A quasi-public good is a resource that provides benefits to the public, but could theoretically be restricted if necessary. This differs from a true public good, which remains accessible to everyone virtually all the time. For example, clean air is a public good, because it is not feasible to restrict access to it. On the other hand, roads are a quasi-public good. While theoretically open to all, they can be restricted with the use of a toll system.
Many nations have a mix of public and quasi-public goods to provide needed services and benefits to their citizens. Some services people may think of as public goods are actually quasi-public in nature, although the possibility of exclusion is not necessarily exercised. Libraries, police forces, and firefighters, for example, could be limited to paying customers instead of being freely open to all. There are clear benefits to not restricting access to these resources, which leads them to operate much like public goods.
With resources like air waves, a nation may make a conscious decision to exclude certain users to make the good more useful for all. Without regulation, transmissions could be sent out by competing parties who might crowd a frequency and generate interference. By assigning specific frequencies and reserving some for private government use, a nation can ensure that the air waves remain accessible for as many people as possible. This quasi-public good functions best when restricted.
Similar tactics can be used with roads and bridges. If everyone had unfettered access, these resources might develop congestion, which would make them less useful. By instituting tolls, a government can cut down on traffic to keep the resource functioning properly. This preserves the quasi-public good by excluding some traffic in the interest of facilitating a smooth flow of vehicles. Likewise, governments might restrict operating hours for certain kinds of vehicles to free up space on the road.
Excludable resources are quasi-public goods, whether a government regulates use of these resources or not. An example of shifting use can be seen in the American West, where historically public land was freely used for grazing by ranchers. Over time, the government recognized that this unrestricted use posed some risks because overgrazing limited available grass and contributed to erosion. In response, it set up a licensing system to oblige ranchers to request permission, illustrating that the range was a quasi-public good that needed to be protected through restrictions.