ERA 2013 Junction & Roundabout Geometry Checker
Evaluate roundabout Inscribed Circle Diameter (ICD), entry deflection radius (R_e), circulatory roadway width, truck apron sizing, design vehicle swept paths, and corner sight triangles (S_tri) based on ERA 2013 guidelines.
INFRADIGITAL CAD — ENGINEERING CALCULATION REPORT
AT-GRADE INTERSECTION GEOMETRIC DESIGN EVALUATION REPORT
Reference Basis: Selected intersection criteria (Referencing ERA 2013 guidelines)
Date: 9/23/2026
Prepared By: User-entered project information
Project: [Insert Project Title]
Section: [Insert Section Details]
Intersection Type: Single-Lane Roundabout
Design Vehicle: DV3 / WB-15 (Semi-Trailer Combination)
Design Standard Manual Option
Geometric Layout & Swept Path Diagram
Single-Lane Roundabout (DV3)ERA 2013 Parameter Verification Checklist
| Parameter | Actual Value | ERA 2013 Requirement | Status |
|---|---|---|---|
| Inscribed Circle Diameter (ICD) Roundabout footprint meets configured criteria. | 36 m | 28 - 45 m | CRITERIA MET |
| Entry Radius (R_e) Good deflection radius. | 20 m | 15 - 25 m | CRITERIA MET |
| Circulatory Roadway Width (W_circ) Adequate clearance provided. | 7 m | ≥ 6 m | CRITERIA MET |
| Central Island Truck Apron Sufficient mounting apron. | 2 m | 1.5 - 3.0 m | CRITERIA MET |
| Unobstructed Corner Sight Distance (S_tri) Keep sight triangle clear of walls, dense trees, or structures for 125m along major corridor. | 125 m | ≥ 125 m | CRITERIA MET |
Intersection Civil Material Takeoff
- Design Vehicle selected: DV3 / WB-15 (Semi-Trailer Combination) (17m length). Ensure swept path templates are checked in CAD per ERA 2013.
- Maintain minimum 2.0% crossfall outwards on circulatory roadways for drainage per ERA 2013 standards.
1. Geometric Evaluation Summary
Overall Assessment Status: CRITERIA MET
Design Speed: Major Road 60 km/h | Minor Road 40 km/h
Unobstructed Corner Sight Distance (S_tri): 125 meters
2. Detailed Parameter Checklist
| Parameter | Actual | Reference Limit | Outcome |
|---|---|---|---|
| Inscribed Circle Diameter (ICD) | 36 m | 28 - 45 m | CRITERIA MET |
| Entry Radius (R_e) | 20 m | 15 - 25 m | CRITERIA MET |
| Circulatory Roadway Width (W_circ) | 7 m | ≥ 6 m | CRITERIA MET |
| Central Island Truck Apron | 2 m | 1.5 - 3.0 m | CRITERIA MET |
| Unobstructed Corner Sight Distance (S_tri) | 125 m | ≥ 125 m | CRITERIA MET |
3. Quantity Takeoff Estimate
| Item Description | Unit | Quantity |
|---|---|---|
| Island & Truck Apron Concrete Paving | m² | 380 |
| Asphalt Concrete Surfacing (Circulatory / Taper Area) | m² | 638 |
| Precast Pre-painted Kerbstones (Mountable / Barrier) | m | 119 |
4. Engineering Notice
Calculation generated by InfraDigital CAD based on the selected methodology and entered parameters. Project-specific engineering review and verification are required before final design, submission or construction use.
Junction & Roundabout Geometry Checker Documentation
Technical specifications, mathematical formulas, and civil guidelines
1. Functional Purpose & Methodology
At-grade intersections and modern roundabouts represent the most critical conflict points on highway networks. The Junction & Roundabout Geometry Checker provides automated geometric design checks based on the ERA Geometric Design Manual 2013 (Chapter 9: Intersections & Roundabouts), ERA Junction Standard Drawings, and NCHRP Report 672.
The tool audits key roundabout dimensions: Inscribed Circle Diameter (ICD), entry radius (R_e), circulatory roadway width, central island truck apron dimensions, entry path deflection to enforce speed reduction (≤ 40 km/h), and corner sight triangles (S_tri) across minor road approaches to prevent truck grounding and collision conflicts.
2. Practical Step-by-Step Instructions
- Select Intersection Category: Choose between Modern Roundabout (Mini, Single-Lane, or Multi-Lane) and Priority T/Cross Intersection.
- Select Design Vehicle: Choose the governing design vehicle class: DV1 (Passenger Car), DV2 (Single Unit Truck), DV3 (Semi-Trailer / WB-15), or DV4 (Articulated Heavy Interstate Hauler).
- Input Geometric Parameters: Enter Inscribed Circle Diameter ICD (m), entry radius R_e (m), entry lane width (m), circulatory roadway width (m), and truck apron width (m).
- Configure Sight Triangles: Enter operating speed of the intersecting roads and minor road setback distance (typically 4.5m to 5.0m from edge of major carriageway).
- Review Compliance Status & Export: The tool checks entry deflection speeds, swept path clearances, and sight triangle adequacy, exporting a clean submittal sheet.
3. Mathematical Formulations & Variable Definitions
Speed control and sight triangle formulations follow standard intersection mechanics:
- Entry Speed (V_entry) from Curvature:V_entry = √[ 127 × R_e × (e + f) ]Where R_e = entry path radius (m), e = entry cross-slope (typically -0.02 m/m away from island), and f = side friction factor (0.20 to 0.25 at low speeds). Entry speed must not exceed 40 km/h for single-lane roundabouts.
- Corner Sight Distance Triangle (S_tri):S_tri = 0.278 × V_major × t_gapWhere V_major = design speed on major road (km/h), and t_gap = critical time gap acceptance (7.5s for passenger cars, 9.5s for heavy articulated trucks DV4 per ERA Chapter 9).
- Circulatory Roadway Width (W_circ): Must satisfy:W_circ ≥ 1.0 to 1.2 × W_entryEnsures adequate maneuvering room for vehicles negotiating the circulatory roadway.
- Truck Apron Outer Slope: Mountable curb with slope 1:5 to 1:10 and height ≤ 75mm to permit rear trailer wheels to track over without overturning.
4. Standard Roundabout Criteria (ERA 2013 Table 9-4)
Geometric threshold limits per ERA 2013 Geometric Design Manual Chapter 9:
| Roundabout Category | Inscribed Circle Diameter (ICD) | Entry Radius (R_e) | Circulatory Width (m) | Target Entry Speed |
|---|---|---|---|---|
| Mini-Roundabout | 13 m - 25 m | 10 m - 15 m | 4.0 m - 6.0 m | ≤ 25 km/h |
| Single-Lane Rural | 35 m - 55 m | 15 m - 25 m | 6.0 m - 8.0 m | ≤ 40 km/h |
| Multi-Lane Rural/Urban | 50 m - 80 m | 20 m - 35 m | 8.0 m - 11.0 m | ≤ 45 km/h |
5. Worked Numerical Example
A single-lane rural roundabout is proposed at the junction of an ERA DS3 corridor and a DS4 link road: Selected Inscribed Circle Diameter: ICD = 45.00 m, Entry Radius: R_e = 22.00 m, Entry Cross-Slope: e = -0.02 m/m, Side Friction: f = 0.22. Major road approach speed is V_major = 60 km/h. Design vehicle is DV3 (Semi-Trailer) with time gap t_gap = 8.5 s.
6. Engineering Assumptions & Limitations
- Swept Path Software Simulation: While this checker verifies entry radius, ICD, and apron width against standard design vehicle dimensions, complex multi-lane entries with articulated trailers (DV4) must be checked using dynamic swept-path software (e.g., AutoTURN or Vehicle Tracking).
- Pedestrian & Cyclist Splitter Island Clearances: Splitter islands must provide a minimum 2.0m wide refuge area for pedestrians and cyclists, situated at least one car length (5.0m to 6.0m) back from the yield line.
- Vertical Gradient Constraints: Roundabouts should ideally be located on grades ≤ 3.0% (and never > 4.0%) to prevent heavy vehicle rollover during circulatory banking maneuvers.
- Professional Verification: Intersection layouts must be evaluated and stamped by a licensed transportation engineer before construction.
7. ERA 2013 vs AASHTO / NCHRP 672 Standard Relationship
ERA 2013 (Chapter 9) adapts the modern roundabout design guidelines from NCHRP Report 672 (Roundabouts: An Informational Guide), standardizing entry deflection speeds to ≤ 40 km/h and mandating mountable central island aprons for Ethiopian freight truck classes (DV3 and DV4).
8. Frequently Asked Questions (FAQs)
Entry path deflection forces approaching drivers to steer into a curved path before entering the circulatory roadway, naturally reducing their speed to below 40 km/h and minimizing collision severity.
A truck apron is a low, mountable concrete collar surrounding the central island that allows the rear wheel assemblies of long articulated trailers to track over without striking high curbs or tipping.
The ICD is the total diameter of the circle inscribed within the outer edge of the circulatory roadway. It defines the overall footprint and capacity of the roundabout.
9. Related Technical Guides & Workflow Integration
For sign and marking requirements at roundabouts, use the Road Signs & Markings Quantity Estimator, and review our guide on Managing Right-of-Way Clearances in Urban Corridors.