  We investigate the optimal design of networks for a general transport system.
Our network is built from a regular two-dimensional ($d=2$) square lattice to
be improved by adding long-range connections (shortcuts) with probability
$P_{ij} \sim r_{ij}^{-\alpha}$, where $r_{ij}$ is the Euclidean distance
between sites $i$ and $j$, and $\alpha$ is a variable exponent. We introduce a
cost constraint on the total length of the additional links and find optimal
transport in the system for $\alpha=d+1$. Remarkably, this condition remains
optimal, regardless of the strategy used for navigation, being based on local
or global knowledge of the network structure, in sharp contrast with the
results obtained for unconstrained navigation using global or local
information, where the optimal conditions are $\alpha=0$ and $\alpha=d$,
respectively. The validity of our theoretical results is supported by data on
the US airport network, for which $\alpha\approx 3.0$ was recently found
[Bianconi {\it et al.}, arXiv:0810.4412 (2008)].
