 10.7 provides justification for added emphasis of the following in multilane roundabout design:

to highlight ranges of common design considerations for various geometric parameters of roundabouts for the four different classifications.

, Section 10.6.4 and Section 10.6.5 detail entry and exit design for roundabouts.

The entry width is measured from the center of the pavement marking that is adjacent to the splitter island to the center of the pavement marking adjacent to the exterior curb, or if a gutter section is present, the width is measured to the edge of the traveled way.

The design vehicle and check vehicle must be able to navigate the entry of a single-lane roundabout with a one-foot shy distance from the wheel path to the face of curb or edge of traveled way. When a WB-62TX is to be accommodated at a single-lane roundabout as the check vehicle, a wider entry width may be required due to the swept path of the rear tandem axle tires relative to the cab’s front tires.

At multilane roundabouts the entry width will depend on the design case. If stay-in-lane design is employed, then the resulting entry width will be greater than the typical straddle lane design entry width to accommodate the check vehicle in-lane while providing the necessary gore width between adjacent entry lanes. Individual lane widths for two-lane entries should remain in the 12-ft to 13-ft range. Maintaining trucks in-lane has safety consequences for other users including longer pedestrian crossings and higher in-lane speeds. Generally, promoting in-lane design for the design vehicle should be restricted to interchanges.

Exit design will rely heavily on single-lane or multilane designation, subsequent performance checks (fastest path and check vehicle), and the exit alignment that connects the roundabout to the adjacent roadway. Straighter exit paths provide ideal sight of the exit crosswalks, but exit speeds can increase, which places a greater emphasis on controlling entry speed.

For single-lane roundabouts, an 18-ft to 20-ft (face of curb to face of curb) circulatory roadway width is typical. For two-lane straddle-lane roundabouts, circulatory width is commonly in the range of 26-ft to 32-ft. For two-lane stay-in-lane design for trucks (“buffered lane design”), the circulatory width may be wider to accommodate the check vehicle being analyzed for both lanes in combination with the gore area separating the two circulatory lanes. Circulating lane widths should split the remaining circulatory roadway evenly.

, Section 10.6.1 and 10.14.2 discusses splitter island types and lengths. Splitter island design discussion can be found throughout due to a splitter island’s influence on pedestrian and bicycle accommodation, speed control, deterring wrong-way driving, and space allocation for traffic control devices. Splitter islands may be bounded by full-height barrier curb, mountable curb, or a flush fully traversable painted area. The context of the intersection should dictate the elements chosen for the splitter island. The minimum length of a splitter island for low-speed urban design is 50-ft. See for additional guidance for splitter islands at highspeed context roundabouts.

, Section 10.5.2 discusses truck aprons. Guidance for truck apron sizing, materials, and even the relationship that truck aprons provide to maintenance can be found within . It is important for the truck apron to have contrasting color and texture from the circulating pavement within the roundabout. This contrast allows drivers, especially during nighttime conditions, an increased ability to identify the limits of the circulatory roadway.

Consider the installation of yellow raised pavement markers spaced at 4-ft to 6-ft along the perimeter of the truck apron curb for mini-roundabouts to enhance nighttime visibility of the central island.

When performing the check vehicle performance checks, the usage of an external truck apron (a.k.a. “perimeter truck apron”) may be required to accommodate the wheel path of the check vehicle. The horizontal geometry of external truck aprons relies on the designer’s understanding of how to navigate the check vehicle through the respective movement (typically the right-turn but may also be required for through movements). , Section 11.6 discusses the use of external truck aprons. The use of external truck aprons should be avoided, if possible, by incremental modifications to the approach and entry geometry, entry radius, exit geometry, and exit radius, such that the modifications still result in a design that is validated by the standard performance checks.

, Section 10.4.7 highlights the three basic types of pedestrian and bicycle crossings at roundabouts, angled, straight, and z-crossing. A best practice at roundabouts, especially where bike ramps are implemented, is the use of tactical directional indicators (TDIs). Refer to , Section 10.4.3 for information regarding TDIs. Key considerations for pedestrian and bicycle crossing design at roundabouts consist of:

Pedestrian circulation paths at roundabouts between crosswalks shall be separated from the curb with landscaping or other nonprepared detectable surface with a minimum width of 24 inches. Exception: If a curb-attached pedestrian circulation path is provided, it shall have a continuous and detectable vertical edge treatment along the street side of the path, from crosswalk to crosswalk. The bottom edge of the vertical edge treatment shall be 15 inches maximum above the pedestrian circulation path. Refer to Section R306.4.1.1. and R306.4.1.2. of the , or latest version.

The validation of a roundabout’s horizontal geometry is one of the most important steps in the design process. Roundabout performance checks should be completed prior to any vertical design or supplemental detailing of a roundabout. , Chapter 9 highlights the various current industry standard performance checks associated with fastest path speeds, sight distance, view angles, vehicle path alignments, design and check vehicles, and bicycle and pedestrian wayfinding and crossing assessment. Appendix A of outlines performance check techniques. Performance checks completed in CAD software are required to remain a part of the project’s design files throughout the project life cycle, including postconstruction, in case an audit of the roundabout occurs after the roundabout is constructed.

A roundabout design performance checks package is required. The documentation should be compiled in a series of design check diagrams as follows:

Series 1 –

Overview and Horizontal Design - A dimensioned general layout drawing showing:

Series 2 -

Geometric design speed (‘fast path’) diagrams for each approach, entry, circulating, and exit are required. Roundabouts must be designed to promote geometric speed control per theoretical fastest path speeds in accordance with , Section 9.4 and . Left offset of the approach alignment increases the space for the approach curve to be well-developed for entry speed reduction. Geometric entry speeds, measured according to , must be less than 25 mph along the fastest through movement path for single-lane roundabouts and less than 28 mph for the fastest through path for multilane roundabout entries. Approach roadway design speeds and fastest theoretical paths are to be shown. Fast path radii and the range of corresponding speeds based on a (+)2.0% cross slope and (- )2.0% cross slope are to be annotated on the design checks submittal Series 2 diagrams.

Series 3 -

Truck swept-path diagrams are to be developed using AutoTURN®, Vehicle Tracking, or similar software, with depiction of the design and check vehicle(s) for all of the anticipated turning movements. Design and check vehicles will validate the width of lanes and the circulatory roadway, e.g., S-BUS-40 or CITYBUS. The check vehicle shall validate the width of the central island truck apron and entry and exit widths using a 1-ft shy distance from face of curb (FOC) to the vehicle wheels. The truck cab of a design and check vehicle will typically stay on the circulatory roadway when running swept-path analysis with all tires remaining at least 1-ft from the face of curb.

Series 4 –

Stopping sight, intersection sight and view angle diagrams accounting for both vertical approach grades and horizontal alignments. Sight distances must account for driver eye height and height of objects. This is for the purpose of user safety and to establish limits to planting scheme/landscaping for the mounded central island and splitter islands. Refer to , Section 9.5.

Series 5 –

Ancillary elements diagrams include light pole locations and traffic control devices such as signing and pavement markings (refer to the ). These elements are generally developed after schematic design; however, with multilane roundabouts, it is necessary to design markings at the concept or schematic stage to validate the use of exclusive lanes and spiraled geometry.

, Section 9.4.1 and Appendix A.1 covers the assessment of speed control for a roundabout’s horizontal geometry. Section 9.4.2 of outlines the process for developing fastest path alignments using the spline technique. Key considerations for fastest path speed assessment for roundabouts consist of:

provides a graphical representation of the corresponding theoretical maximum fastest path speed alignments for a left-turn (R4/V4), through movement (R1/V1, R2/V2, R3/V3), and a right-turn (R5/V5).

Path overlap is a detrimental condition that can arise from inadequate horizontal geometry of multilane roundabout entries and exits, as discussed in , Section 9.6. Providing tangency to adjacent entry and exit lanes has emerged as an important feature of multilane design to discourage vehicle lane changing within the circulating/conflicting conflict zone areas of a roundabout. visualizes the method for checking path overlap. The amount of tangent to avoid entry path overlap and exit path overlap between point A to point B and point B to point C, respectively, is 30-ft to 50-ft and 40-ft to 60- ft. One check of adequate entry geometry is for the tangent point at point A to be located within 5-ft of the entry yield line. Additional tangency beyond 5-ft prior to the entry line should be avoided so the speed-controlling entry radius is not shifted too far in advance of the typical crosswalk location.

, Section 9.7 covers the topic of design vehicles. The largest anticipated non-articulated vehicle that is needed to be fully accommodated in the circulating roadway within the roundabout (the “design vehicle”), and the largest anticipated articulated vehicle subject to use a given roundabout (the “check vehicle”) are key roundabout design parameters. Typical design vehicles are the largest local jurisdiction fire department apparatus or a city bus. An additional check vehicle that includes articulation, such as a WB-40, WB-62TX, or WB-67, should be analyzed in properly sizing entry widths, circulatory roadway widths, truck apron widths, exit widths, and determining the final location of proposed curbing within and adjacent to a given roundabout.

The selection of the design vehicle and the check vehicle to navigate the intersection throughout the life cycle of the intersection is necessary prior to preliminary design. Consideration should be given for individual movements at an intersection. The design vehicle and check vehicle may only need to make thru movements and may not need to make left or right-turn movements on or off the major roadway. Adjacent land use, trip generators, and local and regional routing may factor into the verification of movements of the check vehicle on a case-by-case basis. The selection of the appropriate vehicles should be vetted with TxDOT staff, and possibly with local agency staff, local stakeholders, and potential permitted load routing.

, Chapter 9 and Appendix A.4 outlines vehicle performance checks for design vehicles and check vehicles.

Key considerations for design vehicles and check vehicles at roundabouts consist of:

shows common design and check vehicle specifications based on for a CITY-BUS and a WB-67. The WB-62TX designation, although not an official designation by AASHTO, is a common tractor-trailer configuration in Texas and is recommended to be used as a check vehicle where a WB-67 configuration is not anticipated. of this manual has additional information on Design and Control Vehicles.

Straddle lane design is presumed unless it can be shown that a given intersection is anticipated to accommodate a significant amount of daily or peak period truck traffic. If an intersection is anticipated to have 120 trucks per hour, or more, (one truck every 30 seconds) on a given approach, stay-in-lane design is recommended to be implemented. Contexts for stay-in-lane design will be roundabouts proposed at interchanges, near common OSOW facilities and freight trip generators.

The need for stay-in-lane design increases at roundabouts that are anticipated to experience near- or at-capacity conditions in the peak periods. This is due to the additional time gap needed for large trucks to be able to enter safely into the circulatory while yielding to circulating/conflicting vehicles. A stay-in-lane design in this situation allows passenger vehicles to accept smaller gaps in traffic and proceed through the roundabout while the large truck waits for an extended gap in traffic.

View angle is the angle created by a driver yielding at the entry line while looking to their left to identify oncoming vehicles that will conflict with their entry path. As the angle that a driver is required to turn their head to identify oncoming traffic increases, vision and detection of potential conflicts becomes increasingly difficult. This may especially be the case with drivers with chronic neck pain, injuries, or elderly drivers. View angle is also a factor with large trucks, whereby the side window placement is commonly a constraint for sight to the left at acute angles.

, Exhibit A.16 and depict the arrangement of vehicles for angle of visibility checks. If an oncoming vehicle’s location crosses out of the 105- degree cone of vision for this visibility-to-the-left performance check, drivers will not be able to move close to the yield line for normal sight of oncoming traffic to the left. In such cases, the approach and entry geometry must be modified to improve the view angle. Skewed intersections and right-turn yielding bypass lanes are typical examples where view angle performance checks are vital to providing visibility-to-the-left.

, Section 9.5 documents the relevant sight distance checks for roundabouts. Stopping sight distance, intersection sight distance, and decision sight distance are all relevant to roundabout geometric design. Key considerations for adequate sight distance at roundabouts consist of:

To supplement , the following roundabout design considerations are presented to increase design flexibility for complicated design context of roundabouts in Texas.

Oversized and overweight vehicles are common on Texas roads. Due to the presence of wind turbine tower trailers, heavy construction equipment trailers, and other large transport trucks, it can become necessary to specifically modify a roundabout design to accommodate an OSOW. Coordination should take place between the intersection location’s TxDOT Area Office and the engineer-of-record to determine if permitted loads are present at the subject intersection, and if so, the specifications of the largest anticipated vehicle. , Section 10.5.4 discusses the design of roundabouts for OSOWs.

Note for the general purposes of superelevation methodologies, intermediate speeds are classified as between 50-60 mph (see ). Roundabouts installed in these contexts require recognition of the human factors needs of the driver to detect, recognize, and react to the presence of a roundabout while approaching the intersection at a higher rate of speed.

The primary safety concern in intermediate/high-speed context is clarity of the driving situation, that is, to make drivers aware of the roundabout with ample distance to comfortably decelerate to the appropriate speed. Therefore, designs should abide by these principles:

For approaches to roundabouts see for appropriate superelevation methods.

Guidance provided in , Section 10.14 combined with aids in designing splitter island length, horizontal curves, and superelevation to treat roundabouts in an intermediate/high-speed context.

Spiral circulatory geometry is commonly used when a dedicated left-turn lane is implemented at a multilane roundabout. shows an example of a spiraled movement. Spirals can also be applied for any 2x1 lane configuration roundabout where the single circulating lane spirals to the portion of the roundabout where a resulting two lanes are located within the circulatory. Benefits of using spirals in roundabouts consist of:

The concept of buffered lane design has emerged from discussions in the U.S. of how to adapt turbo roundabout design to conventional U.S. multilane roundabout design. Buffered lane design is also emerging as a retrofit solution where constructed roundabouts may have excessive ICDs and/or circulatory widths.

The benefits of buffered lane roundabouts are evident with improved lane discipline for passenger vehicles, reduced vehicle speeds and failure-to-yield conflicts, while also providing lane discipline for articulated trucks. Where existing multilane roundabouts have been built with a surplus of circulatory width, buffered lane design represents a low-cost solution to narrow circulating lanes which in turn enhances speed reduction and lane discipline without having to consider full-depth reconstruction as part of a safety improvement.

Applying buffered lane design to initial installation is done more deliberately while upholding the basic principles of ICD size, geometric speed control, and approach alignment. shows an example of a buffered lane design roundabout.

By definition, a turbo roundabout includes a radial entry and exit design, employs orbiting geometry within the circulatory to facilitate the origin-destination path through the roundabout, and makes use of raised dividers to channelize vehicles to enforce in-lane driving. Implementation of turbo roundabouts is in its infancy stage in the U.S. since only a handful of turbo roundabouts have been built in the U.S. Designers must exercise caution when selecting this type of roundabout. Refer to Section 10.7.8 in for additional information regarding turbo roundabouts. An example of a constructed turbo roundabout in the U.S. is shown in .

Interim and ultimate lane configuration roundabout analysis may result in a preferred method of using future expandable design. If within the operational analysis a roundabout is shown to benefit from a reduced lane configuration on opening day for the first 10 to 15 years of its existence, then expandable design should be considered. Reducing the number of entry lanes on a given approach will result in less conflict points, shorter crossing distances for pedestrians and bicyclists, and provide for safer operations.

, Section 10.8 discusses the use of expandable design, including commentary on the advantages and disadvantages of expanding outward versus expanding inward.

, Section 12.7 provides guidance on roundabouts near rail crossings. Roundabouts with at-grade rail crossings nearby or within the roundabout have been constructed at many locations in the U.S., including downtown Daingerfield, TX. The decision of whether to install crossing gates at a roundabout may be considered early in the planning or design process. Early and up-front coordination is highly recommended with the governing railroad agency if a roundabout is proposed near an at-grade rail crossing. Risk analysis, traffic queue calculations, train crossing frequency and duration, and proximity of the roundabout to the rail crossing are important factors when considering a potential roundabout installation in this type of context.

Various topics arise when considering the effect of roundabouts and access management. Roundabout corridors can replace median-opening cross-access along an undivided or divided roadway. Motorists can make use of U-turn movements at a roundabout to facilitate what would previously be a left-turn movement to access a property on the opposite side of the street.

A corridor of roundabouts combined with continuous raised medians provides amplified safety benefits by eliminating high severity crash types such as head-on and right-angle or side-impact collisions. Roundabouts provide U-turns for all vehicle types at low speeds, eliminating the risks of mid-block left-turns and U-turns across opposing high-speed traffic. Median width can thereby often be reduced since mid-block turning movements are prohibited. This strategy is called “wide nodes and narrow roads”. Severe crash risks are reduced within the narrowed mid-block roadway section and at the intersections. Roundabout spacing in this type of corridor is important to consider to limit the distance required for a vehicle to make a U-turn to access adjacent property.

Driveway locations relative to a roundabout can typically be moved closer to the intersection with a roundabout, as compared to a conventional traffic signal installation, due to shorter queue lengths. Other factors influencing the location of a driveway relative to a roundabout consist of:

Determination of a concrete or asphalt pavement section is important to the schematic and preliminary design of a roundabout due to the subsequent performance checks for fastest path and design vehicles. Determining whether the roadway will have a monolithic curb, or a separate curb and gutter section may have a direct impact on the curb offset location and the subsequent performance checks performed for a roundabout.

For constructability and grade control, setting up additional alignments and profiles of the edge of pavement produces optimal grade control for retrofit designs. Developing a grading surface that closely follows existing topography or the natural grade of the intersection often results in a tilted plane, also known as “tipping the circle”. Tipping the circle requires gradual transition from sloping in on the high side to sloping out on the low side. The preferred maximum cross-slope of the low side should be 2-percent to prevent truck roll-overs.

Additional considerations for the proper accommodation of OSOWs may be needed for vertical clearance checks of lowboys and other vehicles that have a reduced vertical envelope. In rare cases, the height of the center island truck apron and the circulatory grade may need to factor into the check vehicle vertical clearance.

For additional guidance, refer to , Chapter 11.

Ideally, drainage structures will be installed outside of the limits of the ICD of a roundabout. Inlets upstream of crosswalks in advance of a roundabout and downstream of the ICD typically provide enough capture from runoff within the footprint of a roundabout. Minimum and maximum slopes should be analyzed, including a typical cross slope within the circulatory roadway of 1.5 percent to 3.0 percent. Chapter 10 of provides guidance on drainage best practices and examples.

Pavement marking plans should be separated from signing plans for clarity. Legends are encouraged on pavement marking sheets to convey the TxDOT bid item numbers and an accurate description of the bid item, including color, width, length, and gap length of the pavement marking.

to represent TxDOT’s preferred pavement markings for roundabouts. , Chapter 12 contains supplemental information regarding pavement markings at roundabouts. The also serves as a resource in preparing pavement marking designs. Key considerations for pavement markings at roundabouts consist of:

The most current version of the must be sourced for providing signage at roundabouts. Key considerations for signage at roundabouts include:

Advance overhead lane assignment signage on multilane roundabouts is strongly recommended for wayfinding and driver recognition, especially at interchange roundabouts and roundabouts that are located near other alternative intersections, interstate access points, or where more than two lanes may exist along the approach to a roundabout. Exhibit 12.8 of provides an example of the preferred lane assignment sign assembly and placement for a two-lane roundabout.

, Sections 14.1, 14.2 and 14.3 focus on illumination considerations, lighting levels, and illumination equipment type and location. Since agency illumination policies vary widely across the State, it is important to factor in local restrictions or requirements such as Dark Sky Initiatives.

Section 14.4 of provides guidance for roundabout landscaping. Key considerations for landscaping include:

For roundabout installations where an existing intersection is being reconstructed or modified to accommodate a roundabout, attention to a few factors that influence how the roundabout will be constructed is important. Common considerations consist of:

Oftentimes opposing directions will be narrowly separated by a double-yellow pavement marking. If routing bi-directional traffic through a portion of a constructed multilane roundabout, additional width may be needed to accommodate the swept path of an articulated truck or front/rear overhang of an extended vehicle.

For concrete pavement roundabouts, a jointing plan is commonly prepared by the engineer-of-record or supplied by the contractor. Multilane roundabouts should have panels and joint lines such that a pinwheel pattern is developed to favor yielding of entering traffic and priority to existing traffic. Joint and panel patterns must not contradict lane divisions and pavement markings. The optimal jointing plan reveals where lane divisions are and where vehicles circulating can exit a roundabout without changing lanes.

Exhibit 13.8 of and show the use of a pinwheel pattern jointing plan. Joints must also be installed using conventional minimum and maximum panel sizes. Utility appurtenances, such as manholes, water valves, and curb inlets, should be accounted for in jointing techniques, to minimize cracking and failure of adjacent concrete pavement.

For roundabouts built under traffic conditions, the jointing plan must factor in the location of construction joints based on the phased construction of the roadway. Construction phasing considerations may result in a deviation from a standard jointing pattern, however, the proposed jointing strategy should place longitudinal and transverse joints as close as possible to a jointing design that would be produced in lieu of phased construction.

Bicycle ramps can be beneficial at single-lane roundabouts and are commonly considered at multilane roundabouts where on-street bicycle activity exists or is anticipated. , Exhibit 10.25 and Exhibit 10.26 show an example of how bikes can transition from the on-street facility to the parkway facility and from the parkway facility to the on-street facility.

Based on the publication, multilane pedestrian crossings at roundabouts and channelized turn lanes must have additional treatments that alert motorists to the presence of pedestrians or slow or stop traffic at those crosswalks. Additional treatments can consist of raised crosswalks, pedestrian actuated rectangular rapid flashing beacons (RRFB), pedestrian hybrid beacons (PHB), or a traffic control signal with a pedestrian signal head (R306.4.2).

The August 2023 Rules and Regulations update included additional clarification and guidance for pedestrian facilities consisting of: