This calculator treats the curb return as a circular fillet connecting two curb tangents at an intersection angle θ. It estimates a conservative curb radius that supports a design vehicle tracking near the lane centerline.
- Select your unit system and confirm the intersection angle.
- Choose a design vehicle preset or enter a custom turning radius.
- Enter lane widths and curb offsets for both approaches.
- Add any edge offset and a clearance allowance for comfort.
- Click Calculate to view results above the form.
- Use Download CSV/PDF to save the calculation.
| Scenario | Angle (°) | Vehicle | Lane A / B | Gutter A / B | Edge | Clearance | Radius R (approx) |
|---|---|---|---|---|---|---|---|
| Urban corner | 90 | Passenger car | 3.5 m / 3.5 m | 0.6 m / 0.6 m | 0.3 m | 0.3 m | ≈ 5.0 m |
| Service access | 90 | Single unit truck | 3.6 m / 3.6 m | 0.6 m / 0.6 m | 0.3 m | 0.5 m | ≈ 10.9 m |
| Bus route | 75 | City bus | 3.7 m / 3.7 m | 0.6 m / 0.6 m | 0.3 m | 0.5 m | ≈ 11.2 m |
Why curb return radius matters
Curb return radius controls how smoothly vehicles transition through a corner and how much encroachment occurs. Larger radii reduce tire scrub and steering effort, but they can increase pedestrian crossing distance and raise turning speeds. This calculator balances a target vehicle radius with lane and curb offsets to estimate a practical starting value.
Key inputs that drive the result
The largest influence is the design vehicle turning radius used by your agency or project brief. Lane widths, gutter offsets, and edge offsets shift the vehicle path away from the curb face. A small clearance allowance adds operational tolerance for tracking variation, mirrors, and snowplow or maintenance margins.
Interpreting the setout outputs
Along with the recommended curb radius, the tool reports tangent length, arc length, and chord length. Tangent length supports field layout from the curb PI to each tangent point. Arc length helps estimate curb quantities, while chord length offers a quick geometric check during staking.
Angle and corner geometry considerations
Skewed intersections change the arc geometry even when the radius stays the same. The calculator uses the intersection angle to compute tangent and arc values, so you can compare corner layouts consistently. For very acute angles, verify available frontage and pedestrian refuge width before finalizing a larger return.
How to validate the estimate
Treat the computed radius as a planning baseline, then confirm with swept‑path analysis for the controlling vehicle. Check curb offsets, driveway flares, and adjacent parking or bike facilities. If encroachment is acceptable, you may reduce the radius; if tracking hits the curb, increase radius or adjust offsets.
1) What should I use for design vehicle radius?
Use the radius defined in your local roadway or site access standard for the controlling vehicle. If unsure, select the closest preset and confirm later with a turning template or CAD swept‑path check.
2) Why does the calculator use the smaller of two offsets?
Using the smaller approach offset is conservative because it assumes the vehicle path is closer to the curb on at least one leg. This helps avoid underestimating radius when approaches differ in lane or curb configuration.
3) What clearance value is reasonable?
A modest clearance accounts for tracking variation and operational comfort. Many projects start with about 0.3–0.6 m (1–2 ft), then adjust based on constraints, speed environment, and verification from swept‑path analysis.
4) Does the result replace a swept‑path analysis?
No. It provides a fast estimate for early design and quantity planning. Final geometry should be checked against your standard design vehicle, curb lines, and any encroachment rules using a turning template or software.
5) Why is sector area shown in square meters?
Area is computed internally in metric for consistency and to avoid rounding issues. You can still use it as a comparative indicator between options, even when you enter and review lengths in feet.
6) How do skewed intersections affect curb layout?
Skew changes tangent length and arc length for the same radius, which can impact available sidewalk and corner space. Use the angle input to compare alternatives, then verify pedestrian geometry and sight lines in your plan.