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April 25, 2026 11 min read Highway Design
Comparing AASHTO and ERA Design Speed Thresholds: Minimum Curve Radii and Deflections
Compare horizontal curvature requirements, side friction limits, and design speeds between AASHTO and ERA design guidelines.
Highway design guidelines vary by region. In Ethiopia, the **ERA manual** defines standards tailored to local terrain, while international projects often reference **AASHTO guidelines**.
Here we compare side friction limits and minimum curve radii between the two standards.
1. Minimum Radius Comparison Table (e_max = 8%)
| Design Speed (km/h) | AASHTO Side Friction (f) | AASHTO Min Radius (m) | ERA Side Friction (f) | ERA Min Radius (m) |
|---|---|---|---|---|
| 50 | 0.16 | 79 | 0.16 | 85 |
| 80 | 0.14 | 210 | 0.13 | 230 |
| 100 | 0.12 | 375 | 0.11 | 450 |
2. Design Implications
The ERA standards require slightly larger minimum radii at higher speeds due to local terrain and safety factors. Designers must select the correct standard version based on local project requirements to ensure safety.