Model widths, radii, islands, aprons, and lanes. Compare speed paths, balance, and layout efficiency instantly. Create clearer preliminary junction concepts for safer traffic operations.
These formulas provide a practical preliminary layout model for conceptual roundabout design studies.
Where: e is superelevation in decimal form, and f is side friction factor.
The geometry balance score compares target speed, splitter length, pedestrian offset, and width harmony. It is a screening indicator, not a design code approval value.
| Parameter | Example Value | Example Result or Note |
|---|---|---|
| Number of legs | 4 | Standard four-leg urban roundabout concept |
| Central island diameter | 18.00 m | Provides a clear central deflection feature |
| Truck apron width | 2.00 m | Supports larger vehicles without widening every lane |
| Circulatory width | 8.00 m | Suitable for a typical single-lane layout |
| Inscribed circle diameter | 38.00 m | Computed from island, apron, and circulatory widths |
| Entry speed estimate | 23.90 km/h | Based on 25 m entry radius, 2% e, and 0.16 friction |
| Weaving length estimate | 29.85 m | Approximate spacing between adjacent legs |
| Pedestrian setback | 5.00 m | Usually a practical low-speed offset range |
ICD means inscribed circle diameter. It is the full outside diameter of the roundabout measured across the circulatory roadway. It is one of the main controls for scale, speed, and vehicle accommodation.
The truck apron helps larger vehicles track around the central island without forcing a wider circulatory lane for every car. It can improve compactness while still supporting occasional heavy vehicle movements.
No. They are conceptual estimates based on radius, superelevation, and friction. Actual speeds also depend on markings, entry deflection, approach alignment, visibility, traffic behavior, and local design guidance.
Many low-speed layouts use about 4 to 6 meters from the yield line to the crossing. That offset can help visibility, driver reaction, and pedestrian comfort, but local standards should still govern.
Longer splitter islands are often useful when approach speeds are higher, pedestrian refuge is needed, or stronger approach channelization is desired. They can improve driver guidance and crossing control.
No. It gives a preliminary geometry check only. Detailed design should still confirm turning paths, apron use, curb offsets, drainage, signs, lighting, and safety with more complete engineering tools.
This tool flags multi-lane conditions when approach or circulating lane counts exceed one, or when the overall size becomes larger. That classification is only a practical screening label.
Width harmony helps maintain balanced vehicle paths and more consistent low-speed behavior. Very uneven entry and exit widths can indicate awkward transitions, poor lane guidance, or unnecessary pavement area.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.