Outdoor advertising is subject to governmental regulation at the federal, state and local levels. Regulations generally restrict the size, spacing, lighting and other aspects of advertising structures and pose a significant barrier to entry and expansion in many markets. Federal law, principally the Highway Beautification Act of 1965, or the HBA, regulates outdoor advertising on Federal - Aid Primary, Interstate and National Highway Systems roads. The HBA requires states, through the adoption of individual Federal/State Agreements, to "effectively control" outdoor advertising along these roads, and mandates a state compliance program and state standards regarding size, spacing and lighting. These state standards, or their local and municipal equivalents, may be modified over time in response to legal challenges or otherwise, which may have an adverse effect on our business. The HBA requires any state or political subdivision that compels the removal of a lawful billboard along a Federal - Aid Primary or Interstate highway to pay just compensation to the billboard owner.
All states have passed billboard control statutes and regulations at least as restrictive as the federal requirements, including laws requiring the removal of illegal signs at the owner's expense (and without compensation from the state). Although the Company believes that the number of our billboards that may be subject to removal as illegal is immaterial, and no state in which we operate has banned billboards entirely, from time to time governments have required us to remove signs and billboards legally erected in accordance with federal, state and local permit requirements and laws. Municipal and county governments generally also have sign controls as part of their zoning laws and building codes. We contest laws and regulations that we believe unlawfully restrict our constitutional or other legal rights and may adversely impact the growth of our outdoor advertising business.
Using federal funding for transportation enhancement programs, state governments have purchased and removed billboards for beautification, and may do so again in the future. Under the power of eminent domain, state or municipal governments have laid claim to property and forced the removal of billboards. Under a concept called amortization by which a governmental body asserts that a billboard operator has earned compensation by continued operation over time, local governments have attempted to force removal of legal but nonconforming billboards (i.e., billboards that conformed to applicable zoning regulations when built but which do not conform to current zoning regulations). Although the legality of amortization is questionable, it has been upheld in some instances. Often, municipal and county governments also have sign controls as part of their zoning laws, with some local governments prohibiting construction of new billboards or allowing new construction only to replace existing structures. Although we have generally been able to obtain satisfactory compensation for those of our billboards purchased or removed as a result of governmental action, there is no assurance that this will continue to be the case in the future.
We have continued to expand the deployment of digital billboards, which display static digital advertising copy from various advertisers that change every 6 to 8 seconds. We have encountered some existing regulations that restrict or prohibit these types of digital displays but it has not yet materially impacted our digital deployment. However, new regulations could be enacted to impose greater restrictions on digital billboards due to alleged concerns over aesthetics or driver safety.
The findings of future studies related to the impact of digital billboards on driver safety issues, if any, may result in regulations at the federal or state level that impose greater restrictions on digital billboards. Any new restrictions on digital billboards could have a material adverse effect on both our existing inventory of digital billboards and our plans to expand our digital deployment, which could have a material adverse effect on our business, results of operations and financial condition.