Given:
Tread width, $b = 1.25$ m
Height of CG, $h = 0.85$ m
Speed, $V = 18$ km/h = $5$ m/s
Acceleration due to gravity, $g = 9.81$ m/ssuperscript{2}
Overturning Condition:
\[
\frac{V^2}{gR} = \frac{b}{2h}
\quad \Rightarrow \quad
R = \frac{V^2 \cdot 2h}{g \cdot b}
\]
Substitute the values:
\[
R = \frac{(5)^2 \cdot 2 \cdot 0.85}{9.81 \cdot 1.25}
= \frac{42.5}{12.2625} \approx \boxed{3.47 \, {m}}
\]