# Geometric Design of Suburban Roundabouts

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## Definition

**:**

## 1. Introduction

## 2. Suburban Roundabout Design Elements

#### 2.1. The Approach Alignment

#### 2.2. The Outer Radius

#### 2.3. The Circulatory Roadway

_{bu}” and “R

_{bi}”) for single-lane roundabouts. The cross slope of the circulatory roadway according to [7] is 2.0 to 2.5% and is directed towards the outer edge of the roundabout.

_{k}”). The circulatory roadway is of constant width and a constant cross slope. The circulatory roadway width depends on the size of the outer radius—the widths of the circular ring of intersection (“B

_{k}”) for single-lane roundabouts depending on the size of the outer radius are given in Table 1. For suburban roundabouts, the widths of the circular ring of intersection (“B

_{k}”) shown in the table correspond to the widths of the circulatory roadway, since the aprons are not constructed at these intersections. At suburban intersections with increased truck traffic, it is possible to apply larger widths of the circulatory roadway than those given in the table. The cross slope of the circulatory roadway is directed towards the outer edge of the intersection and must be at least 2.5% to ensure proper surface drainage of the pavement.

_{k}”) arises from the swept path requirements of the design vehicle and the driving conditions. It can be standardized to some extent for typical suburban road network conditions for single-lane and two-lane roundabouts. The standard widths of the roundabout (“b

_{k}”) for single-lane roundabouts (Figure 6) are defined depending on the diameter of the inscribed circle of a roundabout (“D”) and include marginal strips that are 0.20 m wide.

_{p}”) as a function of the slope of the intersection plane (“i

_{Nkr}”) is shown in Table 2. At the same time, the largest total slope is 4% [10].

#### 2.4. The Apron

#### 2.5. The Splitter Island

- Timely warning of drivers about a roundabout in the traffic network.
- Separation of traffic flows.
- Providing space for the installation of traffic signs, lighting, and other traffic equipment.
- Preventing a left turn into a roundabout.

_{g}”), as shown in Table 4. The splitter island on suburban roundabouts (where the outer radius is equal to or larger than 15 m) is shifted to the left to ensure that the approach axis passes through the center point at the top of the island (point C, Figure 12). The minimum width of the splitter island is 2 m, and the usual width at the entrance to the intersection (“B”) is equal to a quarter of the outer radius of the intersection.

- Entry and exit speeds are low due to provided deflection.
- The shape of the splitter island forces the driver at the entrance to the intersection to respect the right of way of the vehicle at the intersection.
- The angle between the entry lane and the circulatory roadway remains 90° (which is advantageous in terms of visibility).

_{m}”) is 10 to 15 m, and the standard width of the island (“B

_{m}”) is 3 m (Figure 13).

#### 2.6. The Entry and Exit Design

_{ul}” and “R

_{iz}”), entry and exit widths (“e” and “e’”), and circulatory roadway width (“u”). The right pavement edge can be designed in two different ways:

- With a shorter effective widening length (“l’”)—the pavement edge is composed of a circular arc and a straight line that is parallel to the side of the triangular splitter island (Figure 19a), and the selected widening (“Δš”) is equal to zero;
- With a longer effective widening length (“l’”)—the pavement edge is composed of a circular arc and a straight line that is not parallel to the side of the triangular splitter island (Figure 19b), and the selected value of widening (“Δš”) is larger than zero.

_{E}”), whereas a radius varying from 15 to 25 m is applied at the exit (exit radius “R

_{A}”), as shown in Figure 21. The radii at the entry are smaller than the radii at the exit to reduce the speed of vehicles at the entrance while facilitating the exit of long vehicles and buses from the intersection. The radii applied must meet the design vehicle swept path requirement.

_{e}”) at single-lane entrances, measured between the boundary lines, is 4 m, and the entrance radius (“R

_{e}”) must always be less than or equal to the outer radius of the roundabout (“R

_{g}”). The standard values of the entry radius range from 10 to 15 m, depending on the position of the approaches. The entry lane is bounded by boundary lines. The standard configuration of the single-lane entrance is shown in Figure 22 (for an outer radius of 20 m). The width of the exit lane (“l

_{s}”) ranges from 4 to 5 m for single-lane approaches and depends on the size of the outer radius of the roundabout (“R

_{g}”). The exit radius (“R

_{s}”) must be larger than the inner radius of the intersection (“R

_{i}”), with a minimum recommended value of 15 m and a maximum of 30 m. The values of these parameters depend on the size of the outer radius of the intersection (“R

_{g}”) and are shown in Table 6.

_{t}”) should be in the range of 3 to 4 m—wider lanes encourage drivers to drive around the roundabout at higher speeds, which reduces safety. The width of the exit lanes (“B

_{a}”) depends on the design vehicle swept path requirements, and ranges from 3.75 to 4.50 m. The standard values of the entry radii (“R

_{t}”) at single-lane suburban roundabouts range from 8 to 12 m, and the exit radii (“R

_{a}”) from 12 to 15 m. Design elements of entry and exit at suburban single-lane roundabouts are given in Figure 23.

_{Z}”) ranges from 3.50 to 4.00 m, whereas the recommended exit lane width (“B

_{A}”) is in the range of 3.75 to 4.50 m (Figure 23). The rounding of the right edge of the pavement at the roundabout entry and exit lane is formed by applying a circular arc, the size of which depends on the desired speed limit and the design vehicle swept path requirements (Figure 24). The standard radius of the circular arc at the entry lane (“R

_{Z}”) ranges from 14 to 16 m, whereas the radius of the circular arc at the exit lane (“R

_{A}”) is in the range of 16 to 18 m. The stated values of the exit radii can be increased by 30% at suburban roundabouts (i.e., “R

_{A}” is in the range of 21 to 23 m).

_{u}”) at single-lane roundabouts is 3.5 to 4.0 m, whereas the standard width of the exit lane (“b

_{i}”) is 3.75 to 4.50 m. Regardless of the required level of traffic-flow channeling, the initial condition for the entry radius (“R

_{u}”) is as follows: The radius (“R

_{u}+ b

_{u}”) can ultimately tangent (but not intersect) the edge of the roundabout, as shown in Figure 25. The standard values of the entry radius (“R

_{u}”) are in the range of 12 to 16 m. The condition of the ratio of entry and exit speeds requires that the exit radius (“R

_{i}”) be larger than the entry radius by 2 m. The standard values of the exit radius are in the range of 14 to 18 m.

- For the highest level of channeling, with a funnel-shaped splitter island, the rounding off the right edge of the pavement at the entry and exit of the intersection is performed using the radii “R
_{u}” and “R_{i},” whereas the transition from lane width at the approaches is performed using the radius “R,” which is at least three times larger than “R_{u}” for the entry and at least three times larger than “R_{i}” for the exit. - For a medium level of channeling, with a triangular splitter island, rounding off the right edge of the pavement at the entry is performed using radii “R
_{u}” and “2R_{u},” and rounding of the right edge of the pavement at the exit is performed using radii “R_{i}” and “2R_{i}.” The transition from the lane width at the approach to the entry lane width is performed by applying the radius “R,” whose tangents are a straight line that defines the edge of the pavement at the entry and a straight line parallel to the edge of the triangular splitter island and at a distance from that edge “b_{u}.” - For the lowest level of channeling, with a radial splitter island, the rounding of the right edge of the pavement at the entry and exit of the intersection is performed using the radii “R
_{u}” and “R_{i},” whereas the transition from the approach lane width to the entry lane width is achieved by applying a curve of length “L_{v}.”

_{e}”) for single-lane roundabouts in terms of safety, whereas a value of 3.5 to 4.5 m is defined as a suitable width of the lane at the exit (“b

_{a}”). According to [12], the right edge of the pavement at the roundabout entry and exit is formed by applying two radii, as shown in Figure 26. At suburban roundabouts, the recommended size of the inner entry radius (“R

_{e2}”) is 12 m, whereas the outer entry radius (“R

_{e1}”) is five times larger. The recommended size of the inner exit radius (“R

_{a2}”) is 14 m, whereas the outer exit radius (“R

_{a1}”) is four times larger.

#### 2.7. The Longitudinal Slopes at Roundabouts

_{o}” from the outer edge of the roundabout (Figure 27, “V

_{ras}” is the design speed). If the longitudinal slopes of the approaches are greater than 2.5%, it is necessary to mitigate them, as shown in Figure 28.

## 3. Comparative Analysis

## 4. Concluding Remarks

- The roundabout outer radius must be selected by considering the spatial limitations and the requirements of the design vehicle swept path.
- The swept path analysis for the design vehicle should be the basis for the determination of the minimum required width of the circulatory roadway.
- When defining the width of a circulatory roadway at single-lane roundabouts, widths above 5.5 m should be avoided so that drivers do not use a wide roadway as two lanes, and so that sufficient deflection is achieved.
- When the swept path conditions for the design vehicle require a large width for the circulatory roadway, it is necessary to introduce an apron.
- The apron is intended for the passage of trucks only—buses use only the width of the circulatory roadway to ensure driving comfort.
- The design of the splitter island, i.e., the channeling of traffic flows, should depend on the traffic conditions (the main traffic flow should be undisturbed), the type of road (the road cross-section outside of the intersection zone), horizontal alignment (good visibility must be provided), and the position of intersection in the network (for suburban roundabouts, the channelization must be stretched and well signaled, otherwise raised islands can be a source of danger due to higher driving speeds).
- The entry and exit radii, along with the width of the circulatory roadway and the approach lanes, should enable the fulfillment of the design vehicle swept path requirements.
- It is recommended to use the smallest possible entry and exit radii since larger radii can result in higher speeds at the intersection.
- The exit radius should be larger than the entry radius to increase the exit speed.
- Particular attention should be paid to wide entrances: An entry width greater than 5.5 m or greater than the width of a circulatory roadway can lead drivers to interpret a wide single-lane entrance as two lanes, which increases the risk of collision when entering a single-lane intersection.

## Author Contributions

## Funding

## Conflicts of Interest

## Entry Link on the Encyclopedia Platform

## References

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**Figure 2.**The approach alignment variants according to [8]: (

**a**) radial alignment geometric elements of single-lane roundabouts, (

**b**) displacement to the left, and (

**c**) displacement to the right. The red “×” marks the center of the roundabout outer radius.

**Figure 4.**Circulatory roadway cross slope according to [5]: (

**a**) recommended cross slope direction and (

**b**) circulatory roadway in one plane. Slope direction is represented by the arrows.

**Figure 5.**Circulatory roadway width (“B”) depending on the outer and inner radius (“R

_{bu}” and “R

_{bi}”) [8].

**Figure 7.**Minimum circulatory roadway width (together with apron width) depending on the roundabout inscribed diameter [12].

**Figure 10.**Standard circulatory roadway cross-section for roundabouts with an outer radius of 15 m, according to [6].

**Figure 11.**Central island design [10].

**Figure 12.**Splitter island elements according to [6]. The center point at the top of the island (point C) is represented by the red circle.

**Figure 14.**Funnel-shaped splitter islands on suburban roundabouts according to [9].

**Figure 15.**Splitter islands for a high level of traffic-flow channeling [10].

**Figure 16.**Splitter islands for a medium level of traffic-flow channeling [10].

**Figure 17.**Splitter islands for a low level of traffic-flow channeling [10].

**Figure 18.**Splitter island according to [15].

**Figure 19.**Entry design according to [4]: (

**a**) shorter effective widening length and (

**b**) longer effective widening length; “v” is the approach lane width, “m” is the splitter island length, “R

_{v}” is the roundabout outer radius, and “R

_{ul}” is the entry radius.

**Figure 20.**Exit design according to [4]: “v’” is the approach lane width, “m” is the splitter island length, “R

_{v}” is the roundabout outer radius, and “R

_{ul}” is the entry radius.

**Figure 21.**Right edge of the pavement at the roundabout entry and exit according to [5].

**Figure 22.**Single-lane approach for an outer radius of 20 m according to [6].

**Figure 23.**Design elements of entry and exit at suburban single-lane roundabouts according to [8].

**Figure 24.**Design elements of roundabout entry and exit according to [9].

**Figure 25.**Initial design parameters for single-lane roundabout entry according to [10].

**Figure 26.**Design elements for single-lane roundabout entry and exit according to [12].

**Figure 27.**Roundabout approach design elements according to [10].

**Figure 28.**Mitigation of longitudinal slopes on roundabout approaches (Rv*—hog vertical curve radius; Rv**—sag vertical curve radius) [10].

Outer Radius (m) | Circular Ring Width “B_{k}” (m) |
---|---|

13 | 9.0 |

15 | 8.0 |

17.5 | 7.0 |

≥20 | 6.5 |

**Table 2.**Circulatory roadway slope [10].

Longitudinal Slope “i_{Nkr}” (%) | Maximum Cross Slope “i_{p}” (%) |
---|---|

0 < i_{Nkr} ≤ 2.5 | −2.5 |

2.5 < i_{Nkr} ≤ 4.0 | 2.5 |

4.0 < i_{Nkr} ≤ 5.7 | 4.0 |

State | Apron Width (m) | Apron Cross Slope (%) |
---|---|---|

Croatia | defined based on the design vehicle swept path | not defined |

Austria | defined based on the design vehicle swept path | not defined |

France | 1.5–2.0 (for outer radius between 12 and 15 m) | 4.0–6.0 |

The Netherlands | defined based on the design vehicle swept path; 1.5–4.0 | 1.0 |

Germany | aprons are not constructed | − |

Serbia | aprons are not constructed | − |

Switzerland | 1.5–2.0 | not defined |

**Table 4.**Splitter island elements according to [6].

Outer Radius “R_{g}” (m) | Splitter Island Length “H” (m) | Splitter Island Width “B” (m) | Shift “d” (m) | Radius at the Top of the Splitter Island “r” (m) |
---|---|---|---|---|

15 | 15 | 3.75 | 0.40 | 0.30 |

20 | 20 | 5.00 | 0.45 | 0.40 |

25 | 25 | 6.25 | 0.50 | 0.50 |

State | Splitter Island Shape | Splitter Island Length (m) | Splitter Island Width (m) | Splitter Island Distance from the Outer Edge of the Roundabout (m) |
---|---|---|---|---|

Croatia | triangle | − | ≥2.0 | ≤0.50 |

funnel | − | ≥2.0 | ≤0.50 | |

Austria | triangle | − | ≥2.0 | ≥0.25 |

France | funnel | 15–25 | ≥2.0 or (outer radius/4) | 1.0 |

The Netherlands | radial | 10–15 | ≥3.0 | 1.0 |

Germany | funnel | − | ≥1.6 | 0.0 |

Serbia | radial | ≥10 | ≥1.5 | 0.25 |

triangle | ≥12 | ≥3.0 | 0.5 | |

funnel | ≥15 | ≥5.0 | 0.5–1.0 | |

Switzerland | funnel | 30–50 | ≥1.2 (5.0) | 0.0 |

**Table 6.**Design elements for roundabout entry and exit according to [6].

Outer Radius “R_{g}” (m) | Entry Width “l_{e}” (m) | Entry Radius “R_{e}” (m) | Exit Width “l_{s}” (m) | Exit Radius “R_{s}” (m) | Transitional Radius “R_{r}” (m) |
---|---|---|---|---|---|

12 | 4 | 12 | 4.0 | 15 | 48 |

15 | 4 | 15 | 4.0 | 20 | 60 |

20 | 4 | 15 | 4.5 | 20 | 80 |

25 | 4 | 15 | 5.0 | 20 | 100 |

State | Entry Width (m) | Exit Width (m) | Entry Radius (m) | Exit Radius (m) |
---|---|---|---|---|

Croatia | 4.00–7.00 | 4.00–7.00 | 8–20 | 10–25 |

Austria | ≥3.75 | ≥4.00 | 12–16 | 15–25 |

France | 3.75–4.00 | 4.00–5.00 | 10–15 | 15–30 |

The Netherlands | 3.50–4.00 | 4.00–4.50 | 8–12 | 12–15 |

Germany | 3.50–4.00 | 3.75–4.50 | 14–16 | 16–18 |

Serbia | 3.50–4.00 | 3.75–4.50 | 3R_{u} or 2R_{u} | 3R_{i} or 2R_{i} |

R_{u} = 8–12 | R_{i} = R_{u} + 2 | |||

Switzerland | 3.00–3.50 | 3.50–4.50 | R_{e1} = 60 | R_{a1} = 56 |

R_{e2} = 12 | R_{a2} = 14 |

State | Maximum Slope of the Intersection Plane (%) | Maximum Longitudinal Slope of the Approaches (%) | Longitudinal Slope Changes–Distance to the Roundabout Outer Edge (m) |
---|---|---|---|

Croatia | ≤4 | ≤4 | ≥6.0 |

Austria | ≤4 | ≤4 | 20.0 |

France | ≤6 | ≤3 | not defined |

The Netherlands | not defined | not defined | not defined |

Germany | ≤6 | ≤6 | not defined |

Serbia | ≤2.5–5.7 | not defined | 7.5 |

Switzerland | ≤5 (7) | ≤5 (7) | not defined |

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Ahac, S.; Dragčević, V.
Geometric Design of Suburban Roundabouts. *Encyclopedia* **2021**, *1*, 720-743.
https://doi.org/10.3390/encyclopedia1030056

**AMA Style**

Ahac S, Dragčević V.
Geometric Design of Suburban Roundabouts. *Encyclopedia*. 2021; 1(3):720-743.
https://doi.org/10.3390/encyclopedia1030056

**Chicago/Turabian Style**

Ahac, Saša, and Vesna Dragčević.
2021. "Geometric Design of Suburban Roundabouts" *Encyclopedia* 1, no. 3: 720-743.
https://doi.org/10.3390/encyclopedia1030056