SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/eng-road-crown-drainage

Why is a straight road surface often crowned or cross-sloped?

The slope makes rainwater drain off the traveled surface.

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  1. To trap water at the center
  2. To keep vehicles centered in the lane
  3. To shed rain toward the edges ✔

Why: A flat surface can let water pond and soften unpaved material or reduce pavement performance. A controlled cross slope gives runoff a path away from traffic lanes.

Huh, didn't know that: Too little crown encourages ponding, while excessive crown can worsen erosion and vehicle stability.

The dad joke
The road wore a crown because it ruled over runoff.
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References

Answer: To shed rain toward the edges. A flat surface can let water pond and soften unpaved material or reduce pavement performance. A controlled cross slope gives runoff a path away from traffic lanes.

  1. FHWA Unpaved Road Dust Management Handbook www.fhwa.dot.gov/clas/ctip/unpaved_roads_dust/ch_6.aspx
    A target crown of 5 percent (± 1%) (Figure 60) assures that the road surface will shed rain.
    Checked 2026-10-10.

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