That's true, you can offer other assets as collateral to secure a loan, but that is typically for unsecured loans (where the loan is not applied to some underlying asset). In this case, the underlying asset, the highway and presumably also the land for the highway, would likely be the collateral on the loan.

In any case, one should be extremely wary of state actors in the case of sovereign debt. The potential geopolitical risk can be great, as we saw in the case of Greece when it tried to default on its debt.