IRC ruling design speeds:
\[
\boxed{
\begin{aligned}
\text{Plain terrain} &\rightarrow 100\ \mathrm{km/h}
\text{Rolling terrain} &\rightarrow 80\ \mathrm{km/h}
\text{Mountainous terrain} &\rightarrow 50\ \mathrm{km/h}
\text{Steep terrain} &\rightarrow 40\ \mathrm{km/h}
\end{aligned}
}
\]