Toll road bonds are a form of municipal bond, usually if not always issued to buy a highway project like a road or a bridge. They're repaid by erecting tollbooths on the project and charging people to use it. These are reasonably safe investments.
They vary from road to road. Some countries - UK, Benelux, Germany, Scandinavia don't have them except for certain bridges & tunnels. Some countries - Switzerland, Austria & the Czech...