14.4.3 Design Process and Design Context

Varying characteristics will determine the selection of a roundabout’s size, lane configuration, and feasibility. Refer to
NCHRP Report 1043
, Chapter 3, Chapter 8, and Chapter 9 for roundabout performance-based planning and design information and roundabout design principles and objectives.
14.4.3.1 Roundabout Implementation Challenges
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:
  • Adjacent features that may lead to queue spillback into the roundabout (overcapacity traffic signal, freeway entrance ramp, overcapacity driveway, etc.);
  • Unfavorable topography that results in limited visibility, excessive cross slopes in the circulatory roadway, or significantly complicates construction;
  • Where the ratio of major roadway traffic to minor roadway traffic exceeds 9:1. Unacceptable delay may result along the minor roadway. Detailed traffic analysis in this situation is recommended. Refer to Chapter 12 of the
    TSAP
    for additional guidance;
  • Significant pedestrian and bicycle movements at a near- or over-capacity roundabout;
  • Within a coordinated traffic signal network. A roundabout may disrupt progression and have a negative impact on the efficiency of a signalized corridor. If possible, converting a corridor to all roundabouts may provide operational efficiencies. Refer to Chapter 9 the
    TSAP
    for more information regarding segment analysis;
  • Roadways with reversible lanes for alternating peak period traffic flows;
  • At or adjacent to an at-grade railroad crossing; and
  • Environmental factors. Proximity of historical sites, ROW constraints, or other environmental constraints adjacent to an intersection.
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
TSAP
, Chapters 10, 11 and 12.
14.4.3.2 Identify Design Considerations
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:
  • Inscribed circle diameter (ICD);
  • The proposed surface pavement section (concrete or asphalt);
  • Mountable/non-mountable curb types;
  • Gutter pan dimensions and slopes, if applicable;
  • Minimum and maximum performance check values;
  • Truck traffic accommodation (straddled lane design vs non-straddled), formerly known as Case 1, Case 2, and Case 3 (refer to
    NCHRP Report 1043
    , Section 9.7.1);
  • Traditional lane geometries, spiraled lanes, buffered lanes, or turbo roundabout design;
  • Pedestrian and bicycle facilities, including buffer width between the back of curb;
  • The inclusion of bike ramps;
  • Proper illumination on the perimeter roadway and within the center landscaped island;
  • Maximum cross slopes in the circulatory;
  • Adjacent driveway accessibility; and
  • If multilane, the use of a traffic control signal with a pedestrian signal head, a pedestrian hybrid beacon (PHB), a pedestrian actuated rectangular rapid flashing beacon (RRFB), or a raised crossing as a crosswalk treatment.
The performance-based geometric design of a roundabout is an iterative process, as outlined in
NCHRP Report 1043
, 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:
  • Traffic patterns and volumes;
  • Existing and/or proposed ROW boundaries;
  • Connecting roadway alignments and centerlines;
  • The footprint of the existing intersection;
  • Circular versus elliptical design;
  • The effect the diameter has on geometric speed control and resulting circulating speeds;
  • If intersecting roadway legs are skewed, or normal to the intersection;
  • The design vehicle and check vehicle(s) – see
    Table 14-1
    for a list of typical design and check vehicles and
    Section 14.4.5.4
    for an explanation of design and check vehicles;
  • Oversize or overweight vehicles (OSOW);
  • Environmental considerations; and
  • Grade considerations.
Table 14-1: Common ICD Ranges
Source: Adapted from NCHRP Report 1043
Roundabout Configuration
Typical AASHTO Design Vehicle
1
Typical AASHTO Check Vehicle
Common ICD Ranges
Mini-roundabout
Local jurisdiction largest apparatus (fire truck or city bus)
2
WB-62TX
70-ft to 90-ft
Compact Roundabout (non-traversable central island)
Local jurisdiction largest apparatus (fire truck or city bus)
2
WB-62TX
90-ft to 110-ft
Single-lane roundabout (non-traversable central island)
Local jurisdiction largest apparatus (fire truck or city bus)
2
WB-62TX, WB-67 or OSOW
110-ft to 150-ft
Multilane roundabout (2 lanes circulating)
3
Local jurisdiction largest apparatus (fire truck or city bus)
2
WB-62TX, WB-67 or OSOW
140-ft to 200-ft
Multilane roundabout (3 lanes circulating)
3
Local jurisdiction largest apparatus (fire truck or city bus)
2
WB-62TX, WB-67 or OSOW
180-ft to 220-ft
Notes:
  1. Assumes 90-degree angles between entries and no more than four legs. List of possible design vehicles is not comprehensive.
  2. Certain Design Vehicles must be able to navigate the roundabout without the use of the truck apron or external truck apron.
  3. The Common ICD Ranges shown for multilane roundabouts represents typical ranges to be used for straddle-lane design. For stay-in-lane design, or OSOW accommodation, slightly larger diameters may be necessary.
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.