Step 1: Identify the force providing centripetal force.
On a curved unbanked road, static friction provides the centripetal force required for circular motion.
Thus,
\[
f=\frac{mv^2}{r}
\]
Maximum static friction is
\[
f_{\max}=\mu mg
\]
Therefore,
\[
\frac{mv^2}{r}=\mu mg
\]
Step 2: Simplify the equation.
Cancelling \(m\),
\[
\frac{v^2}{r}=\mu g
\]
Hence,
\[
r=\frac{v^2}{\mu g}
\]
Step 3: Substitute the given values.
Given:
\[
v=10\text{ ms}^{-1},\quad \mu=0.4,\quad g=10\text{ ms}^{-2}
\]
So,
\[
r=\frac{10^2}{0.4\times 10}
\]
\[
=\frac{100}{4}
\]
\[
=25\text{ m}
\]
Step 4: Final conclusion.
Therefore, the maximum radius of curvature is
\[
\boxed{25\text{ m}}
\]